Title of Invention

POLYTRIMETHYLENE TEREPHTHALATE RESIN AND METHOD FOR PREPARING THE SAME."

Abstract

A polytrimethylene terephthalate resin which comprises 90 to 100 mole % of a trimethylene terephthalate recurring unit and 0 to 10 mole % of at least one monomer unit derived from a monomer which is different from the monomer used for forming said recurring unit and is copolymerizable with at least one monomer used for forming VDLGUHFXUULQJXQLWDQGH[KLELWV $ DOLPLWLQJYLVFRVLW\ >@ RIWR4.0 dl/g, (B) a molecular weight distribution (Mw/Mn) of 2.0 to 2.7, (C) a brightness index L value (L-1) of 70 to 100 and a chromaticness index b* value (b*-1) of -5 to 25, and (D) , after the heat treatment at 180°C for 24 hours in an air atmosphere, a L value (L-2) of 70 to 100 and b* value (b*-2) of -5 to 25. (FIG. - 1)

Full Text

TITLE OF THE INVENTION Polytrimethylene terephthalate resin BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a polytrimethylene terephthalate resin. More particularly, the present in- vention is concerned with a polytrimethylene terephtha- late resin comprised mainly of trimethylene terephthalate recurring units, which has the following characteristics: an intrinsic viscosity of from 0.8 to 4.0 dl/g; a molecu- lar weight distribution (Mw/Mn) of from 2.0 to 2.7; a psychometric lightness L-value (L-l) of from 70 to 100 and a psychometric chroma b*-value (b*-l) of from -5 to 25; and a psychometric lightness L-value (L-2) of from 70 to 100 and a psychometric chroma b*-value (b*-2) of from -5 to 25 as measured after heating the polytrimethylene terephthalate resin at 180 °C for 24 hours in air. By using the polytrimethylene terephthalate resin of the present invention, a shaped article having high strength and excellent color can be stably produced on a commer- cial scale. Further, the present invention is also con- cerned with a method for stably producing the poly- trimethylene terephthalate resin with high productivity on a commercial scale. Prior Art A polytrimethylene terephthalate resin (hereinaf- ter, referred to as "ptt") not only has characteristics similar to those of a nylon (e.g., soft feeling due to the low elasticity of the resin, excellent elastic re- covery and good dyeability), but also has characteris- tics similar to those of a polyethylene terephthalate (hereinafter, referred to as "PET") (e.g., wash and wear property, dimensional stability and discoloration resistance). Therefore, a PTT has been attracting at- tention as a raw material which can be used for produc- ing carpets, clothes, shaped articles and the like. For further expanding the application fields of a PTT, it has been desired to improve the strength and color of the fibers and shaped articles of a PTT. For improving the strength of the fibers and shaped articles of a polymer, it is necessary to in- crease the polymerization degree of the polymer, and to narrow the molecular weight distribution of the polymer so as to reduce the amount of low molecular weight components in the polymer. Further, for improving the color of the fibers and shaped articles of a polymer, it is necessary not only to improve the whiteness of the polymer, but also to improve the heat resistance of the polymer so as to prevent the discoloration of the polymer, which is caused by the thermal history experi- enced by the polymer during the drying, melting and the like. As a polymerization method for producing a PTT, a melt polymerization method is widely known. For exam- ple, Unexamined Japanese Patent Application Laid-Open Specification No. Hei 5-262862 (corresponding to U.S. Patent No. 5,340,909), WO98/23662, WO01/14450 and WO01/14451 disclose a method in which a melt polymeri- zation is performed using a polymerization vessel equipped with a stirrer. The above-mentioned polymeri- zation vessel has advantages in that it exhibits excel- lent volume efficiency and has a simple structure. Such a polymerization vessel can be used on a small scale for efficiently performing a polymerization to produce a polymer having a high polymerization degree. However, when the above-mentioned polymerization vessel is used for performing a polymerization on a commercial scale, the depth of the liquid reaction mixture in the polymerization vessel inevitably becomes deep, leading to a marked occurrence of heat decomposition of the polymer. Thus, a polymer having high polymerization degree cannot be produced on a commercial scale. Various techniques have been proposed for produc- ing a PTT having a high polymerization degree by melt polymerization. Examples of such techniques include a technique in which a lower alcohol diester of terephthalic acid and trimethylene glycol are subjected to a transesterification reaction and a polycondensa- tion reaction in the presence of a titanium compound, wherein the molar ratio of the lower alcohol diester of terephthalic acid to trimethylene glycol is in the range of from 1/1.2 to 1/1.8 (Unexamined Japanese Pat- ent Application Laid-Open Specification No. Sho 51- 140992); a technique in which an organometal catalyst is used as a polycondensation catalyst, and an organic sulfonic acid or an aliphatic carboxylic acid is used as a catalyst auxiliary (U.S. Patent No. 4,611,049); a technique in which a tin catalyst is used as a polycon- densation catalyst (Unexamined Japanese Patent Applica- tion Laid-Open Specification No. Hei 5-262862 (corre- sponding to U.S. Patent No. 5,340,909)),- a technique in which a specific titanium catalyst is used as a poly- condensation catalyst (Unexamined Japanese Patent Ap- plication Laid-Open Specification Nos. 2000-159875 and 2000-159876); a technique in which an antimony compound is used as a polycondensation catalyst (Chemical Fiber International Vol. 46, pp 263-264, 1996); a tech- nique in which heat decomposition of a PTT is sup- pressed by using a hindered phenol-type stabilizer hav- ing a specific structure (Unexamined Japanese Patent Application Laid-Open Specification No. Sho 51-142097); and a technique in which the by-production of acrolein (formed by heating of a prepolyiner and a polymer in air during the polymerization) is suppressed by blocking the terminals of the prepolymer and the polymer with a phosphorus-containing stabilizer and a hindered phenol -type stabilizer (WO98/23662 and WO99/11709). However, the above-mentioned techniques are disadvantageous in that the molecular weight of the obtained PPT is not satisfactorily high, in that a lowering of the molecu- lar weight of the PTT occurs during the molding thereof, and/or in that a discoloration of the PTT occurs. Thus, by the above-mentioned techniques, a PTT having satis- factory properties cannot be obtained. Further, a method is proposed in which, for the purpose of obtaining a high molecular weight PTT which exhibits excellent heat stability during the spinning of the PTT, a solid-phase polymerization of a PTT pre- polymer having a relatively low molecular weight is performed, in which the PTT prepolymer has not suffered heat decomposition and has excellent color (Unexamined Japanese Patent Application Laid-Open Specification No. Hei 8-311177, Japanese Patent Application prior-to- examination Publication (Tokuhyo) No. 2000-502392 and Korean Patent No. 1998-061618). However, the solid- phase polymerization proceeds while releasing trimethylene glycol (hereinafter, referred to as "TMG") from the surface of pellets of the prepolymer during the polymerization reaction. Therefore, the polymeri- zation degree varies depending on the size and shape of the pellets, and also varies depending on the position in the pellets. Therefore, the PTT obtained by this method is markedly non-uniform with respect to the po- lymerization degree. Further, in the solid-phase po- lymerization, the pellets of the solid prepolymer get rubbed with one another over a long period of time, thereby generating a polymer powder which becomes a loss. Furthermore, in the above-mentioned method, the solid-phase polymerization should be conducted after the production of the prepolymer by the melt polymeri- zation and the like, and thus, the entire process for producing a PTT becomes complicated and costly. Still further, the presence of the polymer powder in the spinning process causes breakage or fuzzing of polymer fibers. For removing the polymer powder, an addition step therefor becomes necessary. As a method for producing a PTT having a high po- lymerization degree only by melt polymerization, a technique has been proposed in which the polymerization is performed using a disc ring reactor or a cage type reactor (WO00/64962) or a disc and donut conductor (U.S. Patent No. 5,599,900) to withdraw the TMG efficiently from the reaction system. However, each of the above- mentioned apparatuses is a horizontal agitation-type polymerizer which is equipped with a rotary driving part. Therefore, in the above-mentioned method, when a polymerization is performed under a high vacuum for ob- taining a polymer having a high polymerization degree, it is impossible to seal the driving part completely. Thus, it is impossible to prevent the intrusion of a trace amount of oxygen into the polymer, and hence, a discoloration of the polymer inevitably occurs. Espe- cially, in the case of a PTT, such discoloration mark- edly occurs. When the driving part is sealed with a sealing liquid, it is likely that the sealing liquid gets mixed with the polymer, thereby lowering the qual- ity of the resultant PTT. Further, even when the driv- ing part of the apparatus is tightly sealed at the start of the operation thereof, the tightness of the sealing is lowered during the operation conducted for a long period of time. Thus, the above-mentioned method also has a serious problem with respect to the mainte- nance of the apparatuses. On the other hand, a method for producing a resin (other than PTT) is known in which the polymerization apparatus used therein does not have a rotary driving part, and a polymerization is performed by allowing a prepolymer to fall from a perforated plate (free-fall polymerization method). For example, a method is disclosed in which a polyester prepolymer is allowed to fall in the form of fibers in vacuo in an attempt to obtain a polyester having a desired molecular weight (U.S. Patent No. 3,110,547). In this method, a polymerization reaction is performed in a one pass mode without recycling the polymer, because the recycling of a polymer which has already been allowed to fall in the form of fibers causes the lowering of the quality of the final polyes- ter. However, the above-mentioned method has the fol- lowing disadvantages. The polymer in the form of fi- bers are easily broken during the polymerization reac- tion, thereby causing a disadvantageously large varia- tion in quality of the final condensation polymer prod- ucts. In addition, a low molecular weight condensation polymer is scattered from the polymer fibers during the polymerization reaction to stain the lower surface of the perforated plate. Due to such staining of the lower surface of the perforated plate, it becomes dif- ficult to cause the polymer to fall in the form of fi- bers, so that the polymer fibers contact with one an- other to cause breakage of the polymer fibers or the polymer fibers are combined together to form a thick fiber in which the reaction does not proceed effi- ciently. In order to solve these problems, various methods have been proposed. Examples of such methods include a method in which a polyester or a polyamide is produced by allowing a prepolymer to fall along and in contact with the surface of a perforated guide or a wire guide, which is vertically arranged in a reaction vessel, so that the polymerization of the prepolymer is effected during the fall thereof (Examined Japanese Patent Ap- plication Publication No. Sho 48-8355 and Unexamined Japanese Patent Application Laid-Open Specification No. Sho 53-17569); a method for continuously condensation- polymerizing bis-(ß-hydroxyalkyl) terephthalate (which is an initial-stage condensation product of polyethyl- ene terephthalate (PET)), in which bis-(ß-hydroxyalkyl) terephthalate is allowed to fall along and in contact with wire guides in an atmosphere of inert gas, wherein the wire guides are hung vertically from the holes of a perforated plate, so that the polymerization of bis-(ß- hydroxyalkyl) terephthalate is effected during the fall thereof (Examined Japanese Patent Application Publica- tion No. Hei 4-58806); and a method for producing a melt-polycondensation polymer, such as a polyester, a polyamide and a polycarbonate, in which a melt -polycondensation prepolymer is caused to absorb an in- ert gas, and then, polymerized under reduced pressure (WO99/65970 which also discloses an apparatus used in the method). However, each of the above patent documents only describes a method for producing a polyester, such as a PET, or nylon, and has no proposal or suggestion about the production of a PTT. As a result of the studies of the present inventors, it has been found that, when any of the above-mentioned methods are simply applied to the production of a PTT (that is, when the production of a PTT is conducted by the above-mentioned methods, using raw materials and conditions which are conven- tionally used in the production of a PTT), a foaming of a polymer vigorously occurs, thereby staining the lower surface of the perforated plate or the inner wall of the reaction vessel having the guides provided therein. A PTT is susceptible to heat decomposition, as compared to a PET. Therefore, the stain caused by the above -mentioned vigorous foaming of the polymer is easily decomposed. When the resultant decomposition products get mixed with the polymer, disadvantages are caused in that the quality of the polymer is lowered, in that the desired polymerization degree cannot be obtained, and in that the obtained PTT suffers discoloration. Fur- ther, the simple application of the above-mentioned methods to the production of PTT is accompanied by problems in that it is difficult to achieve a satisfac- torily high polymerization degree, and in that the fi- nal PTT contains low molecular weight polymers, which result in a broad molecular weight distribution of the final polymer and are likely to lower the mechanical strength of an ultimate shaped article. As mentioned above, the conventional methods for producing a PTT have the following problems: (1) It is difficult to produce a PTT having a high polymerization degree only by melt polymerization (i.e., without solid-phase polymerization) on a commercial scale. When the production of a PTT is conducted by solid-phase polymerization, disadvantages are caused in that the molecular weight distribution of the obtained PTT becomes too broad, and in that the production proc- ess becomes complicated and costly (due to the loss caused by the formation of polymer powder). (2) When it is attempted to produce a PTT having high polymerization degree by using a specific catalyst or stabilizer, the obtained polymer is likely to suffer heat decomposition and discoloration. With respect to the free-fall method (in which a polymerization is performed by allowing a prepolymer to fall freely in the form of fibers from a perforated plate) and the guide-wetting fall method (in which a polymerization is performed by allowing a prepolymer to fall along and in contact with a guide), it is known that these methods can be used for producing polyamide and polyesters (such as a PET) other than a PTT. How- ever, the application of the above-mentioned methods to the production of a PTT is not known, and a PTT having satisfactory properties cannot be obtained by a simple application of these methods to the production of a PTT which is different from the above-mentioned polyamides and other polyesters with respect to the melt viscosity, and resistance to heat decomposition, and volatilities of by-products. For these reasons, there has been a demand for the development of a method which can be used for producing an excellent PTT having a high polymerization degree on a commercial scale, which PTT can be used for stably producing a shaped article having excellent strength and color. SUMMARY OF THE INVENTION In this situation, the present inventors have made extensive and intensive studies with a view toward solving the above-mentioned problems accompanying the prior art techniques. As a result, it has unexpectedly been found that, by polymerizing a molten form of a specific trimethylene terephthalate prepolymer by the so-called "guide-wetting fall process" at a temperature of from the crystalline melting point of the prepolymer to 290 ° C, it becomes possible to produce a specific excellent polytrimethylene terephthalate resin which can be used for stably producing a shaped article exhibiting excellent strength and color. The above- mentioned specific polytrimethylene terephthalate resin is composed mainly of recurring units of trimethylene terephthalate, and has the following characteristics: an intrinsic viscosity [n] of from 0.8 to 4.0 dl/g; a molecular weight distribution (Mw/Mn) of from 2.0 to 2.7; a psychometric lightness L-value (L-1) of from 10 to 100 and a psychometric chroma b*-value (b*-l) of from -5 to 25; and a psychometric lightness L-value (L- 2) of from 70 to 100 and a psychometric chroma b*-value (b*-2) of from -5 to 25 as measured after heating the polytrimethylene terephthalate resin at 180 ° C for 24 hours in air. The present invention has been completed, based on these novel findings. Accordingly, it is an object of the present inven- tion to provide a polytrimethylene terephthalate resin which can be used for stably producing a shaped article having excellent strength and color on a commercial scale. The foregoing and other objects, features and ad- vantages of the present invention will be apparent from the following description and appended claims taken in connection with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS In the drawings: Fig. 1 shows an explanatory diagrammatic view of an example of a reaction vessel which can be used in the present invention; Fig. 2 shows explanatory diagrammatic views of ex- amples of an inert gas absorption device and a reaction vessel, which can be used in the present invention; and Each of Figs. 3 to 6 is an explanatory diagram- matic view of one form of a production system used for practicing the method of the present invention. Description of Reference Characters and Numerals A: Polytrimethylene terephthalate prepolymer B: Polytrimethylene terephthalate resin C: Mixture of raw materials (including a starting monomer, a reactant monomer, a catalyst, an additive and the like) D: Exhaust gas E: Inert gas 1, 14, 18, 28, 32, N2 and N7: Transferring pump 2 : Inlet for prepolymer A 3 , N4 : Perforated plate 4 : Observing window 5, N5: Guide 5 " : Polymer falling along and in contact with the guide 6 : Inlet for gas 7, 13, 17, 21, 24, 27 and 31: Vent 8 : Withdrawal pump for polymer 5" 9: Outlet for polymer 5" 10: Polymerizer 11: Esterification reaction vessel 12, 16, 20, 23, 26 and 30: Agitation element 15: First vertical agitation type polymerizer 19: Second vertical agitation type polymerizer 22: Horizontal agitation type polymerizer 25: First transesterification reaction vessel 29: Second transesterification reaction vessel Nl: Inert gas absorption device N3 : Inlet for raw materials N6: Inlet for inert gas DETAILED DESCRIPTION OF THE INVENTION In one aspect of the present invention, there is provided a polytrimethylene terephthalate resin com- prising -. 90 to 100 mole % of trimethylene terephthalate re- curring units, and 0 to 10 mole % of at least one monomer unit se- lected from the group consisting of monomer units ob- tained from comonomers which are other than the mono- mers used for forming the trimethylene terephthalate recurring units and which are copolymerizable with at least one of the monomers used for forming the trimethylene terephthalate recurring units, the polytrimethylene terephthalate resin having the following characteristics (A) to (D): (A) an intrinsic viscosity [r|] of from 0.8 to 4.0 dl/g; (B) a molecular weight distribution of from 2.0 to 2.7 in terms of the Mw/Mn ratio, wherein Mw represents the weight average molecular weight of the poly- trimethylene terephthalate resin and Mn represents the number average molecular weight of the polytrimethylene terephthalate resin; (C) a psychometric lightness L-value (L-l) of from 70 to 100 and a psychometric chroma b*-value (b*-l) of from -5 to 2 5; and (D) a psychometric lightness L-value (L-2) of from 7 0 to 100 and a psychometric chroma b*-value (b*-2) of from -5 to 25 as measured after heating the poly- trimethylene terephthalate resin at 180 ° C for 24 hours in air. For easy understanding of the present invention, the essential features and various preferred embodi- ments of the present invention are enumerated below. 