Title of Invention

"A PROCESS FOR THE PREPARATION OF DIENE ELASTOMERS"

Abstract

The invention concerns a catalytic system used for preparing by polymerisation diene elastomers comprising polyisoprenes and polybutadienes, a method for preparing said catalytic system and a preparation method, using said catalytic system, of said diene elastomers comprising polyisoprenes with high cis-1,4 chaining rate and polybutadienes. The inventive catalytic system is at least based on: a conjugate diene monomer, a salt of one or several rare earth metals of an organic phosphoric acid, an alkylaluminium alkylating agent of formula AlR¿3? or HAIR¿2?, and a halogen donor consisting of an alkylaluminium halide, and such that the salt is suspended in at least an inert and saturated hydrocarbon solvent of the aliphatic or alicyclic type,...

Full Text

Catalytic system and process for the preparation of elastomers by means of this system. The present invention relates to a catalytic system usable for the preparation of diene elastomers comprising polyisoprenes and polybutadienes by polymerisation, to a process for the preparation of said catalytic system and to a process for the preparation, by means of this catalytic system, of such diene elastomers comprising polyisoprenes with high cis-1,4 linkage contents and polybutadienes. As is known, polyisoprenes having a high cis-1,4 linkage content may be prepared using catalytic systems based on: - a rare earth salt in solution in a hydrocarbon solvent, - an alkylating agent of this salt consisting of an alkylaluminium and - an alkylaluminium halide. It is known, for example from the document "Report of the Academy of Sciences of the USSR, volume 234, no. 5, 1977 (Y.B. Monakov, Y.R. Bieshev, A.A. Berg, S.R. Rafikov), to polymerise isoprene using a catalytic system comprising: - a bis(2-ethylhexyl)phosphoric acid salt of neodymium or praseodymium, as the rare earth salt, in solution in toluene, - triisobutylaluminium as the alkylating agent, in an "alkylating agentrare earth salt" molar ratio of 20, and - diethylaluminium chloride as the alkylaluminium halide. Mention may also be made of the document, "Proceedings of China - U. S. Bilateral Symposium on Polymer ChemistryVid Physics, Science Press, pp. 382-398, 1981 (O. Jun, W. Fosong, S. Zhiquan)". This document teaches 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 In its examples of embodiment, US patent specification US-A-3 794 604 describes a catalytic system of the type which has been "preformed" in the presence of a conjugated diene monomer and comprising: - butadiene or isoprene as the conjugated diene monomer, - cerium octanoate as the rare earth salt in solution in benzene, - diisobutylaluminium hydride as the alkylating agent in an "alkylating agentrare earth salt" molar ratio substantially equal to 20 and - ethylaluminium dichloride as the alkylaluminium halide. It will be noted that the only polymerisation examples stated in this US document relate to the polymerisation of butadiene. Japanese patent specification JP-A-60/23406 also describes a catalytic system of the type which has been "preformed" in the presence of butadiene, the system specifically being intended for the polymerisation of butadiene. The catalytic systems tested in the examples of embodiment of said document comprise: - a bis(2-ethylhexyl)phosphoric acid salt of neodymium as the rare earth salt in solution in n-hexane or cyclohexane, -'triisobutyl aluminium or diisobutylaluminium hydride as the alkylating agent in an "alkylating agentrare earth salt" molar ratio ranging from 10 to 30, and - ethylaluminium sesquichloride as the alkylaluminium halide. It will be noted that none of the polybutadienes obtained by means of these catalytic systems simultaneously exhibits a Mooney viscosity ML(l+4) at 100°C greater than or equal to 40 and a polydispersity index of below 2.5. As a result, these polybutadienes are not suitable for use in a tyre tread. Another major drawback of these known catalytic systems is that they exhibit differing levels of activity for the polymerisation of the various conjugated dienes, in particular for the homopolymerisation of isoprene and that of butadiene. Another drawback is the non-reproducible nature of the macrostructural and microstructural properties exhibited by the polymers obtained by means of these catalytic systems, in particular with regard to the content of cis-1,4 linkages, which may vary significantly. 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 and aliphatic or alicyclic hydrocarbon solvent, - an alkylating agent consisting of an alkylaluminium of formula AIRs or HA1R2, the "alkylating agent:rare earth salt" molar ratio ranging from 1 to 5, and - a halogen donor consisting of an alkylaluminium halide, makes it possible to overcome the above-mentioned drawbacks by exhibiting elevated activity for obtaining diene elastomers, such as polyisoprenes and polybutadienes, and in particular polyisoprenes which exhibit, on the one hand, a wide range of viscosities and, on the other, high and reproducible cis-1,4 linkage contents substantially ranging from 98.0% to 98.5%. Of course, the phrase "based on" used to define the constituents of the catalytic system is taken to mean the mixture and/or the reaction product of these constituents. Preferably, said "alkylating agentrare earth salt" molar ratio ranges from 1 to 2. The corresponding catalytic system of the invention in particular makes it possible to obtain polyisoprenes exhibiting the above-stated properties at a very high level of catalytic activity. It will be noted that the catalytic systems of the invention are characterised by an "alkylating agentrare earth salt" molar ratio which is very low in comparison with the molar ratios greater than or equal to 10 or 20 which have hitherto been tested under practical conditions, which surprisingly permits a significant increase in the activity of these catalytic systems catalytic systems for the production of polyisoprenes. 