Title of Invention

"A PROCESS FOR THE PREPARATION OF A SYNTHETIC POLYISOPRENE"

Abstract

The invention concerns synthetic polyisoprenes having a high cis-1,4 chaining rate and the method for preparing same. The inventive polyisoprenes have a cis-1,4 chaining rate, measured according to technique of nuclear magnetic resonance of carbon 13 or by means infrared assay, which is of the order ranging between 99.0 % to 99.6 %. The inventive method for preparing said polyisoprenes consists essentially in reacting a catalytic system in the presence of isoprene, and it consists in: a) using, as catalytic system, a system based on at least a conjugate diene monomer, a salt of one or several rare earths of an organic phosphoric acid, an alkylaluminium alkylating agent of formula AIR3 or HAIR2, and, a halogen donor consisting of an alkylaluminium halide, the mol ratio (alkylating agent/rare earth salt) ranging between 1 and 5; and b) carrying out a polymerising reaction of the isoprene at a temperature not higher than 5 DEG C.

Full Text

The present invention relates to synthetic polyisoprenes having a high content of cis-1,4 linkages and a process for their preparation. For the preparation of polyisoprenes containing a high content of cis-1,4 linkages, it is known to use catalytic systems based on: - a rare earth salt in solution in a hydrocarbon solvent, - an alkylating agent of this salt formed of an alkylaluminium, and - a halide of an alkylaluminium. It is for example known from the document "Report of the Academy of Sciences of the U.S.S.R., volume 234, No. 5, 1977 (Y. B. Monakov, Y. R. Bieshev, A. A. Berg, S. R. Rafikov)" to use, for the polymerisation of isoprene at a temperature of between 20°C and 50°C, a catalytic system comprising: - a bis(2-ethylhexyl)phosphoric acid salt of neodymium or praseodymium, as rare earth salt, in solution in toluene, - triisobutylaluminium as alkylating agent, in a molar ratio (alkylating agent / rare earth salt) of 20, and - diethylaluminium chloride as halide of an alkylaluminium. Mention may also be made of the document "Proceedings of China - U. S. Bilateral Symposium on Polymer Chemistry and Physics, Science Press, pp. 382-398, 1981 (O. Jun, W. Fosong, S. Zhiquan)". This document teaches in particular the use of a bis(2-ethylhexyl)phosphoric acid salt of neodymium, in association with triethylaluminium or triisobutylaluminium, and an alkylaluminium halide of the formula Al2(C2H5)3Cl3. The polyisoprenes which are obtained by means of such a catalytic system have contents of cis-1,4 linkages which vary from 94.2% to 94.7% (tables 4 and 6, pp. 386 and 387). This document also mentions the use of catalytic systems based on: - rare earth naphthenate, the contents of cis-1,4 linkages of the corresponding polyisoprenes being between 93.6% and 96.0%; and based on rare earth trichloride (catalytic system of formula LnCl3 - C=H5OH - Al(C2H5)3, the contents of cis-1,4 linkages of the corresponding polyisoprenes being between 94.1% and 98.0%) (this content of 98% being achieved using ytterbium as rare earth, see table 12 p. 391). In the majority of cases, the determination of the microstructure is accomplished by the technique of medium-wave infrared radiation analysis (abbreviated to MIR) in accordance with the method developed by Ciampelli et al (F. Ciampelli, D. Moreno, M. Cambini, Makromol. Chem., 1963, 61, 250-253). It will be noted that this method, which is based solely on calculations made in the infrared range, does not always provide results of satisfactory accuracy when used in isolation. US patent specification US-A-5,859,156 describes a process for the preparation of polyisoprenes by means of a catalytic system based on titanium tetrachloride, an organoaluminium and an ether. The maximum content of cis-1,4 linkages of the polyisoprenes thus obtained, measured by carbon-13 nuclear magnetic resonance (13C-NMR), is 98.0% (see example 2, column 27, the content of trans-1,4 and 3,4 linkages then being 1.0% each). The object of the present invention is to propose novel synthetic polyisoprenes and a process for their preparation, said polyisoprenes having contents of cis-1,4 linkages which are distinctly greater than those obtained to date. The Applicant has unexpectedly discovered that a catalytic system of the "preformed" type based on at least: - a conjugated diene monomer, - an organic phosphoric acid salt of one or more rare earth metals (metals with an atomic number between 57 and 71 in Mendeleev's periodic table), said salt being in suspension in at least one inert, saturated hydrocarbon solvent of aliphatic or alicyclic type, - an alkylating agent consisting of an alkylaluminium of formula AIR3 or HAIR2, the molar ratio (alkylating agent / rare earth salt) having a value of from 1 to 5, and - a halogen donor consisting of an alkylaluminium halide, makes