1. A polytrimethylene terephthalate resin comprising: 90 to 100 mole % of trimethylene terephthalate re- curring units, and 0 to 10 mole % of at least one monomer unit se- lected from the group consisting of monomer units ob- tained from comonomers which are other than the mono- mers used for forming the trimethylene terephthalate recurring units and which are copolymerizable with at least one of the monomers used for forming the trimethylene terephthalate recurring units, the polytrimethylene terephthalate resin having the following characteristics (A) to (D): (A) an intrinsic viscosity [r\] of from 0.8 to 4.0 dl/g; (B) a molecular weight distribution of from 2.0 to 2.7 in terms of the Mw/Mn ratio, wherein Mw represents the weight average molecular weight of the poly- trimethylene terephthalate resin and Mn represents the number average molecular weight of the polytrimethylene terephthalate resin; (C) a psychometric lightness L-value (L-l) of from 70 to 100 and a psychometric chroma b*-value (b*-l) of from -5 to 2 5; and (D) a psychometric lightness L-value (L-2) of from 70 to 100 and a psychometric chroma b*-value (b*-2) of from -5 to 25 as measured after heating the polytrimethylene terephthalate resin at 180 ° C for 24 hours in air. 2. The polytrimethylene terephthalate resin according to item 1 above, wherein the polytrimethylene terephthalate resin has an intrinsic viscosity [r\] of from 1.25 to 2.5 dl/g. 3. The polytrimethylene terephthalate resin according to item 1 or 2 above, which has a terminal carboxyl group content of from 0 to 20 meq/kg. 4. The polytrimethylene terephthalate resin according to any one of items 1 to 3 above, which has a molecular weight distribution of from 2.0 to 2.6. 5. The polytrimethylene terephthalate resin of any one of items 1 to 4 above, which is in the form of pel- lets. 6. The polytrimethylene terephthalate resin according to item 5 above, wherein the pellets have an average weight of from 1 to 1000 mg per pellet, and wherein the pellets contains a powder of the polytrimethylene terephthalate resin in an amount of 0 to 0.5 % by weight, based on the total weight of the pellets, which powder passes through a 30-mesh filter and does not pass through a 300-mesh filter. 7. The polytrimethylene terephthalate resin according to item 5 or 6 above, wherein the pellets have a crys- tallinity (Xc) of 40 % or less, wherein the crystallin- ity is defined by the following formula: Xc (%) = {?c X (?s - ?a)}/{?s X (?c - ?a)} X 100 wherein pa is 1.300 g/cm3 which is an amor- phous density of trimethylene terephthalate homopolymer, pc is 1.431 g/cm3 which is a crystal density of trimethylene terephtha- late homopolymer, and ps represents a den- sity (g/cm3) of the pellets. 8. A method for producing a polytrimethylene terephthalate resin, which comprises: (1) providing a molten form of a trimethylene terephthalate prepolymer comprising: 90 to 100 mole % of trimethylene terephthalate re- curring units, and 0 to 10 mole % of at least one monomer unit se- lected from the group consisting of monomer units ob- tained from comonomers which are other than the mono- mers used for forming the trimethylene terephthalate recurring units and which are copolymerizable with at least one of the monomers used for forming the trimethylene terephthalate recurring units, the trimethylene terephthalate prepolymer having an intrinsic viscosity [n] of from 0.2 to 2 dl/g, and (2) polymerizing the molten form of a trimethylene terephthalate prepolymer at a temperature which is equal to or higher than the crystalline melting point of the prepolymer and is not higher than 290 °C under reduced pressure by the guide-wetting fall process in which the prepolymer is allowed to fall along and in contact with the surface of a guide so that polymeriza- tion of the prepolymer is effected during the fall thereof. 9. The method according to item 8 above, wherein the molten prepolymer is continuously fed to a polymeriza- tion reaction zone for effecting the polymerization of the prepolymer in the step (2) and the resultant poly- trimethylene terephthalate resin produced in the step (2) is continuously withdrawn from the polymerization zone, so that the step (2) for prepolymer polymeriza- tion is continuously performed. 10. The method according to item 8 or 9 above, wherein the guide has at least one portion selected from the group consisting of a concave portion, a convex portion and a perforated portion. 11. The method according to any one of items 8 to 10 above, wherein the prepolymer falling along and in con- tact with the surface of the guide is in a foaming state. 12. The method according to any one of items 8 to 11 above, wherein the polymerization in the step (2) is performed, while introducing inert gas to the polymeri- zation reaction zone. 13. The method according to item 12 above, wherein the amount of the inert gas introduced to the polymeriza- tion reaction zone is in the range of from 0.05 to 100 mg per gram of the polytrimethylene terephthalate resin withdrawn from the polymerization reaction zone. 14. The method according to item 12 or 13 above, wherein at least a part of the inert gas is introduced to the polymerization reaction zone, independently from the feeding of the trimethylene terephthalate prepoly- mer to the polymerization reaction zone. 15. The method according to any one of items 12 to 14 above, wherein at least a part of the inert gas is in- troduced to the polymerization reaction zone in such a form as absorbed or contained in the trimethylene terephthalate prepolymer. 16. The method according to any one of items 8 to 15 above, wherein the prepolymer has an intrinsic viscos- ity [t|] of from 0.5 to 2.0 dl/g and a terminal carboxyl group ratio of 50 % or less in terms of the molar ratio (%) of the terminal carboxyl groups of the prepolymer to all terminal groups of the prepolymer. 17. The method according to any one of items 8 to 16 above, wherein the prepolymer is produced by at least one polymerization method selected from the following methods (a) to (d): (a) a polymerization method using a vertical agi- tation type polymerizer; (b) a polymerization method using a horizontal agitation type polymerizer; (c) a polymerization method using a free-fall po- lymerizer having a perforated plate; and (d) a polymerization method using a thin film type polymerizer. 18. The method according to item 17 above, wherein the prepolymer is produced by the method (b). 19. A polytrimethylene terephthalate resin produced by the method of any one of items 8 to 18 above. Hereinbelow, the present invention is described in detail. The polytrimethylene terephthalate resin of the present invention comprises: 90 to 100 mole % of trimethylene terephthalate re- curring units, and 0 to 10 mole % of at least one monomer unit se- lected from the group consisting of monomer units ob- tained from comonomers which are other than the mono- mers used for forming the trimethylene terephthalate recurring units and which are copolymerizable with at least one of the monomers used for forming the trimethylene terephthalate recurring units. The trimethylene terephthalate recurring units are formed by reacting a terephthalic acid material with a trimethylene glycol material. Examples of terephthalic acid materials include terephthalic acid, and diesters of terephthalic acid, such as dimethyl terephthalate. Examples of trimethylene glycol materials include 1,3- propanediol, 1,2-propanediol, 1,1-propanediol, 2,2- propanediol, and a mixture thereof. From the viewpoint of stability, 1,3-propanediol is especially preferred, as a trimethylene glycol material. Examples of the above-mentioned comonomers include ester-forming monomers, such as 5-sulfoisophthalic acid sodium salt, 5-sulfoisophthalic acid potassium salt, 4- sulfo-2,6-naphthalenedicarboxylic acid sodium salt, 3,5-dicarboxylic acid benzenesulfonic acid tetramethyl- phosphonium salt, 3,5-dicarboxylic acid benzenesulfonic acid tetrabutylphosphonium salt, 3,5-dicarboxylic acid benzenesulfonic acid tributylmethylphosphonium salt, 3,6-dicarboxylic acid naphthalene-4-sulfonic acid tetrabutylphosphonium salt, 3,6-dicarboxylic acid naph- thalene-4-sufonic acid tetramethylphosphonium salt, 3,5-dicarboxylic acid benzenesulfonic acid ammonium salt, 3,2-butanediol, 1,3-butanediol, 1,4-butanediol, neopentyl glycol, 1,5-pentamethyleneglycol, 1,6- hxamethylene glycol, heptamethylene glycol, octamethyl- ene glycol, decamethylene glycol, dodecamethylene gly- col, 1,4-cyclohexanediol, 1,3-cyclohexanediol, 1,2- cyclohexanediol, 1,4-cyclohexanedimethanol, 1,3- cyclohexanedimethanol, 1,2-cyclohexanedimethanol, ox- alic acid, malonic acid, succinic acid, gultaric acid, adipic acid, heptanedioic acid, octanedioic acid, seba- cic acid, dodecanedioic acid, 2-methylgultaric acid, 2- methyladipic acid, fumaric acid, maleic acid, itaconic acid, 1,4-cyclohexanedicarboxylic acid, 1,3- cyclohexanedicarboxylic acid and 1,2- cyclohexanedicarboxylic acid. The polytrimethylene terephthalate resin of the present invention may further comprise: cyclic or chain oligomers other than polytrimethylene terephthalate resin; monomers, such as dimethyl terephthalate (here- inafter, referred to as "DMT"), terephthalic acid (hereinafter, referred to as "TPA") and trimethylene glycol (hereinafter, referred to as "TMG"); and/or any of various additives, such as a delustering agent, a thermal stabilizer and a flame retardant. For obtaining a fiber or a shaped article, which has excellent strength and color, and which is aimed at in the present invention, it is necessary not only to improve the polymerization degree of the polytrimethyl- ene terephthalate resin while narrowing the distribu- tion of polymerization degree, but also to improve the whiteness of the resin while improving the resistance of the resin against the discoloration at high tempera- tures . As a yardstick of the polymerization degree, the intrinsic viscosity [r|] can be used. For obtaining the above-mentioned excellent fiber or shaped article, which has excellent strength, it is necessary that the resin (used for producing the fiber or shaped article) have an intrinsic viscosity of 0.8 dl/g or more. On the other hand, from the viewpoint of improving the moldability of the resin and the ease in measuring the amount of the resin in a gear pump, the intrinsic vis- cosity should not be too high. For this reason, it is necessary that the intrinsic viscosity [n] be 4.0 dl/g or less. The intrinsic viscosity [n] is preferably in the range of from 1.05 to 3.0 dl/g, more preferably from 1.25 to 2.5 dl/g, still more preferably from 1.3 to 2.0 dl/g. Further, for improving the strength of the above- mentioned fiber or shaped article, not only should the average polymerization degree be high, but also the amount of a low molecular weight polymer should be low, namely, the molecular weight distribution should be narrow. In the present invention, the molecular weight distribution is expressed in terms of a value (Mw/Mn) which is calculated by dividing the weight average mo- lecular weight (Mw) by the number average molecular weight (Mn), wherein each of Mw and Mn is measured by gel permeation chromatography. In the present inven- tion, it is necessary that the Mw/Mn value be 2.7 or less. The Mw/Mn value is preferably 2.6 or less, more preferably 2.5 or less, still more preferably 2.4 or less. In general, the lower limit of the molecular weight distribution of a condensation polymer is 2. With respect to the color of the polytrimethylene terephthalate resin, for suppressing the darkening of the ultimate shaped article and for enabling the resin to ex- hibit a desired color by the use of a dye or a pigment, it is necessary that the polytrimethylene terephthalate resin have a psychometric lightness L-value (L-l) of 70 or more and a psychometric chroma b*-value (b*-l) of -5 or more. From the viewpoint of suppressing the discol- oration of the resin by heat decomposition, it is neces- sary that the polytrimethylene terephthalate resin have a psychometric chroma b*-value of 25 or less. There is no particular limitation with respect to the upper limita- tion of the (L-l) value, but in general, it is 100. The (L-l) value is preferably 75 or more, more preferably 80 or more. The (b*-l) value is preferably from -3 to 15, more preferably from -2 to 10. Further, from the studies of the present inventors, it has been found that, for improving the whiteness of the ultimate shaped article, a polytrimethylene terephthalate resin used for producing a shaped article should exhibit not only excellent whiteness, but also excellent resistance to discoloration during the heat- ing of the resin (e.g., during the high temperature drying, melt molding or the like of the resin). The reason for this is not clear, but it is presumed that the discoloration of the resin is caused by not only the thermal decomposition of the resin per se, but also certain substances or functional groups which are in- evitably contained in the polytrimethylene terephtha- late resin. It is considered that the above-mentioned substances or functional groups are formed by heat de- composition of a prepolymer (used in the production of the polytrimethylene terephthalate resin) and/or the polytrimethylene terephthalate resin. However, espe- cially when the below-mentioned polymerization method proposed by the present inventors is employed, it is possible to obtain a polytrimethylene terephthalate resin which has excellent resistance to the discolora- tion during the heating. The reason for this is con- sidered as follows. The proposed method is advanta- geous not only in that the leakage of oxygen into the polymerization reaction system can be suppressed, thereby preventing the formation of the above-mentioned substances and functional groups, but also in that the surface area of the prepolymer being polymerized in the polymerizer employed in this method is extremely large as compared to the case of polymerizers which are con- ventionally employed for the production of a poly- trimethylene terephthalate resin, and the surface of the prepolymer is efficiently renewed, so that the above-mentioned substances or functional groups, if any, can be easily withdrawn from the reaction system. As a yardstick of the susceptibility to discolora- tion by heat, the color of a polytrimethylene terephthalate resin after heating at 180 °C for 2 4 hours in air can be used. In the present invention, the polytrimethylene terephthalate resin heated under the above-mentioned conditions needs to have a psycho- metric lightness L-value (L-2) of 70 or more, and a psychometric chroma b*-value (b*-2) of from -5 to 25. The psychometric lightness L-value (L-2) is preferably 7 5 or more, more preferably 8 0 or more. The psychomet- ric chroma b*-value (b*-2) is preferably from -4 to 21, more preferably from -3 to 18, still more preferably from -2 to 16. In the present invention, for preventing the hy- drolysis of the polytrimethylene terephthalate resin even when the shaping of the resin which has not been fully dried is performed, and for improving the weatherability of a shaped article of the poly- trimethylene terephthalate resin, it is preferred that the polytrimethylene terephthalate resin of the present invention has a terminal carboxyl group content of not more than 30 meq/kg (based on the weight of the poly- trimethylene terephthalate resin). The terminal car- boxyl group content is preferably not more than 20 meq/kg, more preferably not more than 15 meq/kg, still more preferably not more than 10 meq/kg. It is pre- ferred that the terminal carboxyl group content is as small as possible. The polytrimethylene terephthalate resin of the present invention can be used, for example, for produc- ing an extrusion-molded article, such as a film or a sheet. In the production of such an extrusion-molded article, it is desired that the polytrimethylene terephthalate resin simultaneously satisfies all of such requirements as a very high molecular weight, a very narrow molecular weight distribution and a very low terminal carboxyl group content. Therefore, it is preferred that the polytrimethylene terephthalate resin for use in the production of an extrusion-molded arti- cle has an intrinsic viscosity [r\] of from 1.25 to 2.5 dl/g, an Mw/Mn ratio of 2.6 or less and a terminal car- boxyl group content of 20 meq/kg or less; it is more preferred that the resin has an intrinsic viscosity [n] of from 1.30 to 2.0 dl/g, an Mw/Mn ratio of 2.5 or less and a terminal carboxyl group content of 15 meq/kg or less; and it is still more preferred that the resin has an intrinsic viscosity [?] of from 1.30 to 2.0 dl/g, an Mw/Mn ratio of 2.4 or less and a terminal carboxyl group content of 10 meg/kg or less. When the produc- tion of the polytrimethylene terephthalate resin is conducted by the below-mentioned polymerization method proposed by the present inventors, the polymerization rate is high and the surface area of the prepolymer be- ing polymerized is large. Therefore, not only can the polymerization degree be improved, but also the termi- nal carboxyl group content can be lowered to a level which has never been achieved by the conventional melt polymerization. Further, in this method, the polymeri- zation degree can be improved while maintaining a high piston flowability (the "high piston flowability" means a property that the flowing resin in the polymerizer has no local variation of the flow rate, i.e., the flowing resin as a whole has a uniform flow rate). Thus, by the proposed method, it is possible to obtain a polytrimethylene terephthalate resin having a narrow molecular weight distribution, i.e., a resin which does not contain polymers having largely different molecular weights. When a polytrimethylene terephthalate resin is produced by solid-phase polymerization, the resin produced exhibits a high polymerization degree. How- ever, in the solid-phase polymerization, the polymeri- zation degree varies depending on the reaction site in the pellets (i.e., whether the reaction site is at an inner portion or outer portion of the pellets), and also varies depending on the size and shape of the pel- lets, so that it is very difficult to obtain a polymer having a narrow molecular weight distribution. By the proposed polymerization method, it has, for the first time, become possible to produce a polytrimethylene terephthalate resin which can be suitably used for the commercial scale production of the above-mentioned ex- trusion-molded article. The polytrimethylene terephthalate resin of the present invention, which is in a molten form obtained immediately after the production thereof, can be spun or shaped. Alternatively, the resin can be formed into pellets, and then re-melted at the time of spinning or shaping of the resin. When the resin is used in the form of pellets, it is desired that the amount of loss is small, and that the pellets can be extruded uniformly by means of an extruder. Therefore, it is preferred that each pellet has an appropriate size, and that the amount of polymer powder adhering to the surface of the pellets is small. In the present invention, it is preferred that the average weight of pellets is from 1 to 1,000 mg per pellet. The pellets having such an average weight is advantageous in that uniform extrusion of the pellets by means of a extrusion molding machine becomes easy, that the pellets can be handled with ease at the time of transportation, drying and spinning thereof, and that the drying rate of the pellets becomes fast. The average weight of the pellets is more preferably from 5 to 500 mg per pellet, still more preferably from 10 to 200 mg per pellet. With respect to the shape of the pellet, there is no particular limitation, and the shape of the pellet may be any of a sphere, a rectangle, a cylinder and a cone. However, from the viewpoint of ease in handling of the pellets, it is preferred that the length of the largest portion of each pellet is 15 mm or less, more advantageously 10 mm or less, still more advantageously 5 mm or less. With respect to the polymer powder adhering to the surface of the pellets, it is preferred that the amount of the polymer powder is in the range of from 0 to 0.5 % by weight, based on the total weight of the pel- lets, which