CATALYST SYSTEM FOR THE STEREOSPECIFIC POLYMERIZATION OF DIENES AND USE THEREOF IN A PROCESS FOR THE SYNTHESIS OF DIENE POLYMERS

Description

Technical field

[0001] The present invention relates to a catalytic system for the stereospecific polymerization of conjugated dienes promoting the 1,4-trans insertion of the monomers. The invention relates more particularly to a polymetallic catalytic system for the stereospecific polymerization of conjugated dienes promoting the 1,4-trans insertion of the monomers. The invention also relates to a process for the synthesis of diene polymers with a high 1,4-trans chain rate using a polymetallic catalytic system for the polymerization of the monomers. Prior art

[0002] The use of polymetallic catalyst systems for the synthesis of diene polymers by coordination polymerization has been described previously.

[0003] The literature reports in particular the combination of organic rare earth compounds and organic magnesium compounds for the polymerization of 1,3-diene monomers capable of generating stereospecificity of 1,4-cis or 1,4-trans configurations, and in particular of 1,4-trans configuration.

[0004] For example, the article Macromolecules 2017, 50, 7887-7894, "Regulation of the cis-1,4- and trans-1,4-Polybutadiene Multiblock Copolymers via Chain Shuttling Polymerization Using a Ternary Neodymium Organic Sulfonate Catalyst", Quanquan Dai et al., describe the synthesis of polybutadienes that allow the regulation of the ratio of 1,4-cis and 1,4-trans to achieve a balance between deformation tolerance (attributed to the cis-1,4 part of polybutadiene) and rigidity (attributed to the crystallinity of the trans-1,4-part of polybutadiene). This paper explains that a binary catalyst system based on neodymium trifluoromethane sulfonate and dibutylmagnesium allows the synthesis of polybutadiene by coordination catalysis by promoting the 1,4-trans insertion of butadiene.

[0005] Several publications report the controlled polymerization of isoprene using a catalytic system comprising a neodymium borohydride and a dialkylmagnesium, in the presence or absence of tetrahydrofuran. These catalytic systems make it possible to provide stereoregular polyisoprene with high levels of 1,4-trans. For example, Macromolecules 2005, 38, 3162-3169 "Highly trans-Stereospecific Isoprene Polymerization by Neodymium Borohydrido Catalysts", Fanny Bonnet et al .

[0006] The publication in "Polymer International", 51:208-212 (2002) by Zhang Q. et al. deals with the copolymerization of butadiene with styrene in the presence of a rare metal compound and a dialkylmagnesium.

[0007] Another document EP0091287 describes a catalytic system for the polymerization of butadiene comprising in particular didymium versatate and a dibutylmagnesium making it possible to achieve stereospecific polybutadiene with a high level of 1,4-trans.

[0008] Although catalytic systems used in stereospecific polymerizations of conjugated dienes have already been described in the past, there remains a need for other methods for synthesizing diene polymers, particularly those with high 1,4-trans chain rates, i.e. greater than 65% by weight relative to the diene part of the polymer.

[0009] Furthermore, in certain uses of a diene polymer, it may be advantageous for it to have not only a high 1,4-trans linkage rate, but also a limited 1,2-trans linkage rate. A process for synthesizing diene polymers that can satisfy both of these objectives at the same time would be particularly interesting. Statement of the invention

[0010] The technical problem which arises within the framework of the present invention is to have a process for synthesizing diene polymers, particularly butadiene polymers, promoting the formation of 1,4-trans chains, and also making it possible to limit the formation of 1,2 chains.

[0011] The invention solves this problem by proposing a catalytic system combining a neodymium organophosphate, an organic magnesium compound and an allylic derivative of an alkali metal, which, used for the polymerization of 1,3-dienes, exhibits stereospecificity with respect to the 1,4-trans insertion while maintaining a satisfactory conversion of the polymerization reaction.