1,3-Butadiene may be mentioned as a preferred conjugated diene monomer usable for "preforming" the catalytic system of the invention. Other conjugated dienes which may be mentioned are 2-methyl-1,3-butadiene (or isoprene), 2,3-di(Cl to C5 alkyl)-l,3-butadienes such as, for instance, 2,3-dimethyl-1,3-butadiene, 2,3-diethyl-1,3-butadiene, 2-methyl-3-ethyl-1,3-butadiene, phenyl-l,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 "monomenrare earth salt" molar ratio may have a value ranging 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 suspended is a low molecular weight aliphatic or alicyclic solvent, 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 a high molecular weight aliphatic solvent comprising a paraffmic oil, for example petrolatum oil, and a low molecular weight solvent, such as those mentioned above (for example cyclohexane or methylcyclohexane). This suspension is prepared by dispersive grinding of the rare earth salt in this paraffinic oil in such a manner as to obtain a very fine and homogeneous suspension of the salt. According to another characteristic of the invention, said catalytic system comprises the rare earth metal in a concentration equal to or substantially equal to 0.02 mol/1. According to another characteristic of the invention, said catalytic system is such that said rare earth salt has a mass content of rare earth metal ranging from 12.0% to 13.5%, determined both by complexometric back titration with ethylenediariiinetetraacetic acid (abbreviated to EDTA) and by inductively-coupled plasma atomic emission spectrometry (abbreviated to ICP-AES). The catalytic systems of the invention which are characterised by these rare earth metal contents for said salt advantageously make it possible to obtain polybutadienes which simultaneously exhibit a Mooney viscosity ML(l+4) at 100°C, measured in accordance with Standard ASTM D 1646, which is greater than or equal to 40, and a polydispersity index, measured by size exclusion chromatography (SEC), which is less than 2.5, these combined characteristics making these polybutadienes particularly suitable for use in tyre treads. Said rare earth salt preferably exhibits a mass content of rare earth metal ranging from 12.5% to 13.2%. Advantageously, the polybutadienes obtained simultaneously exhibit a Mooney viscosity ML(l+4) at 100°C of greater than 40 and a polydispersity index of less than 2.0. According to a preferred embodiment of the invention, a tris[bis(2-ethylhexyl)phosphate] salt of the said rare earth metal or metals is used as the salt. Even more preferably, said rare earth salt is neodymium tris[bis(2-ethylhexyl)phosphate]. Alkylating agents usable in the catalytic system of the invention which may be mentioned are alkylaluminiums such as: - trialkylaluminiums, for example triisobutylaluminium, or - dialkylaluminium hydrides, for example diisobutylaluminium hydride. It will be noted that this alkylating agent preferably consists of diisobutylaluminium hydride Halogen donors usable in the catalytic system of the invention which may be mentioned are alkylaluminium halides, preferably diethylaluminium chloride. It will be noted that the "halogen dononrare earth salt" molar ratio may have a value ranging from 2.2 to 3 and, preferably, from 2.6 to 3. According to the invention, the process for the preparation of said catalytic system consists: - in a first stage, of preparing 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 process for the preparation according to the invention of diene elastomers consists in reacting said catalytic system in an inert hydrocarbon solvent and in the presence of the monomer or monomers to be polymerised, to obtain a diene elastomer which may be any homopolymer or copolymer obtained by homopolymerisation or copolymerisation of a conjugated diene monomer having 4 to 12 carbon atoms. Suitable conjugated diene monomers are, in particular, 1,3-butadiene, isoprene, 2,3-di(Cl to C5 alkyl)-1,3-butadienes such as, for instance, 2,3-dimethyl-l,3-butadiene, 2,3 -diethyl-1,3 -butadiene, 2-methyl-3 -ethyl-1,3 -butadiene, 2 -methyl-3 -isopropyl-1,3 -butadiene, an aryl-1,3-butadiene, 1,3-pentadiene and 2,4-hexadiene. The diene elastomer obtained by the polymerisation process of the invention is characterised by a high cis-1,4 linkage content and may, for example, consist of a polyisoprene (IR) or a polybutadiene (BR). Advantageously, the process according to the invention makes it possible to obtain with elevated activity, when polymerisation is carried out at a temperature ranging from 25°C to 55°C, polyisoprenes exhibiting cis-1,4 linkage contents, measured both by carbon 13 nuclear magnetic resonance and by infrared analysis, which fall within a range from 98.0% to 98.5% (disregarding a measurement inaccuracy of ± 0.1% which is inherent to each of these two methods). Furthermore, this process according to the invention also makes it possible to obtain with elevated activity, when polymerisation is carried out at a temperature ranging from 25°C to 100°C, polybutadienes which likewise exhibit high cis-1,4 linkage contents together with an inherent viscosity, measured at a concentration of 0.1 g/dl in toluene, which is greater than 2 dl/g (this inherent viscosity being measured in accordance with Standard ASTM D 1646). Advantageously said "alkylating agentrare earth salt" molar ratio exhibits a value ranging from 1 to 2, in order to obtain polybutadienes with improved catalytic activity which exlibit cis-1,4 linkage contents, measured by near infrared (NIR) analysis, which may be between 98.0% and 99.0%. Reference will be made to the attached appendix 1 for a description of this NIR method. The aforementioned features 