powder passes through a 30-mesh filter and does not pass through a 300-mesh filter. When the amount of the polymer powder is 0.5 % by weight or less, not only is the loss decreased, but also it becomes possible to prevent the clogging of a pneumatic line (i.e., pipe line in which pellets are transferred by gas) or a filter of an air-exhaust ventilator attached to a dryer, and to suppress the pressure variation in an extruder during the spinning, molding or compounding, so that ultimate products having a uniform quality can be easily obtained. It is preferred that the amount of the polymer powder is as small as possible. From a practical point of view, the amount of the polymer pow- der is preferably in the range of from 0 to 0.2 % by weight, more preferably from 0 to 0.1 % by weight, still more preferably from 0 to 0.05 % by weight, based on the total weight of the pellets. Further, it is preferred that the pellets have a crystallinity (Xc) of 0 to 40 %, wherein the crystal- Unity is defined by the following formula: Xc (%) = {?c X (?s - ?a)}/(?s X (?c - ?a)> X 100 wherein pa is 1.300 g/cm3 which is an amor- phous density of trimethylene terephthalate homopolymer, pc is 1.431 g/cm3 which is a crystal density of trimethylene terephtha- late homopolymer, and pE represents a den- sity (g/cm3) of the pellets. The above-mentioned crystal density of trimethyl- ene terephthalate homopolymer (1.431 g/cm3) is a theo- retical value which is calculated from the number of crystal lattices of trimethylene terephthalate ho- mopolymer. The above crystal density value (1.431 g/cm3) is described in "Poritorimechirenterefutareto no Kesshoudanseiritsu (Crystal elasticity of poly- trimethylene terephthalate)" ("Zairyou (Material)", written by Katsuhiko Nakamae, Vol. 35, No. 396, p. 1067, 2000). Further, the amorphous density of trimethylene terephthalate homopolymer (1.300 g/cm3) is obtained by measuring the density of a sample amorphous polymer ob- tained by quenching a trimethylene terephthalate ho- mopolymer in a molten form. (With respect to the sample polymer, it can be confirmed that the sample polymer is amorphous, when no crystal peak is observed in the analysis of the sample polymer by X-ray diffractorae- try. ) When the pellets have the above-mentioned crystal- linity, it becomes possible to prevent a problem which is unique to a PTT and is unlikely to arise in the case of other polyesters, such as a PET and a PBT (polybuty- lene terephthalate), i.e., a problem that pellets be- come brittle and generate a large amount of polymer powder during the transportation of the pellets by means of a pneumatic conveyor or a feeder. It is pre- ferred that the crystallinity is from 0 to 35 %, more advantageously from 0 to 30 %. In the present invention, the crystallinity of a pellet means an average value of crystallinity values measured at different portions of the pellet. Specifi- cally, for example, it is preferred that, when a sur- face portion of the pellet is cut away from a central portion of the pellet and crystallinities of these por- tions are measured, both of the surface portion and the central portion are in the above-mentioned crystallin- ity range. Further, it is preferred that the differ- ence in crystallinity between the surface portion and the central portion is 40 % or less, more advanta- geously 30 % or less, still more advantageously 20 % or less . For obtaining pellets having the above-mentioned crystallinity, it is preferred that a polytrimethylene terephthalate in a molten form, which is obtained by polymerization, is extruded into a strand or a sheet, and, subsequently, the obtained strand or sheet is im- mersed in a coolant, such as water, to cool the strand or sheet, followed by cutting of the strand or sheet to obtain pellets. It is preferred that the temperature of the coolant is 20 °C or less, more advantageously 15 °C or less, still more advantageously 10 °C or less. From the viewpoint of economy and ease in handling of the pellets, it is preferred to use water as a coolant. Naturally, the temperature of water as a coolant is 0 °C or more. It is preferred that the cutting to ob- tain pellets is performed with respect to the strand or sheet solidified by cooling the extruded strand or sheet to 55 ° C or lower within 120 seconds after the extrusion. Hereinbelow, the method for producing the poly- trimethylene terephthalate resin of the present inven- tion is described in detail. The method for producing the polytrimethylene terephthalate resin of the present invention comprises the following steps (1) and (2): (1) providing a molten form of a trimethylene terephthalate prepolymer comprising: 90 to 100 mole % of trimethylene terephthalate re- curring units, and 0 to 10 mole % of at least one monomer unit se- lected from the group consisting of monomer units ob- tained from comonomers which are other than the mono- mers used for forming the trimethylene terephthalate recurring units and which are copolymerizable with at least one of the monomers used for forming the trimethylene terephthalate recurring units, the trimethylene terephthalate prepolymer having an intrinsic viscosity [?] of from 0.2 to 2 dl/g, and (2) polymerizing the molten form of a trimethylene terephthalate prepolymer at a temperature which is equal to or higher than the crystalline melting point of the prepolymer and is not higher than 290 °C under reduced pressure by the guide-wetting fall process in which the prepolymer is allowed to fall along and in contact with the surface of a guide so that polymeriza- tion of the prepolymer is effected during the fall thereof. As mentioned above, a polymerizer which does not have a rotary driving part has been proposed as a poly- merizer for producing resins other than a poly- trimethylene terephthalate. However, the melt -polycondensation reaction for producing a poly- trimethylene terephthalate greatly differs from the melt-polycondensation reactions for producing other types of polyesters, such as a PET and a PBT, and for producing polyamides. Therefore, a practical produc- tion of a polytrimethylene terephthalate cannot be re- alized simply by employing the polymerizers which have been used for the production of the other types of polyesters and for the production of polyamides. The important differences between the polytrimethylene terephthalate, and polyamides and the other types of polyesters (such as a PET and a PBT) are explained be- low. Firstly, both of the melt-polycondensation reac- tion for producing polyamides and the melt -polycondensation reaction for producing the other types of polyesters are equilibrium reactions. However, the equilibrium constants of the above two reactions greatly differ from each other. In general, the equi- librium constants of the melt-polycondensation reac- tions for producing polyamides are in the order of 10 , whereas the equilibrium constants of the melt -polycondensation reactions for producing the other types of polyesters is approximately 1. Thus, despite that both of the reactions for producing polyamides and the reactions for producing the other types of polyes- ters are polycondensation reactions, the equilibrium constants of the reactions for producing the other types of polyesters are extremely small as compared to those of the reaction for producing the polyamides. When an equilibrium constant of a certain reaction is large, the reaction proceeds even without efficiently withdrawing a by-product from the reaction system. Therefore, it is easy to increase the polymerization degrees of polyamides. With respect to the other types of polyesters (such as a PET and a PBT), although the equilibrium constants of the reactions for producing a PET and a PBT are small, the by-products can be easily withdrawn from the reaction systems, so that it is also easy to increase the polymerization degree of each of a PET and a PBT. The reason for this is as follows. In the case of a PET, the PET has a satisfactory heat sta- bility, and hence, a polymerization reaction for pro- ducing the PET can be performed at a temperature (gen- erally from 280 to 300 °C) which is much higher than the boiling point (198 °C) of ethylene glycol which is a by-product of the polymerization reaction. By per- forming the polymerization at such a high temperature, the vapor pressure of ethylene glycol can be increased and, hence, the ethylene glycol can be easily withdrawn from the reaction system. Also in the case of a PBT, 1,4-butanediol, which is a by-product of the polymeri- zation reaction for producing a PBT, can be easily withdrawn from the reaction system. The reason for this has not yet been elucidated, but is considered as follows. In the polymerization reaction system for producing a PBT, 1,4-butanediol (which is a by-product having a high boiling point) is converted into low boiling point substances, such as tetrahydrofuran (formed by hydrolysis) and butadiene (formed by heat decomposition), which low boiling point substances can be easily withdrawn from the reaction system. As in the case of the polymerization reactions for producing the other types of polyesters, the polymeri- zation reaction for producing a polytrimethylene terephthalate has a low equilibrium constant, and hence, the by-produced trimethylene glycol (TMG) needs to be efficiently withdrawn from the reaction system so as to advance the polymerization reaction. The TMG has a boiling point as high as 214 °C. On the other hand, the polytrimethylene terephthalate is susceptible to heat decomposition, so that the polymerization reaction for producing the polytrimethylene terephthalate needs to be performed at a low temperature. Therefore, it is difficult to withdraw the TMG from the reaction system. Further, when the polymerization degree of a poly- trimethylene terephthalate becomes high, the following disadvantage is caused. The viscosity of the poly- trimethylene terephthalate also becomes high and, hence, it becomes difficult to withdraw TMG from the reaction system. In such a case, a heat decomposition of the polytrimethylene terephthalate markedly occurs, so that the reaction rate is lowered, and then, the polymeriza- tion degree of the polytrimethylene terephthalate starts to be lowered. If the polymerization reaction is performed at a high temperature, the withdrawal of the TMG becomes easier; however, the heat decomposition of the polytrimethylene terephthalate markedly occurs, thereby leading to the above-mentioned disadvantage that, when the polymerization degree of a poly- trimethylene terephthalate becomes high and the viscos- ity of the polytrimethylene terephthalate also becomes high, the reaction rate is lowered, and then, the po- lymerization degree of the polytrimethylene terephtha- late starts to be lowered. However, as a result of the studies of the present inventors, it has unexpectedly been found that, when a molten form of a polytrimethylene terephthalate pre- polymer having an intrinsic viscosity within the above- mentioned specific range is polymerized by the above- mentioned guide-wetting fall process at an appropriate temperature under reduced pressure, a polytrimethylene terephthalate can be produced without causing the prob- lems accompanying the conventional polymerization meth- ods for producing the other types of polyesters, such as a PET, and for producing polyamides, i.e., methods in which a polymerization is performed by allowing a prepolymer to fall in the form of fibers or to fall along and in contact with a guide, such as a wire. With respect to the guide-wetting fall process, reference can be made, for example, to U.S. Patent Nos . 5,589,564, 5,840,826, 6,265,526 and 6,320,015. The features of the method of the present inven- tion are described below. Firstly, for obtaining a polytrimethylene terephthalate resin having a high polymerization degree only by melt polymerization, it is required not only to suppress the heat decomposition of the PTT, but also to remove efficiently a TMG (by-product of the reaction for producing the PTT). In the method of the present invention, these requirements are satisfied by perform- ing the polymerization by allowing a prepolymer to fall along and in contact with the guide at an appropriate temperature under reduced pressure to thereby increase the surface area of the prepolymer. Further, by allow- ing the polymer to fall along and in contact with a guide, it becomes possible to prevent the disadvanta- geous fluctuation of qualities of the products, wherein the fluctuation occurs due to the breakage of the poly- mer flow in the polymerizer. Secondly, for preventing the discoloration of the polymer which is caused by the entrance of oxygen and a sealing liquid into the polymer, it is required to em- ploy a polymerizer having no rotary driving part at a portion of the polymerizer, which portion contains a gaseous phase during the polymerization. In the guide- wetting fall process, there is no need for the polymer- izer to have a rotary driving part in such a gaseous phase portion of the polymerizer, and the polymerizer exhibits an excellent sealability under a highly vac- uumized condition, so that a discoloration caused by leakage of oxygen into the polymerizer can be almost completely prevented. Further, since the polymerizer has no rotary driving part, a mixing of a sealing liq- uid into a polymer would not occur, and the maintenance of the polymerizer is easy. Thus, a high quality poly- trimethylene terephthalate which is free from a disad- vantageous discoloration can be obtained. Thirdly, for stably producing a polytrimethylene terephthalate resin on a commercial scale, it is re- quired to suppress the foaming of the prepolymer intro- duced into the polymerization reaction zone, so as to prevent the staining of the lower surface of the perfo- rated plate and inner wall of the polymerizer. In the method of the present invention, this requirement is satisfied by introducing a prepolymer having an in- creased viscosity, more specifically by polymerizing a prepolymer having a high intrinsic viscosity within a specific range at a specific low temperature. By vir- tue of this feature, it becomes possible to suppress the lowering of the quality of the polytrimethylene terephthalate resin, which is caused by the mixing or entrance of the stain into the polytrimethylene terephthalate resin. Thus, by the method of the present invention, the problems accompanying the conventional techniques for performing a melt-polycondensation to produce a poly- trimethylene terephthalate resin can be solved, and it becomes possible to produce resin which is free from a discoloration caused by heat decomposition, and which has high quality and high polymerization degree. Such effects are unexpected from the conventional techniques for performing polymerization reactions for producing polyamides and the other types of polyesters. In the present invention, it is necessary to in- troduce a trimethylene terephthalate prepolymer in a molten form through the holes of the perforated plate into the polymerization reaction zone at a temperature which is equal to or higher than the crystalline melt- ing point of the prepolymer and is not higher than 290 °C, wherein the prepolymer has an intrinsic viscos- ity [r|] of from 0.2 to 2 dl/g. In the present invention, the "trimethylene terephthalate prepolymer" means a polycondensation product which has a molecular weight lower than the fi- nal polytrimethylene terephthalate resin obtained therefrom. In the present invention, it is important to sup- press the scattering of the prepolymer in the polymeri- zation reaction zone, which is caused by the vigorous foaming of the prepolymer. In the present invention, for suppressing the scattering of the prepolymer, and for effectively removing the by-produced TMG from the reaction system, it is essential to introduce the above-mentioned prepolymer having a specific intrinsic viscosity [r|] into the polymerization reaction zone at the above-mentioned specific temperature. When the scattering of the prepolymer is caused by the vigorous foaming of the prepolymer introduced through the holes of the perforated plate into the polymerization reac- tion zone, the polymer adheres to the lower surface of the perforated plate and the inner wall of the polymer- izer, thereby staining them. The prepolymer which ad- heres to the lower surface of the perforated plate and the inner wall of the polymerizer remains in the poly- merizer for a long period of time, and hence, suffers heat decomposition to form a discolored low molecular weight product and/or a discolored modified product. When such discolored products get mixed with the final 48 polytrimethylene terephthalate resin, the quality of the resin is lowered. For suppressing the scattering of the prepolymer caused by the vigorous foaming thereof, it is necessary that the prepolymer has an in- trinsic viscosity [?] of 0.2 or more. Further, for constantly producing a resin having a narrow molecular weight distribution, it is preferred that the prepoly- mer has a high viscosity. The reason for this is as follows. In the guide-wetting fall process employed in the present invention, a fluctuation of polymerization degree of the prepolymer being polymerized may occur due to the fluctuation of the falling rate of the pre- polymer and the fluctuation of the level of surface re- newal of the prepolymer, leading to a broad molecular weight distribution of the final resin. For preventing such fluctuation of the falling rate of the prepolymer and fluctuation of the level of surface renewal of the prepolymer, it is preferred that the viscosity of the prepolymer is high. However, on the other hand, when the prepolymer has too high an intrinsic viscosity, it becomes diffi- cult to withdraw efficiently the by-produced TMG from the reaction system. Further, when the intrinsic vis- cosity is too high, the amount of the by-produced TMG becomes extremely small, so that it becomes difficult to cause an appropriately mild foaming of the prepoly- mer during the fall thereof in the polymerizer, which foaming of the prepolymer is an important feature of the polymerization method of the present invention. Thus, it becomes difficult to improve the polymeriza- tion degree of the polytrimethylene terephthalate resin. For preventing the above disadvantages, it is nec- essary that the prepolymer has an intrinsic viscosity [?] of 2 dl/g or less. It is preferred that the pre- polymer has an intrinsic viscosity [?] of from 0.3 to 1.8 dl/g, more advantageously from 0.4 to 1.5 dl/g. In addition, for preventing disadvantages (e.g., vigorous foaming of the prepolymer, and heat decomposi- tion of the prepolymer) which are caused by the low viscosity of the prepolymer and which lead to a lower- ing of the quality of the polytrimethylene terephtha- late resin, it is necessary that the temperature of the prepolymer introduced into the polymerization reaction zone be 290 °C or less. On the other hand, for uni- formly introducing the prepolymer into the polymeriza- tion reaction zone without solidifying the prepolymer in the holes of the perforated plate, and for causing the prepolymer as a whole to fall uniformly along and in contact with the guide without partially solidifying the prepolymer during the fall thereof, it is necessary that the temperature of the prepolymer introduced into the polymerization reaction zone be equal to or higher than the crystalline melting point of the prepolymer. In the present invention, the crystalline melting point of the prepolymer means a temperature at which an endothermic peak ascribed to the melting of a crystal is observed in a differential scanning calorimetry (DSC) chart of the prepolymer, wherein the DSC chart is obtained by means of an input compensation-type differ- ential scanning calorimeter (trade name: Pyris 1; manu- factured and sold by Perkin Elmer, Inc., U.S.A.) under the following conditions: Measuring temperature: 0 to 280 °C, and Rate of temperature elevation: 10 "C/min. The temperature of the prepolymer introduced into the polymerization reaction zone is preferably 5 ° C or more higher than the crystalline melting point of the prepolymer and not higher than 280 °C, more preferably 10 ° C or more higher than the crystalline melting point of the prepolymer but not higher than 27 5 ° C, still more preferably 15 oC or more higher than the crystal- line melting point of the prepolymer but not higher than 26 5 °C. In the present invention, the above-mentioned per- forated plate is a plate having a plurality of through-holes through which the