it possible to polymerise isoprene with satisfactory activity at polymerisation temperatures which are less than or equal to 0°C, and to obtain at these low temperatures polyisoprenes having a content of cis-1,4 linkages, measured both using the technique of carbon-13 nuclear magnetic resonance and the technique of medium-wave infrared radiation analysis, are strictly greater than 99.0%. This catalytic system according to the invention makes it possible for example to obtain polyisoprenes which are such that said contents of cis-1,4 linkages, measured by one and the other of these techniques, maybe between 99.0% and 99.6%, including 99.6%. It will be noted that the polymerisation operations can be carried out in an inert hydrocarbon solvent, or alternatively without a solvent. Advantageously, this catalytic system makes it possible, at polymerisation temperatures of from -55°C to -20°C, to obtain polyisoprenes the contents of cis-1,4 linkages of which, measured by one and the other of the aforementioned techniques, are equal to or greater than 99.3%, and lie for example in a range from 99.3% to 99.6%. Even more advantageously, this catalytic system makes it possible, at polymerisation temperatures of from -55°C to -40°C, to obtain polyisoprenes the contents of cis-1,4 linkages of which, also measured by one and the other of the aforementioned techniques, are equal to or greater than 99.5%, and are for example equal to 99.6%. It will be noted that these values of contents of cis-1,4 linkages which are very close to the value of 100% which characterises natural rubber have never really been achieved to date. The range of contents of cis-1,4 linkages measured in accordance with the present invention takes into account measurements established by means of, on one hand, the technique of medium-wave infrared radiation analysis after calibration of the samples of polyisoprene which is effected within the scope of C-NMR analysis and, on the other hand, of C-NMR analysis, the measurements obtained by one of these techniques being confirmed by the other (disregarding the inaccuracy of measurement of+/- 0.1%, which is inherent in each of these two techniques). The accuracy of these values of contents of cis-1,4 linkages is thus significantly increased, relative to that of the values of the contents which have been mentioned in the prior art to date. In particular, 13C NMR analysis showed the total absence of 1,2 linkages and trans-1,4 linkages in the samples of polyisoprene according to the invention. It will furthermore be noted that the particularly high content of cis-1,4 linkages obtained for the polyisoprenes according to the invention is independent of the quantity of catalytic system used. It will furthermore be noted that the polyisoprenes thus obtained have a high viscosity. As far as the catalytic systems according to the invention are concerned, it will be noted that they are characterised by molar ratios (alkylating agent / rare earth salt) of between 1 and 5, which, surprisingly, is extremely low compared with the molar ratios equal to or greater than 20 which have been used to date to polymerise isoprene. Mention may be made of 1,3-butadiene as preferred conjugated diene monomer usable for "preforming" the catalytic system according to the invention. Mention may also be made of 2-methyl-l,3-butadiene (or isoprene), 2,3-di(Cl to C5 alkyl)-1,3-butadienes such as, for instance, 2,3-dimethyl-l,3-butadiene, 2,3-diethyl-l,3-butadiene, 2-methyl-3-ethyl-l,3-butadiene, 2-methyl-3-isopropyl-l,3-butadiene, phenyl-1,3-butadiene, 1,3-pentadiene, 2,4-hexadiene, or any other conjugated diene having between 4 and 8 carbon atoms. It will be noted that the molar ratio (monomer / rare earth salt) may have a value of from 25 to 50. According to another characteristic of the invention, said rare earth salt consists of a non-hygroscopic powder having a slight tendency to agglomerate at ambient temperature. - According to a preferred embodiment of the invention, the inert hydrocarbon solvent in which said rare earth salt is in suspension is an aliphatic or alicyclic solvent of low molecular weight, such as cyclohexane, methylcyclohexane, n-heptane, or a mixture of these solvents. - According to another embodiment of the invention, the solvent used to suspend the rare earth salt is a mixture of an aliphatic solvent of high molecular weight comprising a paraffmic oil, for example petrolatum oil, and of a solvent of low molecular weight such as those mentioned above (for example methylcyclohexane). This suspension is produced by dispersive grinding of the rare earth salt in this paraffmic oil, so as to obtain a very fine and homogenous suspension of said salt. According to another characteristic of the invention, said catalytic system comprises the rare earth metal in a concentration equal or substantially equal to 