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, said description being made in conjunction with the attached drawings, in which: Fig. 1 is a graph illustrating the influence of the "alkylating agentrare earth salt" molar ratio on the activity of 8 catalytic systems of the invention for the preparation of polyisoprenes exhibiting an identical inherent viscosity of approximately 4 dl/g, and Fig. 2 is a graph illustrating the influence of the "alkylating agentrrare earth salt" molar ratio on the activity of 6 catalytic systems of the invention for the preparation of polyisoprenes exhibiting an inherent viscosity of between 2.6 and 2.8 dl/g. I. PREPARATION OF CATALYTIC SYSTEMS OF THE INVENTION; > 1) Synthesis of an organic phosphate salt of neodymium of the invention; A plurality of tests were carried out for synthesis of this salt. The same synthesis method, which is described in detail below, was used for each of these tests. a) Synthesis of an aqueous solution of neodymium NdCk. 6H?O: 96 g of Nd2O3 (sold by RHODIA), which has been determined by complexation analysis to have an Nd content of 85.3% (theoretical value 85.7%), so amounting to 0.57 mol of Nd, are weighed out into a "tall" form 600 ml beaker. 80 ml of demineralised water are added. Under a fume hood, 150 ml of 36 wt.% concentrated HC1 (d = 1.18), namely 1.75 mol of HC1 (molar ratio HCl:Nd = 1.75:0.57 = 3.07), are slowly added at ambient temperature while the mixture is stirred with a magnetic stirrer. The reaction Nd2O3 +6 HC1 +9 H2O -» 2 NdCl3, 6H2O is highly exothermic. Once all the hydrochloric acid has been added, the solution is raised to boiling while being stirred with a magnetic stirrer The aqueous NdCl3 solution is clear and mauve in colour. No insoluble product (Nd2O3) remains. This solution is then evaporated until a volume of 130 ml remains in the beaker. The NdCl3, 6H2O is then highly concentrated (it crystallises at ambient temperature). The concentrated solution of NdQ3 is then poured into a 10 litre drum containing 4500 ml of demineralised water at ambient temperature while the mixture is stirred (using a motor with an anchor agitator). The pH of the solution, measured at 25°C, is close to 4. 150 ml of technical grade acetone are then added to the solution. No insoluble product remains and the resultant solution is pink in colour. b) Synthesis of an organic sodium phosphate of formula [RO]iP(0)ONa TR = 2-ethylhexyl): 68 g, or 1.70 mol, of NaOH flakes are 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 number 23,782-5), namely 1.72 mol of this acid, are 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. At ambient temperature and while stirring the mixture by hand with a glass stirrer, the solution of said organic phosphoric acid is poured into the NaOH solution. The reaction is as follows: [RO]2P(O)OH + NaOH ->• [RO]2P(O)ONa + H2O. The reaction is slightly exothermic and a homogeneous 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 the formula - At ambient temperature and while the mixture is being vigorously stirred (motor with anchor agitator), the organic Na phosphate salt obtained in paragraph b) above is poured into the aqueous solution of NdCl3,6H2O obtained in paragraph a) above. A very fine white precipitate forms immediately. Stirring of the resultant mixture is continued for 30 minutes once all the organic Na phosphate has been added (in a molar ratio (RO)2P(O)ONa:NdCl3 = 1.70:0.57 = 2.98). The reaction is as follows: 3 [RO]2P(O)ONa + NdCl3,6H2O -> Nd[OP(O)[OR]2]3 + 3 NaCl + 6 H2O. - The resultant phosphated neodymium salt is recovered and washed in a centrifuge quipped with a "sock". The pH of the mother liquors is between 3 and 4 at 25°C. These mother liquors are olourless and clear.The salt obtained is divided into two samples, then each sample is washed with an acetone/demineralised water mixture, performing the washing cycle described below three times in order to remove 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 using an "Ultra-Turrax" homogeniser for apprbx. 1 minute in order to obtain a milky solution. 4 litres of demineralised water are then added to the bucket and the resultant mixture is homogenised for 3 minutes using the same homogeniser. The resultant mixture is centrifuged and the phosphated neodymium salt is recovered in the "sock". The qualitative analytic test for chlorides is virtually negative for the final washing water (the reaction is as follows: NaCl + AgNOs (HNOa medium) —> AgCl -I + NaNOa). The neodymium salt washed in this manner is dried in an oven under a vacuum and with air-flow for approx. 80 hours. The final yield for each of the synthesis tests performed is between 95% and 98%, depending upon the losses arising during washing, hi each case, approx. 600 g of dry phosphated neodymium salt are obtained. The mass contents of neodymium, determined both by complexometric back titration with ethylenediaminetetraacetic acid (EDTA) and by inductively-coupled plasma atomic emission spectrometry (abbreviated to ICP-AES), are substantially between 12.5% and 12.8% (with a theoretical content T of 13.01% where T - [144.24 / 1108.50] x 100, where 144.24 g/mol = molar mass of neodymium). For each of these two methods, the neodymium content measurements were performed after wet acid mineralisation of the salt, either in a sand bath in an open system or in a microwave oven in a closed system. The complexometnc Dacic titration with EDTA involves back titration with complexation of neodymium with an excess of EDTA (ethylenediaminetetraacetic acid), in which the excess EDTA is determined at pH = 4.6 with zinc sulphate. A coloured indicator was used with photometric detection of the equivalence point. Inductively-coupled plasma atomic emission spectrometry is an elemental analytical method based on the observation of the radiation emitted by atoms