prepolymer is intro- duced into the polymerization reaction zone. There is no particular limitation with respect to the thickness of the perforated plate. However, the thickness of the perforated plate is generally in the range of from 0.1 to 300 mm, preferably from 1 to 200 mm, more preferably from 5 to 150 mm. The perforated plate needs to have a strength sufficient to sustain the pressure inside the chamber of the polymerizer, into which the molten pre- polymer is fed. Also, when the guide(s) in the polym- erization reaction zone of the polymerizer is (are) hung from the perforated plate, it is necessary that the perforated plate can sustain the weight of the guide(s) and the molten prepolymer which is falling along and in contact with the surface of the guide(s). For this reason, it is preferred that the perforated plate is reinforced with a rib. The shape of the hole of the perforated plate is generally selected from a circle, an ellipse, a triangle, a slit, a polygon, a star and the like. The area at the opening of each hole is generally in the range of from 0.01 to 100 cm2, preferably from 0.05 to 10 cm , more preferably from 0.1 to 5 cm . Further, nozzles or the like may be at- tached to the holes of the perforated plate. The dis- tance between mutually adjacent holes of the perforated plate is generally from 1 to 500 mm, preferably from 25 to 100 mm, as measured between the respective centers of the mutually adjacent holes. The perforated plate may have tubes attached thereto, such that the hollow portions of the tubes serve as the holes of the perfo- rated plate. Further, the hole of the perforated plate may have a tapered configuration. It is preferred that the size and shape of the hole are determined so that the pressure loss which occurs when the molten form of a prepolymer passes through the perforated plate is from 0.01 to 5 MPa, more preferably from 0.1 to 5 MPa. When the pressure loss is in the above-mentioned range, it becomes easy to obtain a resin having a further im- proved polymerization degree (the reason for this is not clear). In general, it is preferred that the mate- rial used for the perforated plate is selected from the group consisiting of metallic materials, such as stainless steel, carbon steel, Hastelloy, nickel, tita- nium, chromium and alloys other than mentioned. Further, it is preferred that a filter is provided in the flowing path of the prepolymer in the polymer- izer at a point which is upstream of the perforated plate. The reason for this is that the filter can be used for removing an impurity, if any, which is con- tained in the prepolymer, and which may cause the clog- ging of the holes of the perforated plate. The type of filter is appropriately selected so that the impurity having a size larger than the diameter of the holes of the perforated plate can be removed, and that the fil- ter is not destroyed by the passage of the prepolymer therethrough. Examples of methods for causing the molten pre- polymer to pass downwardly through a perforated plate provided in the polymerizer and fall along and in con- tact with the guide include a method in which the pre- polymer is allowed to fall only by liquid head or by gravity, and a method in which the prepolymer on the perforated plate is pressurized by using a pump or the like to thereby force the molten prepolymer to pass downwardly through a perforated plate. It is preferred that the fluctuation of the amount of falling prepoly- mer is suppressed by means of a pump which has a meas- uring ability, such as a gear pump. There is no particular limitation with respect to the number of holes of the perforated plate. The ap- propriate number of holes of the perforated plate var- ies depending on the polymerization conditions (such as a polymerization temperature and a polymerization pres- sure), the amount of the catalyst used, the range of the molecular weight of the prepolymer, and the like. For example, when it is intended to produce the resin at a rate of 100 kg/hr, it is preferred that the perfo- rated plate has 1 to 104 holes, more advantageously 2 to 102 holes. In the present invention, it is necessary that the prepolymer which has been introduced through the holes of the perforated plate into the polymerization reac- tion zone is polymerized by allowing the prepolymer to fall along and in contact with the guide in the polym- erization reaction zone under reduced pressure. It is preferred that the prepolymer in the polymerization re- action zone is in a foaming state such that the bubbles formed in the prepolymer in the polymerization reaction zone would not be broken instantaneously. It is espe- cially preferred that the prepolymer at a lower portion of the guide is in a foaming state. Needless to say, it is most preferred that the whole of the prepolymer in the polymerization reaction zone is in a foaming state. The guide used in the method of the present inven- tion may be any of a wire, a chain, a wire mesh (each of the chain and the wire mesh is made by combining wires), a jungle gym-like body (having a lattice struc- ture composed of wires), a flat or curved thin plate, a perforated plate, and a filler column (which is formed by regularly or irregularly piling fillers together). For efficiently withdrawing TMG from the reaction system, it is preferred that the surface area of the falling prepolymer is increased. Therefore, it is pre- ferred that the guide is a wire, a chain, a wire mesh or a jungle gym-like body. Further, for more effi- ciently withdrawing TMG from the reaction system to further improve the polymerization rate, it is espe- cially preferred not only to increase the surface area of the guide, but also to form concave portion(s) and/or convex portion(s) on the surface of the guide which are arranged along the length thereof, so as to promote the agitation and surface renewal of the pre- polymer falling along and in contact with the surface of the guide. Thus, it is preferred that the guide has at least one portion selected from the group consisting of a concave portion, a convex portion and a perforated portion. Specifically, it is preferred to use a guide having a structure which hampers the fall of the poly- mer, such as a chain, a jungle gym-like body or a wire having concavo-convex portions on the surface thereof along which the prepolymer falls. Needless to say, the above-mentioned guides can be used in combination. In the present specification, the term "wire" means a body in which the ratio of the length of the body to the average perimeter of the cross-section of the body is very large. There is no particular limita- tion with respect to the cross-sectional area of the wire. However, in general, the cross-sectional area is in the range of from 10-3 to 102 cm2 , preferably from 10-2 to 10 cm2, more preferably from 10"1 to 1 cm2. There is no particular limitation with respect to the shape of the cross-section of the guide, and the shape is generally selected from a circle, an ellipse, a tri- angle, a quadrangle, a polygon, a star and the like. The shape of the cross-section of the wire may be uni- form or may vary along the length of the wire. The wire may be hollow. Further, the wire may be made of a single strand, or made of a plurality of strands, wherein, for example, the strands are twisted together. The surface of the wire may be smooth or may have con- cavo-convex portions as mentioned above, a locally pro- truding portion or the like. There is no particular limitation with respect to the material used for the wire, but the material is generally selected from the group consisting of, for example, stainless steel, car- bon steel, Hastelloy, nickel, titanium, chromium and other alloys. If desired, the wire may be subjected to surface treatment. Examples of surface treatments in- elude plating, lining, passivation, and washing with an acid. The "wire mesh" means a body which is made by com- bining the above-mentioned wires so as to form a lat- tice. The wires can be linear or curved, and the angle between the combined wires can be appropriately se- lected. With respect to the area ratio of the wires of the wire mesh to the open spaces (which ratio is meas- ured with respect to the projected image of the wire mesh), there is no particular limitation, but the area ratio is generally in the range of from 1/0.5 to 1/1,000, preferably from 1/1 to 1/500, more preferably from 1/5 to 1/100. It is preferred that the area ratio of the wire mesh does not vary horizontally relative to the vertical direction of the wire mesh. Further, it is preferred that the area ratio of the wire mesh along the vertical length of the wire mesh does not vary or varies such that, when the wire mesh is provided as the guide in the polymerizer, the area of each open space at a lower portion of the wire mesh becomes smaller than that at an upper portion of the wire mesh (which means that the area ratio at a lower portion of the wire mesh becomes larger than that at an upper portion of the wire mesh). In the present invention, the "chain" means a body in which rings formed by the above-mentioned wires are linked together. The shape of rings can be a circle, an ellipse, a rectangle, a square or the like. The rings can be linked in one dimension, two dimensions or three dimensions. In the present invention, the term "jungle gym -like body" means a material in which the above -mentioned wires are three-dimensionally combined with one another so as to form a lattice. The wires used can be linear or curved, and the angle between the com- bined wires can be appropriately selected. Examples of wires having convex portion(s) and/or concave portion(s) on the surface thereof (along and in contact with which the prepolymer is allowed to fall) include a wire to which a rod having a circular or po- lygonal cross-section is attached such that the rod ex- tends substantially vertically of the wire, and a wire to which a disc-shaped or cylindrical-shaped body is attached such that the wire penetrates through the disc-shaped or cylindrical-shaped body around the cen- ter thereof. It is preferred that the convex portion has a height which is at least 5 mm larger than the diameter of the wire. As a specific example of wires having convex portion(s), there can be mentioned a wire to which a plurality of discs are attached at intervals of from 1 to 500 mm, in which each disc has a diameter which is at least 5 mm larger than the diameter of the wire and not more than 100 mm, and a thickness of from approximately 1 to 10 mm. With respect to the chain, jungle gym-like body and wire having concavo-convex portions on the surface thereof, which are used as the guides, there is no par- ticular limitation on the volume ratio of the skeleton of the guide (e.g., wires used to form the guide) to the open spaces in the guide. However, in general, the volume ratio is in the range of from 1/0.5 to 1/10 , preferably from 1/10 to 1/10 , more preferably from 1/102 to 1/105. It is preferred that the volume ratio does not vary horizontally of the downwardly extending guide. Further, it is preferred that the volume ratio of the downwardly extending guide along the length of the guide does not vary or varies such that, when the guide is provided in the polymerizer, the volume ratio at a lower portion of the guide becomes larger than that at an upper portion of the guide. The above-mentioned guides can be used individu- ally or in combination, depending on the configuration of the guides. When the guide is a wire or a linear chain, the number of guide(s) used is generally in the range of from 1 to 100,000, preferably from 3 to 50,000. When the guide is a wire mesh, a chain formed by two- diraensionally combining the wires, a thin plate or a perforated plate, the number of guide(s) used is gener- ally in the range of from 1 to 1,000, preferably from 2 to 100. When the guide is a chain formed by three- dimensionally combining the wires, a jungle gym-like body or a filler column, the number of the guide(s) can be appropriately selected depending on the sizes of the polymerizer and the polymerization reaction zone where the guide(s) is (are) provided. When a plurality of guides are used, it is pre- ferred to arrange the guides so as for the guides not to contact with each other by using a spacer or the like. In the present invention, in general, the prepoly- mer is introduced through at least one hole of the per- forated plate into the polymerization reaction zone where the prepolymer is allowed to fall along and in contact with the guide. The number of holes of the perforated plate can be appropriately selected depend- ing on the shape of the guide. In the method of the present invention, the prepolymer which has passed through a single hole of the perforated plate can be allowed to fall along and in contact with a plurality of guides. However, for causing the prepolymer to fall uniformly so as to obtain a resin having a narrow mo- lecular weight distribution constantly, it is preferred that the number of guide(s) along which the prepolymer (which has passed through a single hole of the perfo- rated plate) is allowed to fall is small. For example, when the guide is a wire, it is preferred that the num- ber of guide(s) along which the prepolymer (which has passed through a single hole of the perforated plate) is allowed to fall is 3 or less. There is no particu- lar limitation with respect to the position of the guide(s) in the polymerizer so long as the prepolymer can fall along and in contact with the guide(s), and the guide(s) can be provided such that the guide passes through the hole of the perforated plate or is hung be- low the hole of the perforated plate. With respect to the distance over which the molten prepolymer (having passed through the holes of the per- forated plate) falls along and in contact with the sur- face of the guide, the distance is preferably from 0.3 to 50 m, more preferably from 0.5 to 20 m, still more preferably from 1 to 10 m. The flow rate of prepolymer passing through the holes of the perforated plate is preferably in the range of from 10+2 to 10 liters/hr per hole of the perforated plate, more preferably from 0.1 to 50 li- ters/hr per hole of the perforated plate. When the flow rate of prepolymer is within the above-mentioned range, it becomes possible to prevent a marked lowering of the polymerization rate and the productivity of the final resin. In the method of the present invention, it is pre- ferred that the average falling time of the prepolymer is in the range of from 10 seconds to 100 hours, more preferably from 1 minute to 10 hours, still more pref- erably from 5 minutes to 5 hours, most preferably from 20 minutes to 3 hours. In the method of the present invention, as men- tioned above, it is necessary that the polymerization reaction (performed by allowing the prepolymer to fall along and in contact with the guide) be performed under reduced pressure. By performing the polymerization re- action under reduced pressure, TMG (which is by- produced during the polymerization reaction) is effi- ciently withdrawn from the reaction system so as to ad- vance the polymerization reaction. The reduced pres- sure means a pressure which is lower than the atmos- pheric pressure. Generally, it is preferred that the polymerization is conducted under a pressure of 100,000 Pa or less, more preferably 10,000 Pa or less, still more preferably 1,000 Pa or less, most preferably 100 Pa or less. There is no particular limitation with re- spect to the lower limit of the pressure under which the polymerization is conducted. However, from the viewpoint of the size of the equipment for reducing the pressure in the reaction system, it is preferred that the pressure is 0.1 Pa or more. Further, an inert gas which does not adversely af- fect the polymerization reaction can be introduced into the reaction system under reduced pressure, so as to remove the by-produced TMG in such a form as entrained by the inert gas . In the method of the present inven- tion, the inert gas is generally used in an amount of from 0.005 to 100 mg per gram of the polytrimethylene terephthalate resin withdrawn from the polymerization reaction zone. Conventionally, it has been understood that the introduction of inert gas into a polycondensation reac- tion system lowers the partial pressure of a by-product formed during the polycondensation reaction, thereby displacing the equilibrium of the reaction in the di- rection of the desired product formation. However, in the present invention, the inert gas is introduced into the reaction zone only in a very small amount, and hence, the improvement of the polymerization rate by the lowering of partial pressure of a by-product cannot be expected. Thus, from the conventional understanding, the function of the inert gas used in the method of the present invention cannot be explained. From the stud- ies of the present inventors, it has surprisingly been found that the introduction of inert gas into the po- lymerization reaction zone causes an appropriately mild foaming of the molten prepolymer on the guide, so that not only is the surface area of the molten prepolymer greatly increased, but also the surface renewal of the prepolymer vigorously occurs without staining the inner wall of the polymerizer. It is presumed that a vigor- ous movement of the prepolymer at various portions (in- cluding inner and surface portions) thereof causes the remarkable improvement in the polymerization rate. As the inert gas introduced into the polymeriza- tion reaction zone, it is preferred to use inert gas which does not cause discoloration, denaturation or de- composition of the polymer. Preferred examples of in- ert gases include nitrogen gas, argon gas, helium gas, carbon dioxide gas and a lower hydrocarbon gas. Need- less to say, a mixture of the above mentioned gases can also be used in the present invention. As the inert gas, it is more preferred to use nitrogen gas, argon gas, helium gas and/or carbon dioxide gas. Among these gases, nitrogen gas is most preferred from the view- point of availability. In the present invention, the amount of inert gas introduced into the polymerization reaction zone may be very small. Specifically, it is preferred that the amount of inert gas is in the range of from 0.05 to 100 mg per gram of the final resin withdrawn from the po- lymerization reaction zone. By using the inert gas in an amount of 0.05 mg or more per gram of the final res- in withdrawn from the polymerization reaction zone, it becomes possible to foam the polymer satisfactorily so as to increase the polymerization degree. On the other hand, by using the inert gas in an amount of 100 mg or less, it becomes easy to maintain the appropriate reduced pressure in the polymerization reaction zone. It is preferred that the amount of inert gas introduced into the polymerization reaction zone is in the range of from 0.1 to 50 mg per gram of the final resin with- drawn from the polymerization reaction zone, more ad- vantageously from 0.2 to 10 mg. Examples of methods for introducing the inert gas into the polymerization reaction zone include a method in which at least a part of the inert gas is introduced to the polymerization reaction zone, independently from the feeding of the trimethylene terephthalate prepoly- mer to the polymerization reaction zone, and a method in which at least a part of the inert gas is introduced to the polymerization reaction zone in such a form as absorbed and/or contained in the trimethylene terephthalate prepolymer, such that the inert gas is caused to be discharged from the prepolymer under re- duced pressure in the polymerization reaction zone. These two methods can be used individually or in combi- nation . In the present invention, it is preferred to em- ploy the above-mentioned method in which at least a part of the inert gas is introduced to the polymeriza- tion reaction zone in such a form as absorbed by the trimethylene terephthalate prepolymer or in such a form as contained in the trimethylene terephthalate prepoly- mer. The former means that the inert gas is dissolved in the prepolymer, and is not present in the form of bubbles in the prepolymer. On the other hand, the lat- ter means that the inert gas is present in the form of bubbles in the prepolymer. When the inert gas is pre- sent in the form of bubbles in the prepolymer, it is preferred that the size of each bubble is as small as possible. It is preferred that the average diameter of each bubble is 5 mm or less, more advantageously 2 mm or less. When the inert gas is introduced to the