0.02 mol/1. According to a preferred example of embodiment of the invention, a tris[bis(2-ethylhexyl)phosphate] salt of said rare earth metal(s) is used as salt. Even more preferably, said rare earth salt is neodymium tris[bis(2-ethylhexyl)phosphate]. As alkylating agent usable in the catalytic system according to the invention, mention maybe made of alkylaluminiums such as: - trialkylaluminiums, for example triisobutylaluminium, or - dialkylaluminium hydrides, for example diisobutylaluminium hydride. It will be noted that this alkylating agent is preferably formed of diisobutylaluminium hydride (referred to as DiBAH in the rest of the present description). As halogen donor usable in the catalytic system according to the invention, mention may be made of alkyl aluminium halides, preferably diethylaluminium chloride (referred to as DEAC in the rest of the present description). It will be noted that the molar ratio (halogen donor / rare earth salt) may have a value of from 2.6 to 3. According to the invention, the process for the preparation of said catalytic system consists: - in a first stage, of producing a suspension of said rare earth salt in said solvent, - in a second stage, of adding said conjugated diene monomer to the suspension, - in a third stage, of adding said alkylating agent to the suspension comprising said monomer to obtain an alkylated salt, and - in a fourth stage, of adding said halogen donor to the alkylated salt. The aforementioned characteristics of the present invention, as well as others, will be better understood on reading the following description of several examples of embodiment of the invention, which are given by way of illustration and not of limitation. I. PREPARATION OF CATALYTIC SYSTEMS ACCORDING TO THE INVENTION: 1) Synthesis of an organic phosphate salt of neodymium according to the invention: A plurality of tests were carried out for the synthesis of this salt. The same synthesis method was used for each of these tests, and is described in detail hereafter. a) Synthesis of an aqueous solution of neodymium NdCl3, 6H2O: 96 g of Nd203 (sold by RHODIA), which has been determined by complexation analysis to contain 85.3% Nd (85.7% in theory), or having 0.57 mol of Nd, is weighed into a 600 ml beaker of "tall" form. 80 ml of demineralised water is added. Under a fume hood, while being stirred with a magnetic stirrer at ambient temperature, there is slowly added 150 ml of 36% by weight concentrated HC1 (d = 1.18), or 1.75 mol of HC1 (molar ratio HCl/Nd = 1.75/0.57 = 3.07). The reaction Nd203 + 6 HCl + 9 H20→2 NdCl3, 6H20 is highly exothermic. Once all the hydrochloric acid has been added, the solution is brought to boiling with magnetic stirring, in order to eliminate the excess hydrochloric acid. The aqueous solution of NdCl3 is clear and mauve in colour. No insoluble product (Nd2O3) remains. This solution is then evaporated until a volume of approximately 130 ml is obtained in the beaker. The solution of NdCl3, 6H20 is then highly concentrated (it crystallises at ambient temperature). Then the concentrated solution of NdCl3is poured into a 10 litre drum containing 4500 ml of demineralised water with stirring and at ambient temperature (using a motor with anchor agitator). The pH of the solution, measured at 25°C, is close to 4. Then 1500 ml of technical-grade acetone is added to the solution. No insoluble product remains, and the solution thus obtained is pink in colour. b) Synthesis of an organic sodium phosphate of formula [RO]2P(0)ONa (R=2-ethvlhexyl): 68 g of NaOH in pellets, or 1.70 mol, is dissolved in a 5 litre beaker containing 1500 ml of demineralised water. 554 g of an organic phosphoric acid (bis(2-ethylhexyl) phosphoric acid, listed in the "Aldrich" catalogue under the reference 23,782-5), or 1.72 mol of this acid, is dissolved in another, 3 litre, beaker containing 500 ml of acetone. The molar ratio NaOH / organic phosphoric acid is 1.70 /1.72, or 0.99. The solution of said organic phosphoric acid is poured into the NaOH solution at ambient temperature and with manual stirring using a glass stirrer. The reaction is as follows: [RO]2P(0)OH+NaOH → [RO]2P(0)ONa + H20. It is slightly exothermic, and a homogenous solution of a yellowish colour is obtained. The pH of the solution, measured at 25°C, is close to 7. c) Synthesis of a phosphated neodymium salt of formula [[RO]2P(O)O]3Nd: - The solution of organic Na phosphate obtained in paragraph b) above is poured with vigorous stirring (motor with anchor agitator) and at ambient temperature into the aqueous solution of NdCl3,6H20 obtained in paragraph a) above. A very fine white precipitate immediately forms. Stirring of the resultant mixture continues for 30 minutes, once all the organic Na phosphate has been added (in a molar ratio (R0)2P(0)0Na/NdCl3 = 1.70/0.57 = 2.98). The reaction is as follows: 3 [RO]2P(0)ONa + NdCl3,6H20 → Nd[OP(0)[OR]2]3 + 3 NaCl + 6 H20. - The phosphated neodymium salt thus obtained is recovered and washed in a centrifuge fitted with a "sock". The pH of the "mother" liquors is between 3 and 4 at 25°C. These "mother" liquors are colourless and clear. The salt obtained is divided into two samples, then each sample is washed with an acetone/ demineralised water mixture by carrying out the washing cycle described below three times, in order to eliminate all the chlorides. Each washing cycle is performed in a 10 litre plastic bucket initially containing 2 litres of acetone. Each sample is then homogenised with the acetone by means of an "Ultra-Turrax" homogeniser for approximately 1 minute, in order to obtain a solution of milky type. Subsequently, 4 litres of demineralised water is added to the bucket, then the mixture obtained is homogenised for 3 minutes using the same homogeniser. The mixture thus obtained is centrifuged, then the phosphated neodymium salt is recovered in the "sock". For the final batch of washing water, the qualitative analytic test for chlorides is virtually negative (the reaction is: NaCl + AgNO3 (medium F1NO3) → AgCl↓ + NaN03). The neodymium salt thus washed is dried in an oven at 60°C, under a vacuum and with an air current for approximately 80 hours. The final yield for each of the synthesis tests carried out is between 95% and 98%, dependent on the losses due to the washing operations. Each time approximately 600 g of phosphated neodymium salt is obtained in the dry state. The mass contents of neodymium, determined by complexometry, are between 12.9% and 13.0% (for a theoretical content = [144.24 /1108.50] x 100 = 13.01%, with 144,24 g/mol = molar mass of the neodymium). 2) Synthesis of "preformed" catalytic systems according to the invention: a) Composition of catalytic systems according to the invention: Each of these systems comprises a phosphated neodymium salt such as synthesised in accordance with paragraph 1) above, which is in suspension in an inert hydrocarbon solvent of low molecular weight (consisting of cyclohexane, abbreviated to "CH" hereafter, or of methylcyclohexane, abbreviated to "MCH" hereafter). These catalytic systems are characterised by the following relative molar ratios, relative to the neodymium salt: Nd salt / butadiene (Bd hereafter) / DiBAH / DEAC = 1 / 50 /1.8 to 4 / 2.6 or 3. b) Process for synthesising these catalytic systems: - First stage: In order to obtain these catalytic systems, 15.6 g of the neodymium salt, in powder form, is poured into a 1 litre reactor from which the impurities have been removed beforehand. This salt is then subjected to nitrogen bubbling from the bottom of the reactor, for a period of 15 min. - Second stage: 90% (mass fraction) of the solvent mentioned in paragraph a) above is introduced into the reactor containing the neodymium salt. When the solvent used is cyclohexane, the duration of contact of the neodymium salt with this solvent varies from 2 hours to 4 hours, and the temperature of contact varies from 30°C to 60°C. When the solvent used is methylcyclohexane, the duration of contact of the neodymium salt with this solvent is 30 min., and the temperature of contact is 30°C. - Third stage: Butadiene is then introduced into the reactor (in the molar ratio salt / butadiene of 1/ 50 mentioned in paragraph a) above), at a temperature of 30°C, for "preforming" each catalytic system. - Fourth stage: Diisobutylaluminium hydride (DiBAH) is then introduced into the reactor as alkylating agent for the neodymium salt, in a concentration of approximately 1 M, and also a quantity of the solvent mentioned above in the second stage corresponding to a mass fraction of 5% of the entire solvent. The duration of the alkylation is 15 min. and the temperature of the alkylation reaction is 30°C. - Fifth stage: Diefhylaluminium chloride (DEAC) is then introduced into the reactor as halogen donor, in a concentration of approximately 1 M, and together with a quantity of the solvent mentioned above in the second stage corresponding to the remaining mass fraction of 5% of the entire solvent. The temperature of the reaction medium is adjusted to 60°C. - Sixth stage: "Preforming" (or ageing) of the mixture thus obtained is then carried out by maintaining this temperature of 60°C for a period of 2 hours to 4 hours. - Seventh stage: In this manner, approximately 700 ml of a solution of catalytic system is obtained. The reactor is emptied and this solution is transferred to a 750 ml "Steinie" bottle, which has beforehand been washed, dried and subjected to nitrogen bubbling. Finally the catalytic solution is stored under a nitrogen atmosphere in a freezer, at a temperature of -15°C. Table summarising the catalytic systems prepared: (Table Removed) II. Polymerisation of isoprene by means of the aforementioned