raised to an excited state in a plasma. The emitted radiation used for analysis of neodymium corresponds to wavelengths of 406.109 nm and 401.225 ran. This spectrometric method was implemented by previously calibrating the system with "control" neodymium salts having a known neodymium content. The following table shows the Nd contents obtained by means of these two methods (the number of tests performed on each salt sample is shown in brackets). (Table Removed) The results obtained by the two methods are comparable (relative deviation 2) Synthesis of preformed catalytic systems; "control" and according to the invention; a) Composition of "control" catalytic systems t and t'. "Control" catalytic system t is based on: - butadiene as the conjugated diene monomer - neodymium octoate as the rare earth salt, - diisobutylaluminium hydride (hereafter DiBAH) as the alkylating agent, and - diethylaluminium chloride (hereafter DEAC) as the halogen donor. This catalytic system t is characterised by the following relative molar ratios, with respect to the neodymium salt: Nd octoate:butadiene:DiBAH:DEAC = 1:30:1.8:2.6. "Control" catalytic system t1 differs from said catalytic system t solely in that it comprises neodymium acetylacetonate instead of neodymium octoate as the rare earth salts (the relative molar ratios with respect to the neodymium salt being identical). In these two catalytic systems t and t', the neodymium salt is suspended in a low molecular weight hydrocarbon solvent consisting of methylcyclohexane. b) Composition of catalytic systems 1 to 29 according to the invention: Each of these systems 1 to 29 comprises a phosphated neodymium salt as synthesised according to paragraph 1) above. - A first series of catalytic systems according to the invention (hereafter systems 1 to 13 and 27 to 29) is such that the phosphated neodymium salt is suspended in a low molecular weight hydrocarbon solvent (consisting of methyl cyclohexane, n-heptane or cyclohexane). - A second series of catalytic systems according to the invention (hereafter systems 14 to 26) is such that the phosphated neodymium salt is suspended in a mixture of two inert hydrocarbon solvents, one of low molecular weight and the other of high molecular weight (mixture of "Prolabo" petrolatum oil, "Rectapur" grade, and cyclohexane or methylcyclohexane). Suspension in the mixture of these two solvents is achieved by, in a first phase, carrying out dispersive grinding of the phosphated neodymium salt for 1 minute in said oil by means of an "Ultra-Turrax" homogeniser in order to obtain a very fine, homogeneous and relatively stable suspension (it being several days before the onset of any settling of the solid could be observed). Sampling of the mixture is thus facilitated. In a second stage, the cyclohexane or methylcyclohexane is added to the suspension obtained, so substantially raising viscosity and possibly resulting in the formation of a gel of greater or lesser fluidity. Catalytic systems 1 to 29 according to the invention are characterised by the following relative molar ratios, with respect to the neodymium salt: Octoate:butadiene:DiBAH:DEAC = 1:25-5:1.3-4.5:2.6 or 3. c) Common synthesis method for "control" catalytic systems and those according to the invention: - First stage: With the aim of obtaining catalytic systems t, t', 1 to 13 and 27 to 29, 15.6 g of the neodymium salt in powder form are poured into a 1 litre reactor from which the impurities have previously been removed. This salt is then subjected to nitrogen bubbling from the bottom of the reactor for a period of 15 minutes. With the aim of obtaining catalytic systems 14 to 26, the phosphated neodymium salt is suspended as described above in the petrolatum oil in a mass fraction of 10% for said salt. This suspension of the salt is then subjected to nitrogen bubbling for a period of 5 minutes. The resultant suspension is then poured into a reactor identical to the previous one, from which the impurities have been improved and which has been placed under a nitrogen atmosphere. - Second stage: 90% (mass fraction) of the solvent mentioned in paragraphs a) and b) above are introduced into the reactor containing the neodymium salt, this solvent being methylcyclohexane for synthesis of catalytic systems t and t1, methylcyclohexane or n-heptane for catalytic systems 1 to 13 and 29, and cyclohexane or methylcyclohexane for catalytic systems 14 to 28. The period for which and temperature at which this solvent and the neodymium salt are brought into contact are respectively 30 minutes and 30°C for catalytic systems t, t1, 1 to 13 and 29, 4 hours and 60°C for catalytic systems 14 to 27 and 2 hours and 60°C for catalytic system 28. - Third stage: Butadiene is then introduced into the reactor (in the molar ratios already stated in paragraphs a) and b) above) at a temperature of 30°C with the aim of "preforming" each catalytic system. - Fourth stage: Diisobutylaluminium hydride (DiBAH) is then introduced into the reactor as the alkylating agent for the neodymium salt in a concentration of approx. 1 M, together with a quantity of the solvent already stated in the second stage corresponding to a mass fraction of 5% of the entire quantity of said solvent. The alkylation time is 15 minutes for catalytic systems t, f, 1 to 9, 14 to 28 and 30 minutes for the other catalytic systems 10 to 13 and 29 (see paragraph II below). The temperature of the alkylation reaction is 30°C. - Fifth stage: Diethylaluminium chloride (DEAC) is then introduced into the reactor as the halogen donor in a concentration of approx. 1 M, together with a quantity of the solvent already stated in the second stage corresponding to a remaining mass fraction of 5% of the entire quantity of said solvent. The temperature of the reaction medium is adjusted to 60°C. - Sixth stage: The resultant mixture is then "preformed" (or aged) by maintaining this temperature of 60°C for a period of 120 minutes, with the exception of catalytic system 11 where the temperature is maintained for only 60 minutes (see paragraph II). - Seventh stage: In this manner, approx. 