polymeri- zation reaction zone, independently from the feeding of the prepolymer to the polymerization reaction zone, it is preferred to feed the inert gas to the polymerizer at a position remote from the perforated plate and close to the outlet for withdrawing the final resin. Further, it is preferred to feed the inert gas to the polymerizer at a position remote from the vent to which a vacuum gas discharge line is connected. On the other hand, examples of methods for causing the inert gas to be absorbed by and/or contained in the prepolymer include a method using any of conventional absorption devices, such as a packed column type ab- sorption device, a tray-containing column type absorp- tion device, a spraying device-containing column type absorption device (in which a liquid is sprayed in a gas to be absorbed in the liquid), a turbulent contact absorption device, a gas-liquid film cross-contacting type absorption device, a high-speed rotary flow type absorption device, an absorption device utilizing me- chanical force, which are described in "Kagaku Souchi Sekkei"Sousa Shiriizu No. 2, Kaitei Gasu Kyushu (Design and Operation of Chemical Devices, No. 2, Gas Absorp- tion (Revised Version))", pp. 49-54 (published on March 15, 1981 by Kagaku Kogyosha, Inc., Japan); and a method in which the inert gas is injected into the prepolymer in a conduit for transferring the prepolymer to the po- lymerizer. Most preferred is a method using a device in which the prepolymer is allowed to fall along and in contact with the surface of a guide in an atmosphere of inert gas to thereby cause the prepolymer to absorb the inert gas during the fall thereof. In this method, in- ert gas having a pressure higher than the pressure in- side the polymerizer is introduced into the gas absorp- tion device. The pressure of the inert gas introduced into the gas absorption device is preferably from 0.01 to 1 MPa, more preferably from 0.05 to 0.5 MPa, still more preferably from 0.1 to 0.2 MPa. In both of the above-mentioned methods for intro- ducing the inert gas into the polymerization reaction system, it is preferred that at least a part of the prepolymer falling in the polymerization reaction zone is in a foaming state. It is especially preferred that the prepolymer at a lower portion of the polymerization reaction zone is a forming state. Needless to say, it is most preferred that the whole of the prepolymer fal- ling in the polymerization reaction zone is in a foam- ing state. In the present invention, the "foaming state" encompasses both a state in which the formed bubbles are immediately broken and a state in which the formed bubbles are maintained for a relatively long time. In the method of the present invention, it is nec- essary that the temperature in the polymerization reac- tion zone is equal to or higher than the crystalline melting point of the prepolymer, and not higher than 290 °C. By performing the polymerization reaction at a temperature which is equal to or higher than the crys- talline melting point of the prepolymer, it becomes easy to cause the prepolymer to fall stably without causing the prepolymer to get too viscose or solidified. Further, by performing the polymerization reaction at a temperature which is not higher than 290 °C, a discol- oration of the prepolymer caused by heat decomposition of the prepolymer is suppressed, and hence, a poly- trimethylene terephthalate resin having high quality can be easily obtained. It is preferred that the dif- ference between the temperature in the polymerization reaction zone (which is within the above-mentioned range) and the temperature of the molten prepolymer in- troduced into the polymerization reaction zone through the perforated plate is 20 °C or less, more advanta- geously 10 °C or less, still more advantageously 5 °C or less, and it is most preferred that the temperature in the polymerization reaction zone and the temperature of the molten prepolymer introduced into the polymeri- zation reaction zone are the same. The temperature in the polymerization reaction zone can be controlled by adjusting the temperature of a heater or a jacket which is provided on the inner wall of the polymerizer, or by adjusting the temperature of a heater or a heating me- dium which is provided inside the guide. In the present invention, for improving the polym- erization rate, it is preferred that the prepolymer contains a polycondensation catalyst. Examples of polycondensation catalysts include ti- tanium alkoxides, such as titanium tetrabutoxide and titanium tetraisopropoxide; titanium compounds, such as titanium dioxide and a double salt of titanium dioxide and silicon dioxide; antimony compounds, such as dian- timony trioxide and antimony acetate; and tin compounds, such as butyl stannate, butyltin tris(2-ethylhexoate) and tin 2-ethylhexanoate. Of these, titanium tetrabu- toxide and tin 2-ethylhexanoate are especially pre- ferred from the viewpoint of improvement in the polym- erization reaction rate and the color of the final resin. These catalysts can be used individually or in combination. It is preferred that the polycondensation catalyst is contained in the prepolymer in an amount of from 0.001 to 1 % by weight, more advantageously from 0.005 to 0.5 % by weight, still more advantageously from 0.01 to 0.2 % by weight, based on the weight of the prepolymer. In the present invention, for obtaining a poly- trimethylene terephthalate resin having a very high mo- lecular weight, it is preferred that the polymerization degree of the prepolymer is increased, and that the terminal carboxyl group ratio of the prepolymer is low- ered, wherein the terminal carboxyl group ratio is a molar ratio (%) of the terminal carboxyl groups at the terminals of the prepolymer to the total of terminal groups of the prepolymer. It is preferred that the prepolymer has an intrinsic viscosity [?] of 0.5 or more. By using a prepolymer having such a high intrin- sic viscosity, it becomes possible to obtain a desir- able falling rate of the prepolymer and a desirable foaming state of the prepolymer, thereby greatly im- proving the polymerization rate. It is preferred that the prepolymer has an intrinsic viscosity [r|] of 0.55 or more, more preferably 0.60 dl/g or more. On the other hand, it is preferred that the pre- polymer has a terminal carboxyl group ratio of 50 % or less. The terminal carboxyl group ratio is calculated by the following formula: Terminal carboxyl group ratio (%) = (terminal carboxyl group content)/(total terminal group content) x 100 wherein: the terminal carboxyl group content is the molar amount of carboxyl group per kg of a sample, and the total terminal group content is the total mo- lar amount of terminal groups per kg of a sample. When the terminal carboxyl group ratio is 50 % or less, the polymerization reaction rate can be improved, so that a resin having a high molecular weight can be obtained and that a discoloration of the resin can be suppressed. The terminal carboxyl group ratio is more preferably 30 % or less, still more preferably 20 % or less, most preferably 0 %. The above-mentioned prepolymer which is suitable for producing a resin having a high molecular weight has a high intrinsic viscosity. Therefore, when such a prepolymer is used, it becomes difficult not only to withdraw TMG from the reaction system due to the high viscosity of the prepolymer, but also to produce the prepolymer by means of a conventional vertical agita- tion type polymerizer. In addition, for decreasing the terminal carboxyl group ratio of the prepolymer, it is necessary to improve the polymerization rate and sup- press heat decomposition. Therefore, it is preferred that the prepolymer is produced by means of a horizon- tal agitation type polymerizer equipped with one or two stirrers, each having a large surface area and a high surface renewal efficiency. The method of the present invention can be prac- ticed in either a manner in which the prepolymer in a molten form is continuously fed into the polymerizer and introduced into the polymerization reaction zone through the holes of the perforated plate, and the pre- polymer in a molten form is allowed to fall along and in contact with the guide to thereby perform a polym- erization, and all of the resultant resin is withdrawn form the polymerizer; or a manner in which a part of the resin (obtained by the above-mentioned polymeriza- tion performed by allowing the prepolymer to fall along and in contact with the guide) is recycled to the poly- merizer and subjected to further polymerization. How- ever, it is preferred to employ the former (in which, all of the obtained resin is withdrawn from the poly- merizer). When the method of the present invention is practiced in the above-mentioned manner in which a part of the obtained resin is recycled to the polymerizer and subjected to further polymerization, for suppress- ing the heat decomposition of the resin which occurs at a reservoir portion of the polymerizer (i.e., a bottom portion of the polymerizer where the resin obtained by the polymerization is accumulated.) and a conduit for recycling of the resin, it is preferred to reduce the retention times and lower the temperatures at the above-mentioned reservoir portion and conduit. Hereinbelow, the method for producing a poly- trimethylene terephthalate prepolymer which is used in the present invention is described in detail. As representative examples of preferred methods for producing the polytrimethylene terephthalate pre- polymer on a commercial scale, there are the following two methods which differ in the materials used. In one of the methods, a lower alcohol diester of terephthalic acid and TMG are subjected to transesterification reac- tion to obtain bis(3-hydroxylpropyl)terephthalate (hereinafter, referred to as "BHPT") which is an inter- mediate of a PTT, and the obtained BHPT is subjected to a polycondensation reaction, thereby obtaining a PTT prepolymer (hereinafter, this method is referred to as a "transesterification method"). In the other method, terephthalic acid and TMG are subjected to an esterifi- cation reaction to obtain BHPT, and the obtained BHPT is subjected to a polycondensation reaction as in the above-mentioned transesterification method, thereby ob- taining a PTT prepolymer (hereinafter, this method is referred to as "direct esterification method"). Further, the production of the polytrimethylene terephthalate prepolymer can be performed in either a batchwise manner in which all of the raw materials are charged into the polymerizer at once and reacted to- gether simultaneously to obtain a PTT prepolymer, or a continuous manner in which the raw materials are con- tinuously fed into the polymerizer to continuously ob- tain a PTT prepolymer. In the present invention, it is preferred that the production of the PTT prepolymer is performed in a continuous manner, and that the resul- tant prepolymer is continuously polymerized by the method of the present invention. In the present invention, the above-mentioned BHPT may contain unreacted raw materials (such as terephthalic acid, a lower alcohol diester of terephthalic acid and TMG) and a PTT oligomer. However, it is preferred that the BHPT comprises 70 % by weight or more of the BHPT and/or a low molecular weight PTT oligomer, based on the total weight of the BHPT, and the above-mentioned unreacted raw materials and PTT oligomer. Hereinbelow, explanations are made on some exam- ples of the methods for obtaining the BHPT. First, an explanation is made on the "transesteri- fication method". In the transesterification method, the BHPT is produced by subjecting dimethyl terephtha- late (hereinafter, referred to as "DMT") (which is a lower alcohol diester of terephthalic acid) and TMG to a transesterification reaction at 150 to 240 ° C in the presence of a transesterification catalyst. In the transesterification reaction, the DMT used as a raw ma- terial exhibits a high volatility, so that it is pre- ferred to use two or more reactors in combination, and to control the reaction temperature appropriately. It is preferred that the lower alcohol diester of terephthalic acid and the TMG are charged into the re- actor in a molar ratio (a lower alcohol diester of terephthalic acid/TMG molar ratio) of from 1/1.3 to 1/4, more advantageously from 1/1.5 to 1/2.5. When the amount of TMG is too small so that the above-mentioned ratio is larger than 1/1.3, the reaction time is likely to become disadvantageously long. Also when the amount of TMG is too large so that the above-mentioned ratio is smaller than 1/4, the reaction time is likely to be- come disadvantageously long, because it becomes neces- sary to volatilize the excess TMG. In the transesterification method, it is necessary to use a transesterification catalyst. Preferred exam- ples of transesterification catalysts include titanium alkoxid.es, such as titanium tetrabutoxide and titanium tetraisopropoxide; tin compounds, such as tin 2- ethylhexanoate; cobalt acetate; calcium acetate; and zinc acetate. Among these catalysts, titanium tetrabu- toxide and tin 2-ethylhexanoate are preferred because they function also as catalysts in the subsequent poly- condensation reaction to produce the final resin. The amount of transesterification catalyst is preferably in the range of from 0.02 to 1 % by weight, more prefera- bly from 0.05 to 0.5 % by weight, still more preferably from 0.08 to 0.2 % by weight, based on the weight of the diester of terephthalic acid. Next, an explanation is made on the "direct es- terification method". In the direct esterification method, the BHPT is produced by subjecting terephthalic acid and TMG to esterification reaction at 150 to 240 ° C. It is preferred that the terephthalic acid and the TMG are charged into a reactor in a molar ratio (terephthalic acid/TMG molar ratio) of from 1/1.05 to 1/3, more preferably from 1/1.1 to 1/2. When the amount of TMG is too small so that the above-mentioned molar ratio is larger than 1/1.05, the reaction time is likely to become disadvantageously long and the resul- tant prepolymer is likely to be discolored. Also when the amount of TMG is too large so that the above -mentioned molar ratio is smaller than 1/3, the reac- tion time is likely to become disadvantageously long because it becomes necessary to volatilize the excess TMG. In the direct esterification method, free protons derived from terephthalic acid function as a catalyst. Therefore, in the direct esterification method, an es- terif ication catalyst is not always necessary. However, for improving the reaction rate, it is preferred to use an esterification catalyst. Preferred examples of es- terif ication catalysts include titanium alkoxides, such as titanium tetrabutoxide and titanium tetraisopropox- ide, and tin compounds, such as tin 2-ethylhexanoate. It is preferred that the amount of esterification cata- lyst used is from 0.02 to 1 % by weight, more prefera- bly from 0.05 to 0.5 % by weight, still more preferably from 0.08 to 0.2 % by weight, based on the weight of terephthalic acid. For advancing the esterification reaction smoothly, it is preferred to add BHPT to a raw material mixture at the start of the reaction in an amount of 5 to 80 % by weight, based on the total weight of the raw mate- rial mixture and the BHPT. When the production of BHPT is performed in a batchwise manner, the esterification reaction can be initiated by simultaneously charging the terephthalic acid and the TMG (raw materials) into a reactor. On the other hand, when the production of BHPT is performed in a continuous manner, the esterifi- cation reaction can be performed by continuously feed- ing a predetermined amount of a mixture of terephthalic acid and TMG into a reactor to perform a transesterifi- cation reaction, while withdrawing a predetermined amount of the reaction product (BHPT) from the reactor. The BHPT obtained by any of the above-mentioned methods is then subjected to polycondensation, thereby obtaining the prepolymer used in the present invention. The production of the prepolymer by polycondensa- tion is performed by subjecting the BHPT to reaction at a predetermined temperature under reduced pressure or in an inert gas atmosphere, while withdrawing by- produced TMG from the reaction system. It is preferred that such polycondensation reac- tion is performed at 230 to 280 °C. When the reaction is performed at a temperature lower than 230oC, disad- vantages are likely to occur that the formed prepolymer gets solidified and that the reaction time becomes long. On the other hand, when the reaction is performed at a temperature which is higher than 280 °C, a disadvantage is likely to occur that a vigorous heat decomposition of the formed prepolymer occurs, and the resultant pre- polymer cannot be used for producing a polymer having excellent color. It is preferred that the polyconden- sation reaction is performed at a temperature of from 232 to 275 °C, more advantageously from 235 to 270 °C. As mentioned above, the polycondensation reaction can be performed under reduced pressure or in an inert gas atmosphere. When the reaction is performed under reduced pressure, the pressure is appropriately con- trolled, talcing into consideration the sublimation of the BHPT and polycondensation product, and the reaction rate. When the reaction is preformed in an inert gas atmosphere, it is important that the inside of a reac- tor used is always satisfactorily purged with inert gas to withdraw the by-produced TMG efficiently from the reaction system. When the BHPT is subjected to polycondensation, it is preferred to use a polycondensation catalyst. When a polycondensation catalyst is not used, the reaction time is likely to become disadvantageously long. Preferred examples of polycondensation catalysts include titanium alkoxides, such as titanium tetrabutoxide and titanium tetraisopropoxide; titanium dioxide and a double salt of titanium dioxide and silicon dioxide; antimony compounds. such as diantimony trioxide and an antimony acetate; and tin compounds, such as butyl stannate, butyltin tris(2- ethylhexoate) and tin 2-ethylhexanoate. From the view- point of improving the reaction rate and color of the final resin, titanium tetrabutoxide and tin 2- ethylhexanoate are especially preferred. The above- mentioned catalysts can be used individually or in com- bination. The amount of polycondensation catalyst used is preferably from 0.001 to 1 % by weight, more prefera- bly from 0.005 to 0.5 % by weight, still more preferably from 0.01 to 0.2 % by weight, based on the weight of the prepolymer. When a compound which functions as a poly- condensation catalyst is used in the production process of the BHPT, it is preferred that the total amount of the compounds capable of functioning as a polycondensa- tion catalyst is within the above-mentioned range. Examples of devices for conducting such polycon- densation reaction include a vertical agitation type polymerizer, a horizontal agitation type polymerizer equipped with one or two stirrers, a free-fall type thin film polymerizer having trays therein, and a thin film polymerizer in which the prepolymer is allowed to fall on a plane surface of an angled plate. Needless to say, these polymerizers can be used in combination. When a polycondensation of BHPT is performed in a batchwise manner, a single polymerizer can be employed from the start of the polycondensation reaction through the completion of the polycondensation reaction. Need- less to say, two or more polymerizers can be used. On the other hand, when a polycondensation of BHPT is per- formed in a continuous manner, for effectively advanc- ing the reaction, it is preferred to perform the reac- tion of from the polycondensation to the formation of the prepolymer in a stepwise manner, using two or more different polymerizers, wherein the two or more poly- merizers are operated under different temperature -pressure conditions. In the present invention, if desired, various ad- ditives can be incorporated into the PTT resin by co- polymerization or mixing. Examples of additives in- clude a delustering agent, a thermal stabilizer, a flame retardant, an antistatic agent, an anti-foaming agent, an orthochromatic agent, an antioxidant, an ul- traviolet absorber, a nucleating agent and a brightener. These additives can be added at any time during the production of the PTT resin. In the present invention, from the viewpoint of improving the whiteness and melt stability of the PTT resin, and suppressing the formation of organic sub- stances having a molecular weight of 300 or less, such as