[0012] The use of the catalytic system according to the invention in a process for the synthesis of a diene polymer makes it possible to obtain diene polymers, in particular polybutadienes, having high 1,4-trans chain rates, with values ​​of at least 65% by weight relative to the diene part of the polymer, and a low 1,2- chain rate, in particular with values ​​less than or equal to 10% by weight relative to the diene part of the polymer. More particularly, the use of the catalytic system according to the invention allows, in a process for the polymerization of butadiene, the synthesis of a polybutadiene having the following characteristics: a 1,4-trans linkage rate of at least 65% by weight relative to the diene part of the polybutadiene; a 1,2- vinyl linkage rate less than or equal to 10% by weight relative to the diene part of the polybutadiene.

[0013] In a first aspect, the invention relates to such a catalytic system.

[0014] In another aspect, the invention relates to a method of synthesizing a diene polymer using such a catalyst system. Summary of the invention

[0015] The invention, described in more detail below, relates to at least one of the embodiments listed in the following points: 1- Catalytic system based on at least one salt of an organic neodymium phosphoric acid, an organic magnesium compound of formula R 1< R 2< Mg , in which R 1< and R 2< represent, independently of one another, a C 1 -C 10 aliphatic radical, substituted or not, a C 6 -C 20 aromatic radical, substituted or not, an allylic derivative of an alkali metal of formula RM, in which M denotes an alkali metal and R represents an allylic group. 2- Catalytic system according to the preceding embodiment in which the aliphatic radical, in the definition of R 1< and R 2< is a C 1 -C 10 alkyl radical, substituted or not. 3- Catalytic system according to the previous embodiment in which the aliphatic radical, in the definition of R 1< and R 2< is a C 1 -C 10 alkyl radical chosen from a methyl, ethyl radical, propyl, butyl, pentyl, hexyl, heptyl, octyl radicals,nonyls and decyls. 4- Catalytic system according to any one of the preceding embodiments in which the organic magnesium compound of formula R 1< R 2< Mg is n-butylethylmagnesium or n-butyloctylmagnesium. 5- Catalytic system according to embodiment 1 in which the aromatic radical, in the definition of R 1< and R 2< is a C 6 -C 20 aryl radical, substituted or not. 6- Catalytic system according to any one of the preceding embodiments in which the allylic group defining R is a derivative of a mono-alkene or of an allylic diene having lost a hydrogen atom and having at least 4 carbon atoms,preferably a butadienyl. 7- Catalytic system according to any one of embodiments 1 to 5 in which the allylic group defining R is an oligodienyl radical. 8- Catalytic system according to the preceding embodiment in which the oligodienyl radical comprises at least two units derived from a diene monomer. 9- Catalytic system according to the preceding embodiment in which the oligodienyl radical comprises an average number of units derived from a diene monomer varying from 2 to 100. 10- Catalytic system according to any one of embodiments 7 to 9 in which, in the oligodienyl radical, the diene monomer is chosen from conjugated diene monomers,preferably butadiene and isoprene and mixtures thereof. 11- Catalytic system according to any one of the preceding embodiments in which the allylic group R is chosen from the group consisting of oligobutadienyl derivatives having an average number of units derived from butadiene of 2 to 100 and oligoisoprenyls having an average number of units derived from isoprene of 2 to 100, preferably R is an oligobutadienyl derivative having an average number of units derived from butadiene of 2 to 100. 12- Catalytic system according to any one of the preceding embodiments in which the alkali metal M defining the allylic derivative of formula RM is lithium. 13- Catalytic system according to any one of the preceding embodiments in which the allylic derivative of formula RM is an oligodienyllithium,preferably an oligobutadienyllithium 14- Catalytic system according to any one of the preceding embodiments in which the neodymium salt of an organic phosphoric acid is a neodymium tris(organophosphate). 15- Catalytic system according to any one of the preceding embodiments in which the salt of an organic phosphoric acid is chosen from phosphoric acid diesters. 16- Catalytic system according to any one of the preceding embodiments in which the neodymium salt of an organic phosphoric acid is neodymium tris[bis(2-ethylhexyl)phosphate]. 17- Catalytic system according to any one of the preceding embodiments in which the molar ratio (R 1< R 2< Mg / neodymium) has a value of at most 15 / 1, preferably at most 12 / 1, more preferably at most 10 / 1,preferably still strictly less than 10 / 1. 18- Catalytic system according to any one of the preceding embodiments in which the molar ratio (R 1 < R 2 < Mg / neodymium) has a value of at least 1 / 2, preferably at least 1 / 1. 19- Catalytic system according to any one of the preceding embodiments in which the molar ratio (allylic derivative of an alkali metal / neodymium) has a value between 1 / 2 and 20 / 1, advantageously ranging from 1 / 1 to 10 / 1, preferably strictly greater than 1 / 1 and strictly less than 10 / 1. 