catalytic systems: 1) Examples of polymerisation of isoprene at a temperature of-15°C by means of the aforementioned catalytic system 1: a) Polymerisation process used: A 250 ml "Steinie" bottle was used as the polymerisation reactor. Each polymerisation reaction was carried out either under static conditions in a freezer (bottle placed in a bath of glycol), or dynamically (by subjecting the bottle to agitation in a tank of glycol). A steam-cracked C5 naphtha fraction was used, with the aim of extracting isoprene having a purity close to 100% therefrom. To this end, conventional laboratory purification was carried out, consisting successively of: - distillation of this C5 fraction over maleic anhydride to eliminate any residual cyclopentadiene, followed by - passing through a column of alumina to eliminate the polar impurities, and - nitrogen bubbling for 20 min., immediately prior to the polymerisation reaction. The mass fraction of isoprene extracted from this C5 fraction was determined, by the technique of gas phase chromatography (GPC), and is 99.2%. Each isoprene polymerisation reaction (10 g per bottle) was carried out in cyclohexane at -15°C, under an inert nitrogen atmosphere, with amass ratio solvent/monomer (S/M) of 9. In the various examples of polymerisation the neodymium catalyst base is varied from 150 µmol to 500 umol per 100 g of monomer (quantity of neodymium expressed in µMcm hereafter). Tightness of the bottle is ensured by a "septum/open-top seal" assembly permitting the injection of the catalytic system by means of a syringe. At the end of polymerisation, while adding 100 ml of additional solvent to fluidify the medium, acetylacetone is added (1 ml of a solution of a concentration of 1M in cyclohexane) to stop the reaction and N-l,3-dimethylbutyl-N'-phenyl-p-phenylenediamine (abbreviated to 6PPD) as a protection agent (in a volume of 2 ml at a concentration of 10 g/1 in cyclohexane, or amass of 0.02 g). Then the polyisoprene is extracted from each polymer solution thus obtained by steam stripping for 30 min., in the presence of calcium tamolate (using 2 ml of tamol and 50 ml of CaCl2 at 30 g/1). Then each extracted solution is dried for approximately 18 hours in an oven at 50°C under a vacuum (at a pressure of 200 mm Hg), under a gentle stream of nitrogen for approximately 72 hours. b) Results obtained: The conversion rate of isoprene into polyisoprene as a function of the reaction time is measured to describe the polymerisation kinetics. The inherent viscosity ηinh at 0.1 g/dl in toluene and the Mooney viscosity ML(l+4) (measured in accordance with ASTM Standard D-1646) characterise the macrostructure of each polyisoprene obtained. To determine the microstructure of the polyisoprenes, the technique of13C-NMR (absolute method of great accuracy) and MIR (medium-wave infrared) analysis were used, as specified in the attached Appendix 1. These techniques have made it possible to establish, to within an inaccuracy of 0.1%, the content of cis-1,4 and 3,4 linkages ( C-NMR analysis having demonstrated the absence of trans-1,4 or 1,2 linkages). It will be noted that the second technique MIR is of great accuracy in determining the content of 3,4 units, owing to the fact that it uses the samples of polyisoprene having previously been calibrated for the 13C-NMR analysis. Table 1 hereafter sets forth the operating conditions followed for each polymerisation and the macrostructural and microstructural characteristics of each polyisoprene obtained. TABLE 1: (Table Removed) This Table 1 shows that the catalytic system 1 according to the invention makes it possible to obtain with satisfactory activity, at a polymerisation temperature kept at -15°C, polyisoprenes having a content of cis in a range from 99.0% to 99.2%, whatever the quantity of catalytic base used and whatever the conversion rate achieved for a given quantity of catalytic base. As far as the macrostructure of the polyisoprenes obtained is concerned, these results show that stirring has no influence on the final product. Although the reaction rates are relatively low, the activity of the catalytic system 1 is not adversely affected and makes it possible to achieve 100% conversion, even for final polyisoprenes having a high viscosity. 