acrolein and an allyl alcohol, it is preferred that a stabilizer is added at an appropriate stage of the production of the PTT resin, more advantageously before the polycondensation of BHPT. Preferred examples of such stabilizers include pentavalent and/or trivalent phosphorus compounds and hindered phenol compounds. Examples of pentavalent and/or trivalent phospho- rus compounds include trimethyl phosphate, triethyl phosphate, tributyl phosphate, triphenyl phosphate, trimethyl phosphite, triethyl phosphite, tributyl phosphite, triphenyl phosphite, phosphoric acid and phosphorous acid. Among the above-mentioned phosphorus compounds, trimethyl phosphite is especially preferred. It is preferred that the amount of phosphorus compound added is in the range of from 2 to 250 ppm by weight, more advantageously from 5 to 150 ppm by weight, still more advantageously from 10 to 100 ppm by weight, in terms of the weight of phosphorus atom contained in the PTT. The hindered phenol compound is a phenol deriva- tive which has a substituent exhibiting a steric hin- drance at a position adjacent to the phenolic hydroxyl group, and has at least one ester bond in its molecule. Examples of hindered phenol compounds include pen- taerythritol-tetrakis[3-(3,5-di-tert-butyl-4- hydroxyphenyl)propionate] , 1,1,3-tris(2-methyl-4 - hydroxy-5-tert-butylphenyl)butane, 1,3,5-trimethyl- 2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl)benzene, 3,9-bis{2-[3-(3-tert-butyl-4-hydroxy-5-methylphenyl)- propionyloxy]-1,1-dimethylethy1}-2,4,8,10- tetraoxaspiro[5,5 Jundecane, l,3,5-tris(4-tert-butyl-3 - hydroxy-2,6-dimethylbenzene)isophthalic acid, triethyl glycol-bis[3-(3-tert-butyl-5-methyl-4-hydroxyphenyl)- propionate], 1,6-hexanediol-bis[3-(3,5-di-tert-butyl-4- hydroxyphenyl)propionate], 2,2-thio-diethylene-bis[3- (3,5-di-tert-butyl-4-hydroxyphenyl)propionate] and oc- tadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)- propionate]. Among the above-exemplified hindered phenol com- pounds , pentaerythritol-tetrakis[3-(3,5-di-tert-butyl- 4-hydroxyphenyl)propionate] is preferred. It is preferred that the amount of hindered phenol compound added is in the range of from 0.001 to 1 % by weight, more advantageously from 0.005 to 0.5 % by weight, still more advantageously from 0.01 to 0.1 % by weight, based on the weight of the PTT resin. Needless to say, the above-mentioned stabilizers can be used in combination. Next, preferred examples of the polymerizer used in the method of the present invention are described below, referring to the drawings. Fig. 1 shows an explanatory diagrammatic view of a specific example of a polymerizer used in the method of the present invention. In Fig. 1, trimethylene terephthalate prepolymer A is fed through inlet 2 into polymerizer 10 by means of transferring pump 1. Then, prepolymer A is introduced through perforated plate 3 into the interior (polymerization reaction zone) of po- lymerizer 10, and is allowed to fall along and in con- tact with guides 5. The pressure in the interior (po- lymerization reaction zone) of polymerizer 10 is con- trolled so as to maintain a predetermined reduced pres- sure. If desired, inert gas E, such as nitrogen, can be introduced into the polymerizer through inlet 6 for a gas. TMG (which is distilled from the prepolymer) and inert gas E are discharged as exhaust gas D from vent 7. The resultant PTT resin B is withdrawn from the polymerizer by means of withdrawal pump 8 through outlet 9. Polymerizer 10 is heated by means of a heater or a jacket so as to maintain the temperature inside the polymerizer at a desired level. Fig. 2 shows explanatory diagrammatic views of specific examples of an inert gas absorption device and a polymerizer, which can be used in the present inven- tion when the inert gas is introduced to the polymeri- zation reaction zone in such a form as absorbed or con- tained in the PTT prepolymer, and the inert gas is caused to be released from the prepolymer in the polym- erization reaction zone under reduced pressure. In Fig. 2, PTT prepolymer A is continuously fed through inlet N3 into the inert gas absorption device Nl by means of transferring pump N2 . Then, the prepolymer is continu- ously fed through perforated plate 4 into the interior of the inert gas absorption device, into which inert gas E is introduced through inlet N6 for inert gas. In the inert gas absorption device, the prepolymer is al- lowed to fall along and in contact with guides 5. The resultant prepolymer (which contains and/or has ab- sorbed therein the inert gas) is fed through inlet 2 into polymerizer 10 by means of transferring pump 7. The prepolymer is then continuously fed through perfo- rated plate 4 into the interior (polymerization reac- tion zone) of polymerizer 10, and is allowed to fall along and in contact with guides 5. The pressure in the interior (polymerization reac- tion zone) of polymerizer 10 is controlled so as to maintain a predetermined reduced pressure. The inert gas which is absorbed and/or contained in prepolymer A is released from the prepolymer in the polymerization reaction zone of the polymerizer. TMG (which is dis- tilled from the prepolymer) and the inert gas (which is introduced into the polymerizer) are discharged as ex- haust gas D through vent 7. The resultant PTT resin B is continuously withdrawn from outlet 9 by means of withdrawal pump 8. Inert gas absorption device Nl and polymerizer 10 are heated by means of a heater or a jacket so as to maintain the temperature inside the po- lymerizer at a desired level. In each of the above-mentioned methods, the PTT resin (obtained by a polymerization performed by allow- ing the prepolymer to fall along and in contact with the guide) which is accumulated at the bottom portion of the polymerizer is withdrawn from the outlet by means of a withdrawal pump. It is preferred that the withdrawal of the PTT resin from the polymerizer is performed in a manner such that the amount of the PTT resin accumulated at the bottom portion of the polymer- izer becomes as small as possible and constant. When the withdrawal of the PTT resin is performed in such a manner, it becomes easy to prevent the PTT resin from suffering disadvantages, such as discoloration (caused by heat decomposition), lowering of polymerization de- gree, and fluctuation of the quality. The amount of the PTT resin accumulated at the bottom portion of the polymerizer can be controlled by means of transferring pump 1 and withdrawal pump 8, while observing the amount of the PTT resin accumulated at the bottom por- tion of the polymerizer. The polymerizer used in the present invention can be equipped with a stirrer at the bottom portion thereof, but such a stirrer is not necessary. Thus, in the method of the present invention, it is possible to perform a polymerization reaction using a polymerizer having no rotary driving part, so that the polymerizer can be tightly sealed even when a polymerization is performed under high vacuum conditions. The sealabil- ity of the rotary driving part of the withdrawal pump is improved due to the liquid head, as compared to that of a polymerizer having a rotary driving part. The method of the present invention can be con- ducted using either a single polymerizer or a plurality of polymerizers. Further, it is also possible to use a multi-chamber polymerizer prepared by dividing an inner space of a single polymerizer into a plurality of hori- zontally adjacent chambers or a plurality of vertically adjacent chambers. In the present invention, it is preferred to in- crease, in advance, the polymerization degree of the prepolymer (which is to be subjected to the above- mentioned guide-wetting fall polymerization process) to some extent by at least one polymerization method se- lected from the following methods (a) to (d): (a) a polymerization method using a vertical agi- tation type polymerizer; (b) a polymerization method using a horizontal agitation type polymerizer; (c) a polymerization method using a free-fall po- lymerizer having a perforated plate,- and (d) a polymerization method using a thin film type polymerizer. Examples of horizontal agitation type polymeriz- ers include a screw-type polymerizer (such as a single- screw type polymerizer or a twin-screw type polymer- izer) and an independent agitation element type poly- merizer, which are described in "Hanno Kougaku Kenkyu- kai kenkyu Repoto: Riakutibupurosessingu Part 2 (Re- search Group on Reaction Engineering, Research Report: Reactive Processing Part 2)", Chapter 4, edited by The Society of Polymer Science, Japan, 1992. With respect to the free-fall polymerizer having a perforated plate, reference can be made to, for example, U.S. Patent No. 5,596,067. When the free-fall polymer- izer is used, a polymerization is performed by allowing the prepolymer to fall freely from the holes of the perforated plate provided in the polymerizer. More specifically, a polytrimethylene terephthalate prepoly- mer in a molten form is allowed to fall freely from the holes of the perforated plate, thereby improving the polymerization degree of the prepolyraer. Herein, the expression "fall freely" means that the prepolymer is allowed to fall without contacting any materials (such as a guide and an inner wall of the polymerizer) which obstruct the fall of the prepolymer. The prepolymer is allowed to fall freely in the form of a film, a fiber, a droplet, a fog or the like. TMG which is produced during the polycondensation reaction is withdrawn from the reaction system during the fall of the prepolymer. In the above-mentioned method using a free-fall polymerizer, there is no particular limitation with re- spect to the shape of the holes of the perforated plate, and generally, the shape can be a circle, an ellipse, a triangle, a slit, a polygon, a star or the like. The cross-sectional area of each of the holes is generally in the range of from 0.01 to 100 cm , preferably from 0.05 to 10 cm , more preferably from 0.1 to 5 cm . Further, the holes can have a nozzle or a short guide attached thereto. However, it is necessary that the nozzle or the short guide is attached to the holes so that the prepolymer can fall freely after having passed through the nozzle or after having fallen along and in contact with the guide. The distance between mutually adjacent holes of the perforated plate is generally from 1 to 500 mm, preferably from 5 to 100 mm, as meas- ured between the respective centers of the mutually ad- jacent holes. With respect to the distance over which the prepolymer (having passed through the holes of the perforated plate) falls freely, the distance is pref- erably from 0.3 to 50 m, more preferably from 0.5 to 20 m. The amount of prepolymer which is caused to pass through the holes varies depending on the molecular weight of the prepolymer, but is generally in the range of from 10"4 to 104 liters/hr, preferably from 10"2 to 102 liters/hr, more preferably from 0.1 to 50 liters/hr, per hole of the perforated plate. There is no particu- lar limitation with respect to the time for allowing the prepolymer to fall freely from the perforated plate, but in general, it is in the range of from 0.01 second to 1 hour. The thus obtained prepolymer may be with- drawn from the polymerizer. Alternatively, the ob- tained prepolymer may be recycled to the free-fall po- lymerizer, and subjected to further free-fall polymeri- zation. The recycling of the obtained prepolymer to the polymerizer has the following advantage. When the free-fall polymerization is performed while recycling the obtained prepolymer, the area of the prepolymer surface renewed per unit time is large, as compared to that in the case where the obtained prepolymer is not recycled. Therefore, by recycling the obtained polymer to the polymerizer, it becomes easy to achieve a de- sired degree of polymerization. Further examples of vertical and horizontal agi- tation type polymerizers include those which are de- scribed in "Kagaku Souchi Binran (Handbook of Chemical Apparatuses)", Chapter 11, edited by The Society of Chemical Engineers, Japan, 1989. There is no particu- lar limitation with respect to the shape of the vessel, and in general, the shape may be a vertical or horizon- tal cylinder. Further, there is no particular limita- tion with respect to the shape of the agitation element, and the shape of agitation element can be a paddle, an anchor, a turbine, a screw, a ribbon, double wings or the like. Examples of thin film type polymerizers include a wall-wetting fall polymerizer, and polymerizers equipped with a centrifugal thin film type heat ex- changer, a liquid film scraping type heat exchanger or the like. As an example of the above-mentioned wall -wetting fall polymerizer, there can be mentioned a po- lymerizer described in the above-mentioned "Kagaku Souchi Binran (Handbook of Chemical Apparatuses)", Chapter 11, p. 461, published by The Society of Chemi- cal Engineers, Japan, 1989. The thin film type poly- merizer may have a multi-tubular structure. Further, the prepolymer obtained by the wall-wetting fall may be recycled to the polymerizer, and subjected to further wall-wetting fall polymerization. Examples of liquid film scraping type heat exchangers and centrifugal thin film type heat exchangers include those which are de- scribed in "Netsukoukanki Sekkei Handobukku (Handbook for designing a heat exchanger)". Chapters 21-22, pub- lished by Kougakutosho Ltd., Japan, 1974. The production of a prepolymer from the raw mate- rials can be performed in either a batchwise manner or a continuous manner. When the prepolymer is produced in a batchwise manner, all of the raw materials or the whole of a reaction product (i.e., a prepolymer having a molecular weight lower than a desired level) are or is charged into a reaction vessel and reacted for a predetermined period of time, and then, the whole of the resultant reaction product is transferred to an- other reaction vessel. On the other hand, when the prepolymer is produced in a continuous manner, raw ma- terials or a reaction product (i.e., a prepolymer hav- ing a molecular weight lower than a desired level) are or is continuously fed into a reaction vessel, while continuously withdrawing the resultant reaction product from the reaction vessel. For obtaining a large amount of a polytrimethylene terephthalate resin having a uni- form quality, it is preferred that the production of the prepolymer is performed in a continuous manner. With respect to the material of the polymerizer used in the present invention, there is no particular limitation. In general, the material is selected from the group consisting of, for example, stainless steel, nickel and glass lining. Next, preferred examples of combinations of poly- merizers which are used for producing a PTT resin from the raw materials are described below, referring to the drawings. However, the combinations of polymerizers usable in the present invention should not be limited to those examples. Fig. 3 shows an example of a system used for pro- ducing the PTT resin from terephthalic acid and TMG as raw materials, which system comprises a combination of vertical agitation type polymerizers and a polymerizer used for performing the guide-wetting fall process. In Fig. 3, mixture C containing raw materials (terephthalic acid and TMG) and a catalyst is charged into esterifica- tion reaction vessel 11, and subjected to an esterifica- tion reaction for a predetermined period of time, while stirring by means of agitation element 12, thereby ob- taining bis(3-hydroxylpropyl)-terephthalate (BHPT). The atmosphere inside the reaction vessel is an atmosphere of inert gas, such as nitrogen gas, and/or an atmosphere containing water (steam) and/or TMG which are distilled from a reaction mixture in the reaction vessel. In gen- eral, the pressure inside the reaction vessel is con- trolled so as to be around atmospheric pressure. The water and TMG (which are distilled from the reaction mixture) and/or excess nitrogen gas are discharged from vent 13 as exhaust gas D. The BHPT obtained in esteri- fication reaction vessel 11 is transferred by means of transferring pump 14 to first vertical agitation type polymerizer 15, where the BHPT is subjected to polymeri- zation for a predetermined period of time, while stir- ring by means of agitation element 16, thereby obtaining a low molecular weight prepolymer A. The inside of the polymerizer is under reduced pressure, or inert gas (such as nitrogen gas) is flowed through the inside of the polymerizer. The water and TMG (which are distilled from polymer A) and/or excess nitrogen gas is discharged from vent 17 as exhaust gas D. The low molecular weight prepolymer A obtained in first vertical agitation type polymerizer 15 is trans- ferred by means of transferring pump 18 to second ver- tical agitation type polymerizer 19, where the prepoly- mer A is subjected to polymerization for a predeter- mined period of time, while stirring by means of agita- tion element 20, thereby obtaining a prepolymer. The inside of the polymerizer is under reduced pressure, or inert gas (such as nitrogen gas) is flowed through the inside of the polymerizer. The water and TMG (which are distilled from the prepolymer A) and/or excess ni- trogen gas is discharged from vent 21 as exhaust gas D. The prepolymer A having an increased molecular weight, which is obtained in second vertical agitation type po- lymerizer 19, is transferred and continuously fed through inlet 2 into polymerizer 10 by means of trans- ferring pump 1. In polymerizer 10, the prepolymer A is caused to pass through perforated plate 3, and is in- troduced into the interior (polymerization reaction zone) of the polymerizer, where the prepolymer A is al- lowed to fall along and in contact with guides 5. The pressure in the polymerization reaction zone is con- trolled so as to be a predetermined reduced pressure. TMG (distilled from the prepolymer A) and inert gas E (which is optionally introduced into the polymerizer through inlet 6 for a gas) if any, are discharged from vent 7. The obtained PTT resin B is continuously with- drawn from outlet 9 by means of withdrawal pump 8. Esterification reaction vessel 11, first vertical agitation type polymerizer 15, second vertical agita- tion type polymerizer 19, polymerizer 10, conduits and transferring pumps are heated by means of a heater or a jacket so as to maintain the temperatures of the reac- tion vessel, polymerizers, conduits and pumps at de- sired levels. Fig. 4 shows an example of a system used for pro- ducing the PTT resin from terephthalic acid and TMG as raw materials, which system comprises a combination of a vertical agitation type polymerizer, a horizontal agitation type polymerizer and a polymerizer for per- forming the guide-wetting fall process. In Fig. 4, mixture C of raw materials (terephthalic acid and TMG) and a catalyst is charged into esterification reaction vessel 11, and subjected to an esterification reaction for a predetermined period of time, while stirring by means of agitation element 12, thereby obtaining BHPT. The atmosphere inside the reaction vessel is an atmos- phere of inert gas, such as nitrogen gas, and/or an at- mosphere containing water (steam) and/or TMG which are distilled from a reaction mixture in the reaction ves- sel. In general, the pressure inside the reaction ves- sel is controlled to approximately atmospheric pressure. The water and TMG (which are distilled from the reac- tion mixture) and/or excess nitrogen gas are discharged from vent 13 as exhaust gas D. The BHPT obtained in esterification reaction vessel 11 is transferred by means of transferring pump 14 to first vertical agita- tion type polymerizer 15, where the BHPT is subjected to polymerization for a predetermined period of time, while stirring by means of agitation element 16, thereby obtaining a low molecular weight prepolymer A. The inside of the polymerizer is under reduced pressure, or inert gas (such as nitrogen gas) is flowed through the inside of the polymerizer. The water and TMG (which are distilled from polymer A) and/or excess ni- trogen gas is discharged from vent 17 as exhaust gas D. The low molecular weight prepolymer A obtained in first vertical agitation type polymerizer 15 is trans- ferred by means of transferring pump 20 to horizontal agitation type polymerizer 22, where the prepolymer A is subjected to polymerization for a predetermined pe- riod, while stirring by means of agitation element 23, thereby obtaining a prepolymer A having an increased molecular weight. The inside of the polymerizer is un- der reduced pressure, or inert gas (such as nitrogen gas) is flowed through the inside of the polymerizer. The water and TMG (which are distilled from polymer A) and/or excess nitrogen gas is discharged from vent 24 as exhaust gas D. The prepolyraer A having an increased molecular weight, which is obtained in horizontal agi- tation type polymerizer 22, is transferred and continu- ously fed through inlet 2 into polymerizer 10 by means of transferring pump 1. In polymerizer 10, the pre- polymer A is caused to pass through perforated plate 3, and is introduced into the interior (polymerization re- action zone) of the polymerizer, where the prepolymer A is allowed to fall along and in contact with guides 5. The pressure in the polymerization reaction zone is controlled to a predetermined reduced pressure. TMG (distilled from the prepolymer A) and inert gas E (which is optionally introduced into the polymerizer through inlet 6 for a gas) if any, are discharged from vent 7. The obtained PTT resin B is continuously with- drawn from outlet 9 by means of withdrawal pump 8. Esterification reaction vessel 11, vertical agi- tation type polymerizer 15, horizontal agitation type polymerizer 22, polymerizer 10, conduits and transfer- ring pumps are heated by means of a heater or a jacket so as to maintain the temperatures of the reaction ves- sel, polymerizers, conduits and pumps at desired levels. Each of Figs. 5 and 6 shows an example of a sys- tem used for producing the PTT resin from DMT and TMG as raw materials . In each of Figs . 