700 ml of a solution of catalytic system t, t' or 1 to 29 are obtained. The reactor is emptied and the contents transferred into a 750 ml "Steinie" bottle, which has previously 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. II. Polymerisation of isoprene by means of catalytic systems t, t' and 1 to 16: 1) Operating method for the various polymerisations: A 250 ml "Steinie" bottle was used as the polymerisation reactor (except for the second and third polymerisation examples using catalytic system 5, where a 750 ml "Steinie" bottle was used, see table below). Each polymerisation reaction was carried out by subjecting this bottle to agitation in a water tank. A steam-cracked C5 naphtha fraction was used with the aim of extracting isoprene therefrom with a purity approaching 100%. To this end, a conventional laboratory purification process was used comprising the following successive steps: - distillation of this C5 fraction over maleic anhydride to eliminate any residual cyclopentadiene, followed by - passage through an alumina column to remove polar impurities, and - nitrogen bubbling for 20 minutes, immediately prior to the polymerisation reaction. The mass fraction of isoprene extracted from this C5 fraction was determined at 99.2% by gas phase chromatography (GPC). Each isoprene polymerisation reaction (10 g of isoprene is used per bottle, except for the second and third examples with catalytic system 5, where 36 g of isoprene were used) is carried out in cyclohexane at 50°C (the temperature is 30°C for said third example with catalytic system 5) and under an inert atmosphere (nitrogen). A "polymerisation solvent (cyclohexane)monomer (isoprene)" mass ratio of 9 was used (said mass ratio is hereafter denoted "S:M"). The quantity of neodymium catalyst base ranges from 90 umol to 600 umol per 100 g of isoprene, depending upon the test performed (this quantity is stated in uMcm in the summary tables below). It will be noted that this quantity of neodymium is adjusted on the basis of the "alkylating agentirare earth salt" ratio with the aim of obtaining final inherent viscosity values which are substantially identical for the polyisoprenes obtained. Tightness of the bottle is ensured by a "septum/open-top seal" assembly which thus permits addition of each catalytic system using a syringe. Acetylacetone is used in a volume of 1 ml as a polymerisation reaction shortstopping agent and N-l,3-dimethylbutyl-N'-phenyl-phenylenediamine (6PPD) as a protection agent (in a volume of 2 ml at a concentration of 10 g/1 in cyclohexane, giving a mass of 0.02 g). The polyisoprene is then extracted from the resultant polymer solution by steam stripping for 30 minutes in the presence of calcium tamolate (using 2 ml of tamol and 50 ml of CaCl2 at 30 g/1). This extracted solution is then dried for approx. 18 hours in an oven at 60°C under a vacuum (at a pressure of 200 mm of Hg), under a gentle stream of nitrogen. The conversion rate of isoprene to polyisoprene as a function of reaction time is measured to describe the polymerisation kinetics. As for inherent viscosity T}^ at 0.1 g/dl in toluene, this parameter characterises the macrostructure of each polyisoprene obtained. 2) Details of polymerisation reactions carried out by means of each catalytic system, the "controls" t»t' and 1 to 16 according to the invention; The tables below state details of: - the characteristics of each catalytic system used in terms of: • solvation conditions (solvent(s) used, contact time and temperature); CH = cyclohexane, MCH = methylcyclohexane, • "preforming monomer (in this case butadiene, abbreviated to Bd):rare earth salt (abbreviated to Nd)" molar ratio, • "alkylating agent (Al):rare earth salt (Nd)" molar ratio, • alkylating conditions (time and temperature T), • molar ratio "halogen donor (Cl):rare earth salt (Nd)", • preforming or ageing conditions (time and temperature T); - characteristics of each polymerisation reaction performed in terms of: • quantity of neodymium catalytic base used (Nd in uMcm), • S:M mass ratio (cyclohexane solventrmonomer to be polymerised) and polymerisation temperature T, • conversion rate (abbreviated to conv. rate) corresponding to determined reaction times; and - the characteristics of various of the polyisoprenes obtained in terms of: • inherent viscosity r^ and Mooney viscosity ML (1+4) at 100°C (measured in accordance with Standard ASTM: D-1646), • number-average molecular weight Mn and polydispersity index Ip, measured by size exclusion chromatography (SEC), see appendix 2, • content of cis-1,4 linkages, measured by carbon 13 nuclear magnetic resonance (13C NMR) and by mid-infrared (MIR) analysis, which methods are respectively indicated "*" and "**" in the following tables. Reference will be made to the attached appendix 1 for a description of these methods. (Table Removed) By way of indication, at a cis-1,4 linkage content of 98.1% in the polyisoprene, which was measured by I3C NMR (indicated "*" above), the same method was used to measure trans-1,4 and -3,4 linkage contents of 0.2% and 1.7% respectively (the content of 1,2 linkages being zero). 3) Conclusions: - The polymerisation examples using "control" catalytic systems t and t1 show that, in a catalytic system characterised, by an Al:Nd molar ratio of between 1 and 2, using a salt consisting of neodymium octoate or neodymium acetylacetonate instead of a rare earth salt according to the invention, such as neodymium tris[bis(2-ethylhexyl)phosphate], imparts zero activity for isoprene polymerisation to the corresponding catalytic system. - The polymerisation examples using the catalytic systems according to the invention show that the catalytic activities achieved with Al:Nd molar ratios of between 1 and 2 (see in particular catalytic systems 3 to 7 and 9 to 13) are particularly high. In fact, for all these catalytic systems, 100% isoprene conversion is achieved after approx. 