2) Examples of polymerisation of isoprene at a temperature of -25°C by means of the aforementioned catalytic systems 2, 3.4 and 5: a) Polymerisation process used: A 750 ml "Steinie" bottle was used as the polymerisation reactor, and all the polymerisation reactions of the isoprene were implemented statically in a freezer at -25°C (bottle in a bath of glycol). The quality of the isoprene is as described in paragraph 1) above. The conversion rate is 100% in all cases (for at least 64 hours of reaction time). Each polymerisation was effected as indicated in paragraph 1), except for the fact that a plurality of polymerisation solvents and of solvent/isoprene monomer (S/M hereafter) mass ratios were tested, for the tests carried out. Owing to the viscosity of the polyisoprene solutions obtained, solvent was added thereto before adding the stoppage and protection agents mentioned in paragraph 1). Then the polyisoprene was extracted from each polyisoprene solution thus "fluidified", then each extracted solution was dried, all in the manner described in paragraph 1). b) Results obtained: Table 2 hereafter shows the results obtained (refer to paragraph 1) above for the measurement conditions). TABLE 2: (Table Removed) These results show that the catalytic systems 2 to 5 according to the invention make it possible to obtain with satisfactory activity, at a temperature kept to -25°C, polyisoprenes having contents of cis-1,4 linkages which are on average equal to 99.4%. Insofar as this polymerisation temperature remains constant, the presence or absence of solvent, the nature of this solvent (aliphatic or alicyclic) and the quantity of solvent have no effect on the content of cis-1,4 linkages. As far as the catalytic systems used are concerned, it will be noted that these contents of cis-1,4 linkages are independent of the molar ratios DiBAH/Nd and DEAC/Nd. As far as the macrostructure characteristics of the polyisoprenes obtained are concerned (measurements taken by means of the SEC technique, see attached appendix 2), the polyisoprene obtained for test G (catalytic system 2) with a quantity of Nd of 500µMcm has: - a number-average molecular weight Mn of 338,475 g/mol, and - a polymolecularity index Ip of 2.389. As for the polyisoprene obtained for test K (catalytic system 3), it has: - a number-average molecular weight Mn of 423,472 g/mol, and - a polymolecularity index Ip of 2.483. 3) Examples of polymerisation of isoprene at a temperature of-45°C by means of the aforementioned catalytic system 3: a) Polymerisation process used: The same polymerisation conditions as those set forth in paragraph 2) above were used, except for the fact that the polymerisation temperature was kept at a value of -45°C (instead of-25°C). b) Results obtained: Table 3 hereafter shows the results obtained (reference will be made to paragraph 1) above for the measurement conditions). TABLE 3: (Table Removed) These results show that the catalytic system 3 according to the invention has a sufficient activity to polymerise isoprene at a temperature kept at the constant value of -45°C, despite the reduced reaction rate which it provides at this very low temperature. It will be noted that the polyisoprenes thus obtained each have an content of cis-1,4 linkages of 99.6%, which is an extremely high amount. 4) Examples of polymerisation of isoprene at a temperature of 0°C by means of the aforementioned catalytic systems 5 and 6: a) Polymerisation process used: The same polymerisation conditions as those set forth in paragraph 1) above (250 ml "Steinie" bottle with 10 g isoprene per bottle) were used, except for the fact that the polymerisation temperature was kept at a value of 0°C and that the polymerisation was implemented with stirring in a tank of glycol. b) Results obtained: Table 4 hereafter shows the results obtained (reference will be made to paragraph 1) for the measurement conditions). TABLE 4: (Table Removed) These results show that the catalytic systems 5 and 6 according to the invention make it possible to obtain with satisfactory activity, at a temperature kept to 0°C, polyisoprenes having contents of cis-1,4 linkages which lie within a range from 99.0% to 99.1%. For a ratio of polymerisation solvent/ monomer (cyclohexane/ isoprene) equal to 9 (namely 10% concentration), it will be noted that the polyisoprenes obtained with the catalytic system 5 according to the invention have after 18 hours (100% conversion) a high, reproducible Mooney viscosity of approximately 85. As far as the macro structure characteristics of the polyisoprenes obtained are concerned (measurements taken by means of the SEC technique, see attached appendix 2), the polyisoprene obtained for test N (catalytic system 6) has: - a number-average molecular weight Mn of 930,299 g/mol, and - a polymolecularity index Ip of 2.46. APPENDIX 1; Determination of the microstructure of the polyisoprenes. 