5 and 6 , mixture C of raw materials and a catalyst is charged into first transesterification reaction vessel 25, and the resul- tant reaction product is transferred to second trans- esterif ication reaction vessel 29. In each of first transesterification reaction vessel 25 and second transesterification reaction vessel 29, mixture C of raw materials is subjected to a transesterification re- action for a predetermined period of time, while stir- ring by means of agitation element (26 or 30), thereby obtaining BHPT. The atmosphere inside the reaction vessel is an atmosphere of inert gas, such as nitrogen gas, and/or an atmosphere containing methanol and/or TMG which are distilled from a reaction mixture in the reaction vessel. In general, the pressure inside the reaction vessel is controlled to approximately atmos- pheric pressure. The vent of each of the reaction ves- sels is connected to a fractionating column. TMG dis- tilled from the fractionating column is recycled to the reaction vessel. Methanol and excessive nitrogen are discharged from the fractionating column. The obtained BHPT is subjected to polycondensation in the same man- ner as mentioned above in connection with the systems shown in Figs. 3 and 4, thereby obtaining prepolymer A and then, PTT resin B. BEST MODE FOR CARRYING OUT THE INVENTION Hereinbelow, the present invention will be de- scribed in more detail with reference to the following Examples and Comparative Examples, which should not be construed as limiting the scope of the present inven- tion . In the following Examples and Comparative Examples, various measurements and analyses were conducted by the following methods. (1) Intrinsic viscosity [?] Intrinsic viscosity [?] of a polymer (i.e., a pre- polymer or a final polytrimethylene terephthalate resin) was measured by means of an Oswald viscometer. Specifically, with respect to each of o-chlorophenol solutions of a polymer, which have different concentra- tions [C] (g/100 ml) of the resin, the relative viscos- ity [?sp] was measured at 35 °C. The obtained {?sp/C) values were plotted against the concentrations of the resin, and the resultant gradient is extrapolated into the zero (0) concentration to thereby obtain an intrin- sic viscosity [?] of the polymer. That is, the intrin- sic viscosity [?] of the polymer was calculated by the following formula: 102 [?] = lim (T]sp/C) . (2) Crystalline melting point The crystalline melting point of a prepolymer was measured by means of a differential scanning calorime- ter (trade name-. Pyris 1; manufactured and sold by Perkin Elmer, Inc., U.S.A.) under the following condi- tions : Measuring temperature: 0 to 280 ° C Rate of temperature elevation: 10 • C/min Specifically, a temperature at which an endother- mic peak ascribed to the melting of a crystal is ob- served in the obtained differential scanning calo- rimetry (DSC) chart was defined as the crystalline melting point of the prepolymer, wherein the determina- tion of the peak was conducted using an analytic soft- ware attached to the calorimeter. (3) Terminal carboxyl group content 1 g of a polymer (i.e., a prepolymer or a final polytrimethylene terephthalate resin) was dissolved in 25 ml of benzyl alcohol, followed by addition of 25 ml of chloroform, thereby obtaining a mixture. The ob- talned mixture was subjected to a titration with a 1/50 N solution of potassium hydroxide in benzyl alcohol. From the obtained titration value VA (ml) and a blank test value Vo (ml) which is obtained by a titration conducted in the absence of the polymer, the terminal carboxyl group content was calculated by the following formula: Terminal carboxyl group content (meq/kg) = (VA - Vo) x 20 . (4) Total terminal group content The total amount of terminal groups per kg of a sample is defined as the total terminal group content. Specifically, the total terminal group content (meq/kg) is calculated from the intrinsic viscosity [?] by the following formula: Total terminal group content (meq/kg) = 1,000/(polymerization degree x 206) x 2 x 1,000 wherein: the polymerization degree = intrinsic viscosity [?] x 144.6 - 26.2. (5) Molecular weight distribution The ratio of the weight average molecular weight (Mw) to the number average molecular weight (Mn) (Mw/Mn ratio) was used to evaluate the molecular weight dis- tribution of a polytrimethylene terephthalate resin. The Mw and Mn of a polytrimethylene terephthalate resin were determined by gel permeation chromatography (GPC). Specifically, the GPC was conducted under the following conditions: Apparatus: chromatograph model HLC-8120 (manufac- tured and sold by Tosoh Corporation, Japan); Columns: HFIP804-803 (30 cm) (manufactured and sold by Showa Denko K.K., Japan) (x 2); Carrier: hexafluoroisopropanol; Measurement temperature: 40 °C; and Flow rate : 0.5 ml/min. A calibration curve used in the determination of the Mn and Mw was obtained by using standard polymethyl methacrylate (PMMA) samples (manufactured and sold by Polymer Laboratories Ltd., U.K.). The molecular weights of the PMMA samples used were 620, 1,680, 3,805, 7,611, 13,934, 24,280, 62,591 and 186,000, respectively. (6) Color (L-value and b*-value) A pellet of a polytrimethylene terephthalate resin (PTT) was heated at 100 °C for 10 minutes to partially crystallize the pellet. The color (in terms of L-value and b*-value thereof) of the obtained partially crys- tallized pellet was measured by means of a color meas- uring computer (manufactured and sold by SUGA TEST IN- STRUMENTS Co., Ltd., Japan). Separately from the above, another pellet of PTT resin was heated at 180 °C for 24 hours, and then, the color thereof was measured in the same manner as men- tioned above, except that the heating at 100 °C for partially crystallizing the PTT resin was not conducted since the pellet was already crystallized by the above -mentioned heating at 180 °C. The thus obtained L-value and b*-value of the PTT resin were used as a yardstick of the discoloration of the PTT resin caused by the heating. (7) Pellet size Approximately 2 g of pellets were used as sample pellets . The total weight of the sample pellets was accurately weighed, and the number of sample pellets was counted. From the total weight and the number of the sample pellets, the average weight of the pellets was calculated. (8) Polymer powder The amount of polymer powder attached to the sur- face of pellets was measured as follows. 1. 1 kg of pellets were placed In a beaker filled with water. 2. The pellets in the beaker were stirred for 5 minutes to wash away the polymer powder from the surface of the pellets. 3. The contents of the beaker were filtered through a 30-mesh filter. Then, the pellets on the filter were repeatedly washed with water so that broken pieces of pellets and/or polymer powder would not remain on the pellets. 4. The resultant filtrate obtained in step 3 above was filtered again through a 300-mesh filter. The residue on the filter was dried at 80 "C under reduced pressure, namely, under a pres- sure of 1 kPa. The dried residue was weighed, and the measured weight was defined as the weight of the polymer powder. (9) Crystallinity The density of the pellets was measured in accor- dance with JIS-L-1013, in which the density of the pellets was measured by gradient density tube method in which a gradient density tube prepared using tetrachlorocarbon and n-heptanone was used. The meas- urement was conducted with respect to 10 pellets, and the average of the measured values was defined as the density of the pellets. Using the obtained value (ps) of the density, the crystallinity of the pellets was calculated by the following formula: Xc (%) = {?c x (?s - ?a)}/(?s x (?c - ?a)> x 100 wherein pa is 1.300 g/cm3 which is an amor- phous density of trimethylene terephthalate homopolymer, pc is 1.431 g/cm3 which is a crystal density of trimethylene terephtha- late homopolymer, and ps represents a den- sity (g/cm3) of the pellets. Example 1 Using the device as shown in Fig. 1, production of polytrimethylene terephthalate resin B was conducted as follows. A polytrimethylene terephthalate (PTT) pre- polymer A having an intrinsic viscosity [?] of 0.5 dl/g, a terminal carboxyl group ratio of 7 % and a crystal- line melting point of 230 ° C was fed through, prepolymer feeding inlet 2 into polymerizer 10 by means of trans- ferring pump 1 . In polymerizer 10 , PTT prepolymer A was caused to pass through the holes of perforated plate 3 in a molten form at 260 °C (temperature of the molten polymer) and at a rate of 10 g/min per hole, and then, was allowed to fall along and in contact with guides 5 at an atmospheric temperature of 260 oC, which is the same as the temperature of the molten prepolymer (having passed through the holes of perforated plate 3), under reduced pressure, namely, under a pressure of 10 Pa, to thereby perform a polymerization to obtain PTT resin B. The obtained PTT resin B was withdrawn from outlet 9 by means of withdrawal pump 8. The perforated plate had a thickness of 50 mm and nine holes, each having a diameter of 1 mm, in which the holes of the perforated plate are arranged such that a checkered pattern is formed when lines connecting the holes are drawn on the surface of the perforated plate. The guide was a wire made of stainless steel, which wire had a circular cross- section, and had a diameter of 3 mm and a length of 5 m. Guides 5 were attached to per- forated plate 4 so that each hole of perforated plate 5 had one guide 5 attached thereto. The withdrawal pump was operated while observing the polymer inside the po- lymerizer through observing window 4, so that almost no polymer was accumulated at the bottom of the polymer- izer. (The above-mentioned prepolymer A contained ti- tanium tetrabutoxide (polymerization catalyst) and trimethyl phosphate (stabilizer) in amounts of 0.1 % by weight and 100 ppm by weight (in terms of the weight of phosphorus), respectively, both of which are based on the weight of the prepolymer.) The results are shown in Table 1. In the above-mentioned polymerization, the reten- tion time was 60 minutes. The retention time herein is a value calculated by dividing the total amount of the prepolymer and polymer inside the polymerizer by the feeding rate of the prepolymer. With respect to the staining of the lower surface of the perforated plate, which was caused by the foam- ing of the prepolymer immediately below the holes of the perforated plate, the level of staining was low. The obtained PTT resin had a high molecular weight, a narrow molecular weight distribution, a low terminal carboxyl group content, and excellent color. Further, the degree of discoloration of the obtained PTT resin occurring by heating was very small. The obtained PTT resin was solidified in cool wa- ter having a temperature of 5 °C, and then, cut into pellets, each having a weight of 20 rag. The amount of polymer powder (which had adhered to the pellets) was as low as 0.01 % by weight, and the pellets had a crys- tallinity of 5 %. Therefore, the obtained pellets were not easily broken and easy to handle. Examples 2 to 4 In each of Examples 2 to 4, polymerization was performed in substantially the same manner as in Exam- ple 1, except that polymerization was performed under the conditions indicated in Table 1. The results are shown in Table 1. In each of Examples 2 to 4, with re- spect to the staining of the lower surface of the per- forated plate, which was caused by the foaming of the prepolymer immediately below the holes of the perfo- rated plate, the level of staining was low. The ob- tained PTT resins had a high molecular weight, a narrow molecular weight distribution, a low terminal carboxyl group content, and excellent color. Further, with re- spect to each of the obtained PTT resins, the degree of discoloration occurring by heating was very small. Comparative Examples 1 to 4 In each of Comparative Examples 1 to 4, polymeri- zation was performed in substantially the same manner as in Example 1, except that the polymerization was performed under the conditions indicated in Table 1 . The results are shown in Table 1. In Comparative Example 1, the temperature of the molten preploymer introduced into the polymerization reaction zone was too high, so that a vigorous foaming of the prepolymer occurred just below the holes of the perforated plate, thereby markedly staining the lower surface of the perforated plate. The obtained PTT resin was discolored to assume a yellow color and the color of the PTT resin thereof was non-uniform. Fur- ther, the PTT resin suffered severe discoloration by heating. In Comparative Example 2, the temperature of the molten preploymer introduced into the polymerization reaction zone was too low, so that the prepolymer was solidified, and hence, the prepolymer could not pass through the holes of the perforated plate. In Comparative Example 3, the prepolymer had an intrinsic viscosity [?] of 0.18 dl/g, which was too low, so that a vigorous foaming of the prepolymer occurred just below the holes of the perforated plate, thereby markedly staining the lower surface of the perforated plate, and the inner wall of the polymerizer. The ob- tained PTT resin contained a large amount of black im- purities (heat deterioration products). Further, the obtained PTT had a low molecular weight. In Comparative Example 4. polymerization in the polymerizer was performed under atmospheric pressure. As a result, it was found that the polymerization de- gree of the obtained PTT was not increased, but rather lowered by the heat decomposition. Example 5 A polymerization was performed in substantially the same manner as in Example 1, except that the guide was changed to a jungle gym-like body, in which wires (each having a diameter of 3 mm) were three -dimensionally connected with one another at intervals of 30 mm as viewed in the vertical direction and at in- tervals of 50 mm as viewed in the horizontal direction. The upper end portions of the wires extending in the vertical direction were attached to the holes of the perforated plate. The results are shown in Table 1. With respect to the staining of the lower surface of the perforated plate, which was caused by the foaming of the prepolymer immediately below the holes of the perforated plate, the level of staining was low. The obtained PTT resin had a high molecular weight a narrow molecular weight distribution, and excellent color. Further, the degree of discoloration of the ob- tained PTT resin occurred even by heating. Example 6 Polymerization was performed in substantially the same manner as in Example 1, except that the guide was changed to a wire mesh, in which wires (each having a diameter of 3 mm) were two-dimensionally connected with one another at intervals of 30 mm as viewed in the ver- tical direction and at intervals of 50 mm as viewed in the horizontal direction. The upper end portions of the wires extending in the vertical direction were at- tached to the holes of the perforated plate. The re- sults are shown in Table 1. With respect to the staining of the lower surface of the perforated plate, which was caused by the foam- ing of the prepolymer immediately below the holes of the perforated plate, the level of staining was low. The obtained PTT resin had a high molecular weight, a narrow molecular weight distribution, a low terminal carboxyl group content, and excellent color. Further, the degree of discoloration of the obtained PTT resin occurring by heating was very small. Example 7 Using the production system as shown in Fig. 3, 130 kg of a polytrimethylene terephthalate (PTT) resin was continuously produced per day, in which terephthalic acid and TMG were used as raw materials. With respect to the apparatuses used in the above- mentioned production system, each of esterification re- action vessel 11, first vertical agitation type poly- raerizer 15 and second vertical agitation type polymer- izer 19 was a vertical agitation type polymerizer equipped with a stirrer having paddle-shaped agitation blades, and polymerizer 10 was the same as used in Ex- ample 5 . Specifically, production of the PTT resin was per- formed as follows. Terephthalic acid and TMG were mixed together (terephthalic acid/TMG molar ratio = 1/1.5), followed by addition of 0.1 % by weight of ti- tanium tetrabutoxide, based on the weight of terephthalic acid, thereby obtaining a mixture (in the form of a slurry). The obtained mixture was continu- ously charged into esterification reaction vessel 11, and a polymerization was performed in substantially the same manner as in Example 1, except that the polymeri- zation was performed under the conditions indicated in Tables 1 and 2, thereby obtaining a polytrimethylene terephthalate (PTT) resin. During the polymerization. trimethyl phosphate (stabilizer) was continuously added to first vertical agitation type polymerizer 15 in an amount of 20 ppm by weight, based on the weight of the polymer. The results are shown in Table 1. The prepolymer which was fed into the final poly- merizer 10 satisfied the requirements of the present invention, and the obtained PTT resin had a high mo- lecular weight, a narrow molecular weight distribution, a low terminal carboxyl group content, and excellent color. Further, the degree of discoloration of the ob- tained resin occurring by heating was very small. Example 8 Using the production system as shown in Fig. 4, 130 kg of a polytrimethylene terephthalate (PTT) resin was continuously produced per day, wherein terephthalic acid and TMG were used as raw materials. With respect to the apparatuses used in the above-mentioned produc- tion system, each of esterification reaction vessel 11 and first vertical agitation type polymerizer 15 was a vertical agitation type polymerizer equipped with a stirrer having paddle-shaped agitation blades; horizon- tal agitation type polymerizer 22 was equipped with a uniaxial stirrer having disc-shaped agitation blades; and polymerizer 10 was the same as used in Example 5. Specifically, production of the PTT resin was per- formed as follows. Terephthalic acid and TMG were mixed together (terephthalic acid/TMG molar ratio = 1/1.5), followed by addition of 0.1 % by weight of ti- tanium tetrabutoxide, based on the weight of terephthalic acid, thereby obtaining mixture C (in the form of a slurry). The obtained mixture C was continu- ously charged into esterification reaction vessel 11, and polymerization was performed in substantially the same manner as in Example 1, except that the polymeri- zation was performed under the conditions indicated in Tables 1 and 3, thereby obtaining a polytrimethylene terephthalate (PTT) resin. During the polymerization, trimethyl phosphate (stabilizer) was continuously added to first vertical agitation type polymerizer 15 in an amount of 20 ppm by weight, based on the weight of the polymer. The results are shown in Table 1. The prepolymer which was fed into the final poly- merizer 10 satisfied the requirements of the present invention, and the obtained PTT resin had a high mo- lecular weight, a narrow molecular weight distribution, a low terminal