60 minutes with a final inherent viscosity of, on average, between 4 and 4.5 dl/g. At a given inherent viscosity of the order of 4 dl/g for the polyisoprenes (at 100% conversion), it will be noted that catalytic activity rises when the Al:Nd molar ratio falls, as is shown by Fig. 1, which relates to the preparation of polyisoprenes by means of said catalytic systems 8 and 10 to 16. It will be noted in particular that the catalytic system 13, which is characterised by an Al:Nd molar ratio of 1.3, constitutes a preferred embodiment of the invention, in that it allows a conversion rate of 96% to be achieved in 20 minutes and a final inherent viscosity of approx. 4.4 dl/g with a polyisoprene exhibiting a cis-1,4 linkage content of 98%. - It will be noted that the reproducibility of the activity of the catalytic systems according to the invention is excellent, despite the possible variation with regard to the quality of the isoprene and solvent used in particular. It will also be noted that the polyisoprenes obtained exhibit substantially the same macrostructural and microstructural features (in particular cis-1,4 content of 98%), despite the variation in the quantity of catalytic base (Nd) and in the butadiene:Nd molar ratio. In particular, the tests performed with catalytic systems 3 to 6 show that the values of said latter ratio (ranging from 25 to 50) have no significant influence upon activity and the above-mentioned characteristics. - It will be noted that the polyisoprenes obtained by means of the catalytic systems according to the invention exhibit particularly low polydispersity indices. By way of example, the polyisoprenes obtained by means of catalytic system 5 after said tests (1), (2) and (3) exhibit polydispersity indices ranging between 2.1 and 2.3. - Catalytic systems 14 to 16, which comprise a mixture of an oil and cyclohexane as solvent, exhibit activities which are relatively close to those of catalytic systems 1 to 13, which comprise a single alicyclic solvent, such as methylcyclohexane (independently of the quantity of Nd catalytic base). - As can be seen for the polymerisations performed by means of catalytic systems 10 to 13, which are characterised by'an Al:Nd ratio of below 2, it will be noted that achieving optimum catalytic activity entails reacting the rare earth salt with the alkylating agent for a longer period (30 min. instead of 15 min. at 30°C) and reducing the Cl:Nd ratio (from a value of 3 for catalytic systems characterised by Al:Nd ratios of greater than or equal to 2, to a value of 2.6). - In the light of the results achieved by catalytic systems 10 and 11, it will be noted that preforming conditions (temperature and time) have no effect on catalytic activity, with polymerisation kinetics being substantially identical at a given Al:Nd ratio (of 1.8) for identical viscosity (it being essential to adjust the quantity of Nd). - In general, it will be noted that the cis-1,4 linkage content depends upon polymerisation temperature (see third polymerisation example using catalytic system 5, characterised by a polymerisation temperature of 30°C, which brings about cis-1,4 linkage contents of 98.4 or 98.5% in the polyisoprene obtained which are higher than those achieved at a polymerisation temperature of 50°C using this catalytic system 5). III. Polymerisation of butadiene using catalytic systems 14 to 29: 1) Operating method for the various polymerisations: A 250 ml "Steinie" bottle was used as the polymerisation reactor. Each polymerisation reaction was carried out by subjecting this bottle to agitation in a water tank. Each butadiene polymerisation reaction (10 g of butadiene are used per bottle) is performed in cyclohexane at 30°C, 50°C or 60°C under an inert atmosphere (nitrogen). A "polymerisation solvent (cyclohexane):monomer (butadiene)" mass ratio of 7 or 9 was used (said mass ratio is hereafter denoted "S:M"). The quantity of neodymium catalyst base ranges from 250 umol to 2000 umol per 100 g of butadiene, depending upon the test performed (this quantity is stated in uMcm in the summary tables below). It will be noted that this quantity of neodymium is adjusted on the basis of the "alkylating agentrare earth salt" ratio with the aim of obtaining final inherent viscosity values which are substantially identical for the polybutadienes obtained. Tightness of the bottle is ensured by a "septum/open-top seal" assembly which thus permits addition of each catalytic system using a syringe. Acetylacetone is used in a volume of 1 ml to shortstop the polymerisation reactions corresponding to catalytic systems 14 to 20 and in an acetylacetone:neodymium molar ratio of 21 to shortstop the reactions corresponding to catalytic systems 28 and 29. Methanol, for its part, is used to shortstop the polymerisation reactions corresponding to catalytic systems 21 to 27. N-ljS-dimethylbutyl-N'-phenyl-p-phenylenediamine (6PPD) is used as a protection agent (in a mass of 0.5 g) and, in the case of the polymer solution obtained with catalytic system 28, 6PPD is used in a quantity of 0.2 phr in association with 0.2 phr of a protection agent designated "AO2246". The polybutadienes are then extracted from the polymer solutions thus obtained by steam stripping in the presence of calcium tamolate following the example of the polyisoprenes prepared in paragraph II. Drying is performed for approx. 