20- Catalytic system according to any one of the preceding embodiments in which at least one of the following characteristics is respected, at least two, at least three, at least four, at least five and preferably all: the allylic derivative of an alkali metal is an allylic derivative of lithium, the allylic radical is an oligodienyl radical, preferably an oligobutadienyl,the neodymium salt is a diester of neodymium phosphoric acid, preferably neodymium tris [bis (2-ethylhexyl) phosphate], the organic magnesium compound is a dialkylmagnesium, each of the alkyl radicals being C 1 -C 10 , substituted or not, preferably n-butylethylmagnesium or n-butyloctylmagnesium, the molar ratio (R 1 < R 2 < Mg / neodymium) has a value of at most 15 / 1, preferably at most 12 / 1, more preferably at most 10 / 1, more preferably strictly less than 10 / 1, the molar ratio (allylic derivative of an alkali metal / neodymium) has a value of between 1 / 2 and 20 / 1, advantageously ranging from 1 / 1 to 10 / 1, preferably strictly greater than 1 / 1 and strictly less than 10 / 1. 21- Catalytic system according to any one of the preceding embodiments in which at least one of the following characteristics is respected, at least two, at least three,at least four and preferably all: the allylic derivative of an alkali metal is an oligobutadienyllithium having an average number of diene-derived units of 2 to 100, the neodymium salt is neodymium tris [bis (2-ethylhexyl) phosphate], the organic magnesium compound is n-butylethylmagnesium or n-butyloctylmagnesium the molar ratio (R 1 < R 2 < Mg / neodymium) has a value of at most 15 / 1, preferably at most 12 / 1, more preferably at most 10 / 1, more preferably strictly less than 10 / 1, the molar ratio (allylic derivative of an alkali metal / neodymium) has a value of between 1 / 2 and 20 / 1, advantageously ranging from 1 / 1 to 10 / 1,preferably strictly greater than 1 / 1 and strictly less than 10 / 1. 22- Process for preparing a diene polymer comprising the polymerization reaction of at least one conjugated diene monomer in a reactor in the presence of a catalytic system as defined in one of the preceding embodiments. 23- Process according to embodiment 22 comprising, in a manner offset from the introduction of the polymerization catalytic system into the reactor, the addition to the reactor of at least one organic magnesium compound of formula R 3< R 4< Mg, in which each of R 3< and R 4< represents, independently of one another, a C 1 -C 10 aliphatic radical, substituted or not, a C 6 -C 20 aromatic radical, substituted or not. 24- Method according to the preceding embodiment in which the molar ratio (organic magnesium compounds / neodymium) is at most 15 / 1, preferably at most 12 / 1, more preferably at most 10 / 1,more preferably strictly less than 10 / 1, the molar quantity of organic magnesium compounds being the sum of the molar quantities of the magnesium compounds R 1 < R 2 < Mg and R 3 < R 4 < Mg. 25- Process according to any one of embodiments 22 to 24 in which the conjugated diene monomer to be polymerized is a 1,3-diene monomer. 26- Process according to the preceding embodiment in which the conjugated diene monomer to be polymerized is a 1,3-diene monomer having 4 to 15 carbon atoms, preferably 1,3-butadiene or isoprene, more particularly 1,3-butadiene. 27- Process according to any one of the preceding embodiments 22 to 26 in which the conjugated diene monomer to be polymerized is copolymerized with at least one other monomer. 28- Process according to the preceding embodiment in which the conjugated diene monomer to be polymerized is copolymerized with at least one other monomer chosen from conjugated diene monomers having 4 to 15 carbon atoms. 29- Process according to embodiment 27 or 28 in which the conjugated diene monomer to be polymerized is copolymerized with at least one other monomer chosen from vinylaromatic compounds, preferably styrene. 30 - Process according to any one of the preceding embodiments 22 to 29 in which at least one of the following characteristics is respected, at least two, at least three, at least four, at least five, at least six and preferably all: the conjugated diene monomer to be polymerized is a 1,3-diene having 4 to 15 carbon atoms, preferably 1,3-butadiene,an organic magnesium compound of formula R 3 < R 4 < Mg, preferably n-butylethylmagnesium or n-butyloctylmagnesium, is added to the reactor independently and in a staggered manner from the catalytic system in a molar ratio (organic magnesium compounds / neodymium) of at most 15 / 1, preferably at most 12 / 1, more preferably at most 10 / 1, more preferably strictly less than 10 / 1, the molar amount of organic magnesium compounds being the sum of the molar amounts of the magnesium compounds R 1 < R 2 < Mg and R 3 < R 4 < Mg, the organic phosphoric acid salt of neodymium of the catalytic system is a phosphoric acid diester of neodymium, preferably tris [bis (2-ethylhexyl) phosphate] of neodymium, the allylic derivative of an alkali metal is an oligodienyllithium, preferably a oligobutadienyllithium, having an average number of diene units of 2 to 100,the organic magnesium compound R 1 < R 2 < Mg of the catalytic system is a dialkylmagnesium, each of the alkyl radicals being C 1 -C 10 , substituted or not, preferably n-butylethylmagnesium or n-butyloctylmagnesium, in the catalytic system the molar ratio (R 1 < R 2 < Mg / neodymium) has a value of at most 15 / 1, preferably at most 12 / 1, more preferably at most 10 / 1, more preferably strictly less than 10 / 1, in the catalytic system the molar ratio (allylic derivative of an alkali metal / neodymium) has a value ranging from 1 / 1 to 10 / 1, preferably strictly greater than 1 / 1 and strictly less than 10 / 1. Definition