carboxyl group content, and excellent color. Further, the degree of discoloration of the ob- tained PTT resin occurring by heating was very small. Example 9 Using the production system as shown in Fig. 5, 130 kg of polytrimethylene terephthalate (PTT) resin was continuously produced per day, wherein DMT and TMG were used as raw materials . With respect to the appa- ratuses used in the above-mentioned production system, each of first transesterification reaction vessel 25 and second transesterification reaction vessel 29 was a vertical agitation type polymerizer equipped with a stirrer having turbine blades (26 or 30); each of first vertical agitation type polymerizer 15 and second ver- tical agitation type polymerizer 19 was equipped with a stirrer having paddle-shaped agitation blades (16 or 20); and polymerizer 10 was the same as the polymerizer used in Example 5, except that the number of holes of the perforated plate was changed to four (wherein the four holes were arranged such that a square is formed when lines connecting the holes are drawn on the sur- face of the perforated plate); the length of each guide 5 was changed to 9 m, and the length of the polymerizer casing was increased accordingly. During the operation of polymerizer 10, the prepolymer was caused to pass through the holes of the perforated plate at a rate of 23 g/min per hole. Specifically, production of the PTT resin was per- formed as follows. DMT and a mixture of TMG and tita- nium tetrabutoxide (amount of titanium tetrabutoxide = 0.1 % by weight, based on the weight of DMT) (DMT/TMG molar ratio = 1/1.5) were continuously charged into es- terification reaction vessel 11, and polymerization was performed under the conditions indicated in Tables 1 and 4, thereby obtaining a polytrimethylene terephtha- late (PTT) resin. During the polymerization, trimethyl phosphate (stabilizer) was continuously added to first vertical agitation type polymerizer 15 in an amount of 20 ppm by weight, based on the weight of the polymer. The results are shown in Table 1. Prepolymer A which was fed into the final polymer- izer 10 satisfied the requirements of the present in- vention, and the obtained PTT resin B had a high mo- lecular weight, a narrow molecular weight distribution, a low terminal carboxyl group content, and excellent color. Further, the degree of discoloration of the ob- tained resin occurring by heating was very small. The obtained polytrimethylene terephthalate resin was immersed in cool water having a temperature of 5 ° C to thereby solidify the PTT resin. The solidified PTT resin was cut into pellets, each having a weight of 20 mg. The amount of polymer powder (which had adhered to the pellets) was as low as 0.01 % by weight, and the pellets had a crystallinity as low as 5 %. Therefore, the obtained pellets were not easily broken and easy to handle. Example 10 Polymerization was performed in substantially the same manner as in Example 9, except that nitrogen gas E was introduced through inlet 6 into polymerizer 10 in an amount indicated in Table 1, thereby obtaining a polytrimethylene terephthalate (PTT) resin (the condi- tions employed in this Example are indicated in Tables 1 and 4). The results are shown in Table 1. Prepolymer A which was fed into the final polymer- izer 10 satisfied the requirements of the present in- vention, and the obtained PTT resin B had a high mo- lecular weight, a narrow molecular weight distribution, a low terminal carboxyl group content, and excellent color. Further, the degree of discoloration of the ob- tained resin occurring by heating was very small. Example11 Polymerization was performed in substantially the same manner as in Example 9, except that the production system as shown in Fig. 6 was employed instead of the production system as shown in Fig. 5 (i.e., except that horizontal agitation type polymerizer 22 equipped with a uniaxial stirrer having disc-shaped agitation blades 23 was used instead of second vertical agitation type polymerizer 19), thereby obtaining a polytrimethylene terephthalate resin (the conditions employed in this Example are indicated in Tables 1 and 5). The results are shown in Table 1. Prepolymer A which was fed into the final polymer- izer 10 satisfied the requirements of the present in- vention, and the obtained PTT resin B had a high mo- lecular weight, a narrow molecular weight distribution, a low terminal carboxyl group content, and excellent color. Further, the degree of discoloration of the ob- tained resin occurring by heating was very small. Example 12 Polymerization was performed in substantially the same manner as in Example 11, except that nitrogen gas E was introduced through inlet 6 into polymerizer 10 in an amount indicated in Table 1, thereby obtaining a polytrimethylene terephthalate (PTT) resin (the condi- tions employed in this Example are indicated in Tables 1 and 5). Prepolymer A which was fed into the final polymer- izer 10 satisfied the requirements of the present in- vention, and the obtained PTT resin B had a high mo- lecular weight, a narrow molecular weight distribution, a low terminal carboxyl group content, and excellent color. Further, the degree of discoloration of the ob- tained resin occurring by heating was very small. Examples 13 and 14 In each of Examples 13 and 14, polymerization was performed in substantially the same manner as in Exam- ple 12 (in which the production system as shown in Fig. 6 was employed), except that, in Example 13, guides 5 provided in polymerizer 10 were changed to chains, each formed by the combination of elliptical rings (the di- ameter of a wire forming each ring was 3 mm; the major axis of the ellipse defined by each ring was 50 mm; and the curvature of each ring was 2 0 mmF)) , and that, in Example 14, guides 5 provided in polymerizer 10 were changed to wires, each having a diameter of 5 mm and having welded thereto discs (each having a diameter of 20 mmF) and a thickness of 3 mm) at intervals of 200 mm in a manner such that the wire penetrates the center of each disc, thereby obtaining a polytrimethylene terephthalate resin (the conditions employed in these Examples are indicated in Tables 1 and 6) . The results are shown in Table 1. Prepolymer A which was fed into the final polymer- izer 10 satisfied the requirements of the present in- vention, and the obtained PTT resin B had a high mo- lecular weight, a narrow molecular weight distribution, a low terminal carboxyl group content, and excellent color. Further, the degree of discoloration of the ob- tained resin occurring by heating was very small. Comparative Example 5 The prepolymer obtained in Example 13 was immersed in cool water having a temperature of 5 °C to thereby solidify the prepolymer. The solidified prepolymer was then cut into pellets, and the pellets were dried at 120 °C in air. 100 kg of the dried pellets were charged into a 300 liter tumbling solid-phase polymer- izer, and solid-phase polymerization was performed at 205 °C for 72 hours, while flowing nitrogen gas into the polymerizer at a rate of 100 liters/hr, thereby obtaining a polytrimethylene terephthalate (PTT) resin. The results are shown in Table 1. The obtained PTT resin had a satisfactorily high molecular weight. However, the obtained resin had a broad molecular weight distribution. Further, the ob- tained pellets not only had attached thereto polymer powder in an amount as large as 1.0 % by weight, but also had a crystallinity as high as 55 %, and hence. the obtained, pellet was brittle. If it is attempted to transfer the obtained pellets by means of a feeder or a pneumatic conveyer, the pellets would be broken, thereby forming a large amount of polymer powder. Comparative Example 6 Polymerization was performed in substantially the same manner as in Example 12, except that the amount of a polytrimethylene terephthalate resin produced per day was reduced to 75 kg, and that polymerizer 10 was not used, thereby obtaining a polytrimethylene terephtha- late resin. The results are shown in Table 1. The obtained PTT resin had a low polymerization degree, a broad molecular weight distribution and a high terminal carboxyl group content. Further, the de- gree of discoloration of the obtained resin occurring by heating was very large Example 15 Polymerization was performed in substantially the same manner as in Example 1, except that the system as shown in Fig. 2 was employed in which inert gas was in- troduced into the polymerizer by means of inert gas ab- sorption device Nl, and that the polymerization was performed under the conditions indicated in Table 1. Perforated plate N4 provided in inert gas absorption device Nl had nine holes, each having a diameter of 1 mm, in which the holes of the perforated plate are ar- ranged such that a checkered pattern is formed when lines connecting the holes are drawn on the surface of the perforated plate. Each of guides N5 used in inert gas absorption apparatus Nl was a wire made of stainless steel, which had a circular cross-section, and had a diameter of 5 mm and a length of 3 m. Guides N5 were attached to perforated plate N4 so that each hole of perforated plate N5 had one guide N5 attached thereto. Nitrogen gas E was fed into the gas absorp- tion apparatus so that the internal pressure thereof was 0.11 Pa. Prepolymer N5" was allowed to fall along and in contact with guides N5 so as to cause the pre- polymer to absorb and contain nitrogen gas. Transfer- ring pump N7 was operated while observing the prepoly- mer inside the gas absorption apparatus through the ob- serving window, so that almost no prepolymer was accu- mulated at the bottom of the gas absorption apparatus. Prepolymer A withdrawn from inert gas absorption appa- ratus Nl contained very small bubbles. After conduct- ing the production of the PTT resin for a while in the above-mentioned manner, the feeding of nitrogen gas E into inert gas absorption apparatus Nl was stopped. and the difference in the internal pressure of inert gas absorption apparatus Nl before and after the stop- ping of the feeding of nitrogen gas was measured. As a result, it was found that the difference in the amount of nitrogen gas was 0.5 mg per gram of the prepolymer. This difference in the amount of nitrogen gas was de- fined as the amount of nitrogen gas which was absorbed by and contained in the prepolymer. Using the thus ob- tained amount of nitrogen gas absorbed by and contained in the prepolymer, the amount of nitrogen gas intro- duced into the polymerizer was calculated on the as- sumption that all nitrogen gas contained in the pre- polymer was introduced into the polymerizer. The re- sults are shown in Table 1. When the prepolymer fal- ling in polymerizer 10 was observed through observing window 4, it was found that the prepolymer was in a foaming state and contained a large amount of bubbles. The resultant polytrimethylene terephthalate resin had a high molecular weight, a narrow molecular weight dis- tribution and a low terminal carboxyl group content, and excellent color. Further, the degree of discolora- tion of the obtained resin occurring by heating was very small. Example 16 Polymerization was performed in substantially the same manner as in Example 9, except that the second vertical agitation type polymerizer was replaced by a free-fall polymerizer (which is the same as polymerizer 10 of Fig. 5 except that guides 5 were removed so that the prepolymer was caused to fall freely from the holes of perforated plate 3), thereby obtaining a poly- trimethylene terephthalate resin (the conditions em- ployed in this Example are indicated in Tables 1 and 4). The free-fall polymerizer was operated at 260 ° C under a pressure of 100 Pa. The results are shown in Table 1 Prepolymer A which was fed into the final polymer- izer 10 satisfied the requirements of the present in- vention, and the obtained PTT resin B had a high mo- lecular weight, a narrow molecular weight distribution, a low terminal carboxyl group content, and excellent color. Further, the degree of discoloration of the ob- tained resin occurring by heating was very small. Example 17 Polymerization was performed in substantially the same manner as in Example 9, except that the second vertical agitation type polymerizer was replaced by a thin film type polymerizer (which is the same as poly- merizer 10 of Fig. 5 except that the perforated plate had 4 holes arranged, in a line, and that a flat board is vertically provided as guide 5, so that prepolymer A is allowed to fall in the form of a film on the flat board), thereby obtaining a polytrimethylene terephtha- late resin (the conditions employed in this Example are indicated in Tables 1 and 4). The thin film type poly- merizer was operated at 260 ° C under a pressure of 100 Pa. The results are shown in Table 1. Prepolymer A which was fed into the final polymer- izer 10 satisfied the requirements of the present in- vention, and the obtained PTT resin B had a high mo- lecular weight, a narrow molecular weight distribution, a low terminal carboxyl group content, and excellent color. Further, the degree of discoloration of the ob- tained resin occurring by heating was very small. INDUSTRIAL APPLICABILITY The polytrimethylene terephthalate resin of the present invention can be stably produced on a commer- cial scale without performing solid-phase polymeriza- tion. Further, the polytrimethylene terephthalate resin of the present invention has an intrinsic viscos- ity within an appropriate range, a narrow molecular weight distribution, and excellent color, and hence, can advantageously be used for stably producing a fiber or shaped article which has high strength and excellent color on a commercial scale. We claim: 1. A poly trimethylene terephthalate resin comprising: 90 to 100 mole % of trimethylene terephthalate recurring units, and 0 to 10 mole % of at least one comonomer unit said trimethylene terephthalate recurring units, said poly trimethylene terephthalate resin having the following characteristics (A) to (D): (A) an intrinsic viscosity (?) of from 0.8 to 4.0 dl/g; (B) a molecular weight distribution of from 2.0 to 2.7 in terms of the Mw/Mn ratio, wherein Mw represents the weight average molecular weight of said poly trimethylene terephthalte resin and Mn represents the number average molecular weight of said poly trimethylene terephthalate resin; (C) a psychometric lightness L-value (L-1) of from 70 to 100 and a psychometric chroma b*-value (b*-l) of from -5 to 25; and (D) a psychometric lightness L-value (L-2) of from 70 to 100 and a psychometric chroma b*-value (b*-2) of from -5 to 25 as measured after heating said poly- trimethylene terephthalate resin at 180 " C for 24 hours in air. 2. The polytrimethylene terephthalate resin according to claim 1, wherein said polytrimethylene terephthalate resin has an intrinsic viscosity [?] of from 1.25 to 2.5 dl/g. 3. The polytrimethylene terephthalate resin according to claim 1 or 2, which has a terminal carboxyl group content of from 0 to 20 meq/kg. 4. The polytrimethylene terephthalate resin according to any one of claims 1 to 3, which has a molecular weight distribution of from 2.0 to 2.6. 5. The polytrimethylene terephthalate resin of any one of claims 1 to 4, which is in the form of pellets, 6. The polytrimethylene terephthalate resin according to claim 5, wherein said pellets have an average weight of from 1 to 1000 mg per pellet, and wherein said pel- lets contains a powder of said polytrimethyfene terephthaiate resin in an amount of 0 to 0.5% by weight,, based on the total weight of said peliets , which powder passes through a 30-mesh filter and does not pass through a 300-mesh filter. 7. The polytnm ethyiene terephthaiate resin according to claim 5 or 6, wherein said peiiets have a crystallinity (Xc) of 40% or less, wherein said crystallinity is defined by the following formula: Xc (%) - { ? X (?s-?a) }/ { ?, X (?c-?a)} X 100 wherein ?a is 1.300 g/cm3 which is an amorphous density of trimethylene terepbthalate homopolymer, ? is 1 431 g/cm3 which is a crystal density of trimethylene terephthaiate homopoiymer, an ?s represents a density (g/cm3) of said pellets, 8. A method for producing a polytrimethyiene terephthaiate resin, which comprises: (1) providing a molten form of a trimethylene terephthaiate prepolymer comprising: 90 to 100 mole % of trimethylene terephthaiate recurring units, and 0 to 10 mole % of at least one comanomer unit other than said trimethylene terephthaiate recurring units, said trimethylene terephthaiate prepolymer having an intrinsic viscosity [?] of from 0.2 to 2 dl/g. and (2) polymerizing said molten form of a trimethylene terephthaiate prepolymer at a temperature which is 5°C or more higher than the crystalline melting point of said prepolymer but is not higher than 280°C under reduced pressure by the guide wetting fall process in which said prepolymer is allowed to fall along and in contact with the surface of a guide so that polymerization of said prepolymer is effected during the fall thereof. 9. The method according to claim 8, wherein said molten prepolymer is continuously fed to a polymerization reaction zone for effecting the polymerization of said prepolymer in said step (2) and the resultant poly trimethylene terephthaiate resin produced in said step (2) is continuously withdrawn from said polymerization zone, so that said step (2) for prepoiymer polymeriza- tion is continuously performed. 10. The method according to claim 8 or 9, wherein said guide has at least one portion selected from the group consisting of a concave portion, a convex portion and a perforated portion. 11. The method according to any one of claims 8 to 10, wherein said prepolymer falling along and in contact with the surface of said guide is in a foaming state. 12. The method according to any one of claims 8 to 11, wherein the polymerization in said step (2) is performed, while introducing inert gas to said polymerization reac- tion zone. 13. The method according to claim 12, wherein the amount of said inert gasN introduced to said polymeriza- tion reaction zone is in the range of from 00.5 to 100 mg per gram of said polytrimethylene terephthalate resin withdrawn from said polymerization reaction zone. 14. The method according to claim 12 or 13, wherein at least a part of said inert gas is introduced to said po- lymerization reaction zone in a manner wherein the in- troduction of the part of said gas is conducted sepa- rately from the feeding of the trimethylene terephtha- late prepolymer to the polymerization reaction zone. 15. The method according to any one of claims 12 to 14, wherein at least a part of said inert gas is introduced, to said polymerization reaction zone in such a form as absorbed or contained in said trimethylene terephthalate prepolymer. 16. The method according to any one of claims 8 to 15, wherein said prepolymer has an intrinsic viscosity [?] of from 0.5 to 2.0 dl/g and a terminal carboxyl group ratio of 50 % or less in terms of the molar ratio (%) of the terminal carboxyl groups of the prepolymer to all terminal groups of the prepolymer. 17. The method according to any one of claims 8 to 16, wherein said prepolymer is produced by at least one po- lymerization method selected from the following methods (a) to (d): (a) a polymerization method using a vertical agita- tion type polymerizer; (b) a polymerization method using a horizontal agi- tation type polymerizer; (c) a polymerization method using a free-fall poly- merizer having a perforated plate; and (d) a polymerization method using a thin film type polymerizer. 18. The method according to claim 17, wherein said pre- polymer is produced by said method (b). 19. A polytrimethylene terephthalate resin produced by the method of any one of claims 8 to 18. A polytrimethylene terephthalate resin comprising: 90 to 100 mole % of trimethylene terephthalate recurring units, and 0 to 10 mole % of at least one monomer unit 5 obtained from a comonomer other than the monomers used for forming the recurring units and copolymerizable with at least one of the monomers used for forming the recur- ring units, which resin has the following characteris- tics: (A) an intrinsic viscosity [?] of from 0.8 to 4.0 10 dl/g; (B) an Mw/Mn of from 2.0 to 2.7; (C) a psychomet- ric lightness L-value (L-1) of from 70 to 100 and a psychometric chroma b*-value (b*-l) of from -5 to 25; and (D) a psychometric lightness L-value (L-2) of from 70 to 100 and a psychometric chroma b*-value (b*-2) of 15 from -5 to 25 as measured after heating the resin at 180 °C for 24 hours in air.

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