1) By the technique of carbon-13 nuclear magnetic resonance (13C NMR analysis): a) Preparation of the samples: 2 g of polyisoprene is extracted with acetone at reflux for 8 hours. The extracted polyisoprene is then dried at ambient temperature under vacuum for 24 hours. This dried polyisoprene is then redissolved in chloroform. The polyisoprene solution is filtered and the solvent is eliminated in a rotary evaporator for 4 hours (the temperature of the bath is 40°C). For the purposes of the analysis, approximately 600 mg of the polyisoprene thus prepared is solubilised in CDCI3 (2 ml), directly in a 13C-NMR tube. b) Characteristics of the apparatus: - Spectrophotometer sold under the name "BRUKER AM250". - Resonance frequency (SFO) = 62.9 MHz. - Pulse programme: INVGATE.AU (suppression of the "NOE" effect for quantitative NMR analysis of 13C). - Duration of pulse: 9µs (90°). - Duration of relaxation: 10 s. - Number of accumulated transients (NS) = 8192. c) Assignment of the peaks of the spectrum: The peaks were identified in accordance with: Quang Tho Pham, R. Petiaud, H. Waton, M.F. Llauro Darricades, "Proton and NMR Spectra of Polymers", 1991, Penton Press. d) Integration method: - No 1,2 structural units detected. - The ratio between the contents of 3,4 and of 1,4 is determined by means of the ethylenic carbons. The content of trans-1,4 and cis-1,4 linkages in the polyisoprene is calculated from the aliphatic carbons. 2) By the technique of medium-wave infrared radiation analysis (MIR): a) Preparation of the samples: For this infrared analysis, the polyisoprene as prepared in paragraph 1) above is used, [while] for the NMR the sample is extracted with acetone then is dried in an oven. A solution of the polyisoprene of exactly 10 g/1 in CCI4 is analysed using a KBr cell of a thickness of 0.2 mm. b) Apparatus: - Spectrophotometer sold under the name "BRUKER IFS88". - Recording conditions: beam aperture: maximum; resolution: 2 cm-1; speed of the moving mirror: 0.639 cm.s-1; detector: DTGS; accumulations: 64 scans; purge time: 3 mn; spectral window: 4000 to 400 cm-1; transmission spectra recorded; reference: CCI4 solvent. - Processing of the spectra: transfer to microcomputer; processing with "OPUS" software from "BRUKER". c) Assignment of the peaks of the spectrum: Spectral studies and the contents of the following documents made it possible to determine the characteristic bands of the different linkage modes: - Y. Tanaka, Y. Takeuchi, M. Kobayashi, H. Tadokoro, Journal of Polymer Science, Part A-2, 1971, 9(1), 43-57. - J.P. Kistel, G. Friedman, B. Kaempf, Bulletin de la Societe Chimique de France, 1967, No. 12. - F. Asssioma, J. Marchal, C. R. Acad. Sc. Paris, Ser C, 1968, 266(22), 1563-6 mdSerD, 1968, 266(6), 369-72. - T.F. Banigan, A.J. Verbiscar, T.A. Oda, Rubber Chemistry and technology, 1982, 55(2), 407-15. The 3-4 conformation has two characteristic bands: - a high intensity band at 880 cm"1 corresponding to the out-of-plane deformation vibrations (δ C-H) of the terminal hydrogens of the vinyl group (=CH2). - a band at 3070 cm-1 corresponding to the v C-H stretching of this same group (=CH2). The 1-4 cis conformation has a characteristic band around 3030 cm-1. This band corresponds to the v C-H stretching vibrations of the =CH group. The band corresponding to the symmetrical deformation vibrations of the methyl groups δ CH3) is a complex band which incorporates all three configurations. The absorption corresponding to the δ CH3 of the trans-1,4 conformation is at its maximum around 1385 cm-1 ; this is a shoulder of the band. d) Integration method: The bands of the cis-3,4 and 1,4 are integrated by the tangential area method. The absorption maximum of the trans-1,4 is located on the shoulder of the intenseδ CH3 band. The most suitable method in this case is to measure the height of band using the tangent of the δ CH3 band as baseline. e) Calibration curves: Statement of the Beer-Lambert law: Do(v or δ ) = ε(v or δ ) e c where: Do(v or δ ) = optical density of the band v or 8 ; ε(v or δ) = molar extinction coefficient of the analyte responsible for the band v or 8 ; c = molar concentration of the analyte; and e = thickness of the sample. Commercial polyisoprenes (sold under the names "IR305", "NATSYN 2200" and "SKI-3S"), a polyisoprene synthesised in the laboratory (MC78) and natural rubber (NR) are used as standards. Compared at isoconcentration (solutions), the law may therefore be written: Dx = KX with: Dx = integration value of the band corresponding to the structural unit X, X = amount of structural unit X in the rubber (determined by 13C NMR), and K = calibration constant. The calibration curves Dx = f(X) may therefore be traced for each of the structural units. APPENDIX 2: Determination of the distribution of the molecular weights of the elastomers obtained by the technique of size exclusion chromatography (SEC). a) Principle of the measurement: Size exclusion chromatography or SEC makes it possible physically to separate macromolecules according to their size in the swollen state in columns filled with porous stationary phase. The macromolecules are separated by their hydrodynamic volume, the bulkiest being eluted first. Although not an absolute method, SEC does enable an assessment to be made of the molecular weight distribution of a polymer. On the basis of commercially available standards, the various number-average (Mn) and