18 hours in an oven at 60°C under a vacuum (at a pressure of 200 mm of Hg), under a gentle stream of nitrogen. The conversion rate of butadiene to polybutadiene as a function of reaction time is measured to describe the polymerisation kinetics. As for inherent viscosity r\-lnh at 0.1 g/dl in toluene, this parameter characterises the macrostructure of each polybutadiene obtained. 2) Details of polymerisation reactions carried out by means of each catalytic system. 14 to 29 according to the invention; On the basis of the example of paragraph II, the following tables state details of each catalytic system used, each polymerisation reaction performed and the polybutadienes obtained in terms of: (Table Removed) The inherent viscosity values indicated (*) are "crude" viscosities, i.e. they correspond to the viscosity of the sample together with its residual oil content from the catalytic system. The inherent viscosity values indicated (**) are viscosities "without oil", i.e. they state the viscosity of the preceding sample once its oil content has been washed out by coagulation in methanol. The cis-1,4 linkage contents shown in the two tables below were determined by "near infrared" analysis (also abbreviated to "NIR" by the person skilled in the art, see appendix 1). With regard to the tests performed by means of catalytic systems 28 and 29, the Mooney viscosity ML(l+4) and polydispersity index Ip (measured by SEC) were also measured on the polybutadienes obtained. The results are as follows: (Table Removed) The results from these tests show that the catalytic systems according to the invention make it possible to produce polybutadienes which simultaneously exhibit a Mooney viscosity ML(l+4) of greater than 40 and a polydispersity index of below 2, so making them particularly suitable for tyre treads. 3) Conclusions: - At a given inherent viscosity of between 2.6 and 2.8 dl/g for the polybutadienes obtained (at substantially 100% conversion), it will be noted that catalytic activity rises when the Al:Nd molar ratio falls, as is shown by Fig. 2, which relates to the preparation of polybutadienes by means of said catalytic systems 14, 15, 16, 24, 25, 26. It may be seen in Fig. 2 that reducing the Al:Nd ratio from 4.5 to 1.7 allows a very substantial increase in catalytic activity (tests performed by means of catalytic systems 14 and 26, respectively, where the quantity of Nd was changed from 250 to 680 umol per 100 g of butadiene in order to obtain a similar viscosity of between 2.6 and 2.8 dl/g). It will be noted that catalytic system 26, which is characterised by an Al:Nd molar ratio of 1.7, constitutes a preferred embodiment of the invention, in that it allows a conversion rate of 100% to be achieved in 16.5 minutes for a polybutadiene exhibiting an inherent viscosity of approx. 2.8 dl/g and a cis-1,4 linkage content of 98.5%. - It will be noted that the reproducibility of the activity of these catalytic systems for polymerising butadiene is very satisfactory (see in particular the test carried out with catalytic systems 17 and 20, the polybutadienes obtained exhibiting substantially identical macro- structural and microstructural features). - It will also be noted that the polybutadienes obtained by means of these catalytic systems exhibit particularly low polydispersity indices (Ip substantially between 1.40 and 1.80). - The tests performed by means of catalytic systems 21, 22, 23 and 27, which comprise as the aliphatic or alicyclic solvent a mixture of a high molecular weight oil and a low molecular weight solvent, such as cyclohexane or methylcyclohexane, or alternatively only cyclohexane, show that the nature of the aliphatic or alicyclic solvent associated with the oil to form a gel with the rare earth salt has virtually no impact upon the activity of the catalytic system. Furthermore, the presence or absence of said oil in this solvent causes virtually no change in catalytic activity (see test performed with catalytic system 27). APPENDIX 1: Determination of the microstructure of the elastomers obtained. I/ For polyisoprenes; 1) By carbon 13 nuclear magnetic resonance analysis (13C NMR analysis): a) Sample preparation: 2 g of polyisoprene are extracted in refluxing acetone for 8 hours. The extracted polyisoprene is then dried at ambient temperature under a vacuum for 24 hours. This dried polyisoprene is then redissolved in chloroform. The polyisoprene solution is filtered and the solvent removed in a rotary evaporator for 4 hours (bath temperature is 40°C). For the purposes of the analysis, approx. 600 mg the of polyisoprene prepared in this manner are solubilised in CDC13 (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 program: INVGATE.AU (suppression of "NOE" effect for quantitative analysis of13CbyNMR). - Pulse duration: 9 \is (90°). - Relaxation time: 10 s. - Cumulative number of scans (NS) = 8192. a) Assignment of spectrum peaks: Peaks were identified after: 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 3,4- and 1,4- contents 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 midrinfrared (MIR) analysis: a) Sample preparation: The polyisoprene as prepared in paragraph 1) above is used for this infrared analysis, while for NMR the sample is extracted with acetone and then dried in an oven. A polyisoprene solution of exactly 10 g/1 in CCU is analysed using a KBr cell with a pathlength of 0.2 mm. b) Apparatus: - Spectrophotometer sold under the name "BRUKERIFS88". - Recording conditions: beam opening: maximum; resolution: 2 cm"1' moving mirror speed: 0.639 cm.s"1' detector: DIGS; accumulations: 64 scans; purge time: 3 min; spectral window: 4000 to 400 cm"1' transmission spectra recorded; reference: CCl^ solvent. - Spectrum processing: transfer to microcomputer; processing with "OPUS" software from "BRUKER". a) Assignment of spectrum peaks: Spectral studies and the contents of the following documents made it possible to determine the characteristic bands of the various linkage modes: - Y. Tanaka, Y. Takeuchi, M. Kobayashi, H. Tadokoro, Journal