[0016] The terms "radical", "group" and "grouping", in the singular or plural, are equivalent and interchangeable.

[0017] The expression "in C x -C y " for a hydrocarbon radical means that said radical comprises x to y carbon atoms.

[0018] In the present description, unless expressly indicated otherwise, all the percentages (%) indicated are percentages (%) by weight (also called percentage (%) by mass). The percentages (%) expressing the microstructure of the diene polymer (for example the relative distribution of the 1,2-, 1,4-trans and 1,4-cis butadiene units) are percentages by weight relative to the diene part of the polymer. Thus a 1,4-trans linkage rate of more than 70% in a diene polymer corresponds to a rate of 1,4-trans diene units of more than 70% by weight relative to the weight of the diene part of the polymer.

[0019] On the other hand, any interval of values ​​designated by the expression "between a and b" represents the domain of values ​​going from more than a to less than b (i.e., excluding the limits a and b) while any interval of values ​​designated by the expression "from a to b" means the domain of values ​​going from a to b (i.e., including the strict limits a and b).

[0020] The term "based on" used to define the constituents of the catalytic system means the mixture of these constituents and / or the product of the reaction between these constituents.

[0021] The compounds mentioned in the description comprising carbon may be of fossil or bio-sourced origin. In the latter case, they may be, partially or totally, derived from biomass or obtained from renewable raw materials derived from biomass. In the same way, the compounds mentioned may also come from the recycling of materials already used, that is to say they may be, partially or totally, derived from a recycling process, or obtained from raw materials themselves derived from a recycling process. The monomers concerned more particularly in the present application.

[0022] Thus, for example, butadiene can advantageously be derived in a known manner directly from biomass or be obtained from a bio-sourced precursor, for example bio-sourced ethanol. Isoprene can advantageously be derived in a known manner directly from biomass or be obtained from a bio-sourced precursor, for example from bio-sourced isobutene. Styrene can, for example, advantageously be derived in a known manner directly from biomass or from the recycling of polystyrene. Detailed description of the invention

[0023] The subject of the invention is a catalytic system based on at least a neodymium salt of an organic phosphoric acid, an organic magnesium compound of formula R 1< R 2< Mg , in which R 1< and R 2< represent, independently of each other, a C 1 -C 10 aliphatic radical, substituted or not, a C 6 -C 20 aromatic radical, substituted or not, and an allylic derivative of an alkali metal of formula RM, in which M denotes an alkali metal and R represents an allylic group.