weight-average molecular (Mw) weights can be determined and the polydispersity index calculated (Ip = Mw/Mn). b) Preparation of the polymer: There is no particular treatment of the sample of polymer before analysis. It is simply solubilised in tetrahydrofuran, at a concentration of approx. lg/1. c) SEC analysis: The apparatus used is a "WATERS model 150C" chromatograph. The elution solvent is tetrahydrofuran, the flow rate is 0.7 ml/min, the temperature of the system is 35°C and the duration of analysis is 90 min. A set of four columns in series is used, of the trade names "SHODEX KS807", "WATERS type STYRAGEL HMW7" and two "WATERS STYRAGEL HMW6E". The volume of the solution of polymer sample injected is 100 µl. The detector is a "WATERS model RI32X" differential refractometer, and the software for processing the chromatographic data is the system "WATERS MILLENNIUM" (version 3.00). WE CLAIM: 1. A process for the preparation of a synthetic polyisoprene having a content of cis-1,4 linkages which is strictly greater than 99%, said process consisting essentially of reacting a catalytic system in the presence of isoprene, wherein it consists of: • using, as catalytic system, a system based on at least: a conjugated diene monomer, an organic phosphoric acid salt of one or more rare earth metals, - an alkylating agent consisting of an alkylaluminium of the formula AIR3 or HAIR2, and - a halogen donor consisting of an alkylaluminium halide, said salt being in suspension in at least one inert saturated hydrocarbon solvent of aliphatic or alicyclic type which is included in said catalytic system, and characterized in that the molar ratio (alkylating agent/ rare earth salt) being in a range from 1 to 5, and • implementing the polymerisation reaction of the isoprene at a temperature less than 0°C in an inert hydrocarbon polymerisation solvent or without a solvent, so that said polyisoprene has a content of cis-1,4 linkages, measured using the technique of carbon-13 nuclear magnetic resonance and using the technique of medium-wave infrared radiation analysis, which is strictly greater than 99.0%. 2. A process as claimed in claim 1 for the preparation of a synthetic polyisoprene, wherein it consists of carrying out said polymerisation reaction at a temperature from -55°C to -20°C, so that said polyisoprene has a content of cis-1,4 linkages, measured using the technique of carbon-13 nuclear magnetic resonance and using the technique of medium-wave infrared radiation analysis, which is equal to or greater than 99.3%. 3. A process as claimed in claim 1 for the preparation of a synthetic polyisoprene, wherein it consists of carrying out said polymerisation reaction at a temperature from -55°C to -40°C, so that said polyisoprene has a content of cis-1,4 linkages, measured using the technique of carbon-13 nuclear magnetic resonance and using the technique of medium-wave infrared radiation analysis, which is equal to or greater than 99.5%. 4. A process for the preparation of a synthetic polyisoprene as claimed in claims 1 to 3, wherein it consists of using, as catalytic system, a system which is such that said rare earth salt is a rare earth tris[bis(2-ethylhexyl)phosphate]. 5. A process for the preparation of a synthetic polyisoprene as claimed in claim 4, wherein it consists of using, as catalytic system, a system which is such that said rare earth salt is neodymium tris[bis(2-ethylhexyl)phosphate]. 6. A process for the preparation of a synthetic polyisoprene as claimed in claims 1 to 5, wherein it consists of using, as catalytic system, a system comprising said rare earth metals in a concentration equal or equal to 0.02 mol/1. 7. A process for the preparation of a synthetic polyisoprene as claimed in claims 1 to 6, wherein it consists of using, as catalytic system, a system which is such that the molar ratio (halogen donor/salt) lies within a range from 2.6 to 3. 8. A process for the preparation of a synthetic polyisoprene as claimed in claims 1 to 7, wherein it consists of using, as catalytic system, a system which is such that the molar ratio (conjugated diene monomer/ salt) lies within a range from 25 to 50. 9. A process for the preparation of a synthetic polyisoprene as claimed in claims 1 to 8, wherein it consists of using, as catalytic system, a system which is such that said conjugated diene monomer is butadiene. 10. A process for the preparation of a synthetic polyisoprene as claimed in claims 1 to 9, wherein it consists of using, as catalytic system, a system which is such that said alkylating agent is diisobutylaluminium hydride. 11. A. process for the preparation of a synthetic polyisoprene as claimed in claims 1 to 10, wherein it consists of using, as catalytic system, a system which is such that said halogen donor is diethylaluminium chl oride

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