of Polymer Science, PartA-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 and Ser D, 1968, 266(6), 369-72. - T.F. Banigan, AJ. Verbiscar, T.A. Oda, Rubber Chemistry and Technology, 1982, 55(2), 407-15. The 3-4 conformation exhibits two characteristic bands: - a high intensity band at 880 cm"1 corresponding to the out-of-plane deformation vibrations (5 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 The cis-1.4 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 (8 CHs) is a complex band incorporating all three conformations. Absorption corresponding to the 5 CHs of the trans- 1.4 conformation is at its maximum around 1385 cm" *; this is a shoulder of the band. d) Integration method: The cis-3,4 and 1,4 bands are integrated by the tangential area method. The 1 ,4-trans absorption maximum is located on the shoulder of the intense 8 CHa band. The most suitable method in this case is to measure the height of the band using the tangent of the 8 CHs band as the baseline. e) Calibration curves: Statement of Beer-Lambert law: Do(v or 5) = s(v or 5) e c where: Do(v or 8) - optical density of the band v or 8 ; s(v or 8) = molar extinction coefficient of the analyte responsible for the band v or 8 ; c = molar concentration of the analyte; and e = sample thickness. Commercial polyisoprenes (sold as "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 thus be written: where: Dx = integration value of the band corresponding to structural unit X, X = content of structural unit X in the rubber (determined by 13C NMR), and K = calibration constant. Calibration curves Dx = f(X) may thus be plotted for each of the structural units. Ill For the polybutadienes; "Near infrared" (NIR) analysis was used. This is an indirect method making use of "control" elastomers whose microstructure has been measured by 13C NMR. The quantitative relationship (Beer-Lambert law) prevailing between the distribution of the monomers in an elastomer and the shape of the elastomer's NIR spectrum is exploited. This method is carried out in two stages: 1) Calibration: Spectra of the "control" elastomers are acquired. A mathematical model is constructed which associates a microstructure to a given spectrum using the PLS (partial least squares) regression method, which is based on a factorial analysis of the spectral data. The following two documents provide a thorough description of the theory and practice of this "multi-variant" method of data analysis: (1) P. GELADI and B. R. KOWALSKI "Partial Least Squares regression: a tutorial", Analytica Chimica Acta, vol. 185, 1-17 (1986). (2) M. TENENHAUS "La regression PLS - Theorie et pratique" Paris, Editions Technip (1998). 2) Measurement: - The spectrum of the sample is recorded. The microstructure is calculated. APPENDIX 2: Determination of the distribution of molecular weights of the elastomers obtained by size exclusion chromatography (SEC). a) Measurement principle: SEC (size exclusion chromatography) makes it possible physically to separate macromolecules by their size in the swollen state in columns filled with a 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 (Mw) molecular weights may be determined and the polydispersity index calculated (IP = Mw/Mn). b) Preparation of the polymer: The polymer sample is not subjected to any particular treatment prior to analysis, but is simply solubilised in tetrahydrofuran at a concentration of approx. 1 g/1. c) SEC analysis: The apparatus used is a "WATERS model 150C" chromatograph. The elution solvent is tetrahydrofuran, the flow rate 0.7 ml/min, the temperature of the system 35°C and the duration of analysis 90 min. A set of four columns is used in series, the columns having the commercial names "SHODEX KS807", "WATERS type STYRAGEL HMW7" and two "WATERS STYRAGEL MHW6E". The volume of polymer sample solution injected is 100 ul. The detector is a "WATERS model RI32X" differential refractometer and the chromatographic data processing software is "WATERS MILLENNIUM" (version 3.00). WE CLAIM: 1) A process for the preparation of diene elastomers comprising polyisoprenes and polybutadienes consisting in reacting a catalytic system in an inert hydrocarbon solvent and in the presence of a conjugated diene to be polymerised, wherein said process consists in using a catalytic system as claimed in one of claims 1 to 11. 2) A process for the preparation of diene elastomers as claimed in Claim 1, wherein it consists in homopolymerising isoprene at a temperature ranging from 25°C to 55°C in order to obtain polyisoprenes exhibiting cis-1,4 linkage contents, measured both by carbon 13 nuclear magnetic resonance and mid-infrared analysis, which fall within a range from 98.0% to 98.5%. 3) A process for the preparation of diene elastomers as claimed in Claim 1, wherein it consists in homopolymerising or copolymerising butadiene at a temperature ranging from 25°C to 100°C with the assistance of a catalytic 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 AIRa or HAlRa and - a halogen donor consisting of an alkylaluminium halide, said salt being suspended in at least one inert and saturated aliphatic or alicyclic hydrocarbon solvent, the "alkylating agent: rare earth salt" molar ratio ranging from 1 to 5 and said salt exhibiting a mass content of rare earth metal or metals ranging from 12.0 to 13.5%, said content being determined both by complexometric back titration with ethylenediaminetetraacetic acid and by inductively-coupled plasma atomic emission spectrometry, in order to obtain homopolymers or copolymers of butadiene simultaneously exhibiting a Mooney viscosity ML(l+4) at 100°C of greater than or equal to 40, measured in accordance with Standard ASTM D 1646, and a polydispersiry index of less than 2.5, measured by size exclusion chromatography.

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