[0024] The catalytic system according to the invention is based on at least one neodymium salt of an organic phosphoric acid. The organic phosphoric acid salt of neodymium is advantageously a neodymium tris(organophosphate), the organophosphate possibly being chosen from phosphoric acid diesters of general formula (R'O)(R"O)PO(OH), in which R' and R", which are identical or different, represent a C 1 -C 10 alkyl radical or a C 6 -C 20 aryl radical.Examples include neodymium tris[dibutyl phosphate], neodymium tris[dipentyl phosphate], neodymium tris[dioctyl phosphate], neodymium tris[bis(2-ethylhexyl)phosphate], neodymium tris[bis(1-methylheptyl)phosphate], neodymium tris[bis(p-nonylphenyl)phosphate], neodymium tris[butyl(2-ethylhexyl)phosphate], neodymium tris[(1-methylheptyl)(2-ethylhexyl)phosphate], neodymium tris[(2-ethylhexyl)(p-nonylphenyl)phosphate], neodymium tris[bis(2-ethylhexyl)phosphate], neodymium tris[bis(oleyllyl)phosphate] or neodymium tris[bis(lineolyl) neodymium phosphate.

[0025] According to embodiments of the invention, the organic phosphoric acid salt of neodymium is a tris[bis(2-ethylhexyl)phosphate] of neodymium.

[0026] Neodymium salt can be in the form of a powder, a solution in an inert hydrocarbon solvent, a suspension in an inert hydrocarbon solvent or a gel in an inert hydrocarbon solvent.

[0027] The catalytic system according to the invention is also based on at least one organic magnesium compound of formula R 1 < R 2 < Mg

[0028] In the formula R 1< R 2< Mg, R 1< and R 2< can be the same or different.

[0029] In the formula R 1< R 2< Mg, each of R 1< and R 2< may be a C 1 -C 40 aliphatic radical, substituted or unsubstituted. As an aliphatic radical representing R 1< and R 2<, mention may be made of C 1 -C 10 alkyl groups, C 2 -C 10 alkenyl groups and C 2 -C 10 alkynyl groups, whether cyclic or non-cyclic. In particular, when one of R 1< and R 2< is an aliphatic radical, it is a non-cyclic C 1 -C 10 alkyl radical chosen from the methyl radical, ethyl radical, propyl radical, butyl radical, pentyl radical, hexyl radical, heptyl radical, octyl radical, nonyl radical and decyl radical.

[0030] In the formula R 1< R 2< Mg, each of R 1< and R 2< may be a C 6 -C 20 aromatic radical, substituted or not. As aromatic radical representing each of R 1< and R 2<, mention may be made of C 6 -C 20 aryl groups.

[0031] By substituted aliphatic radical or substituted aromatic radical is meant a radical substituted by one or more hydrocarbon groups having from 1 to 10 carbon atoms, for example chosen from C 1 -C 10 alkyls, C 7 -C 12 aralkyls, C 2 -C 10 alkenyls and C 2 -C 10 alkynyls.

[0032] According to particular embodiments of the invention, in the formula R 1< R 2< Mg, R 1< and R 2< each denote a non-cyclic C 1 -C 10 alkyl radical. According to this embodiment, the organic magnesium compound of formula R 1< R 2< Mg is n-butylethylmagnesium or n-butyloctylmagnesium.

[0033] According to particular embodiments of the invention, the molar ratio (R 1 < R 2 < Mg / neodymium) in said catalytic system has a value of at least 1 / 2, preferably at least 1 / 1. There is no maximum value for this ratio which is linked to the 1,4-trans chaining rate to be achieved. However, in order to ensure that achieving a high 1,4-trans chaining rate greater than 65% by weight relative to the diene part of the polymer is not to the detriment of the 1,2- chaining rate, the molar ratio (R 1 < R 2 < Mg / neodymium) in said catalytic system is preferably at most 15 / 1. Preferably the molar ratio (R 1 < R 2 < Mg / neodymium) is at most 12 / 1, more preferably at most 10 / 1, more preferably strictly less than 10 / 1, to maintain a 1,2-linkage rate of less than 10% by weight relative to the diene part of the polymer.

[0034] Furthermore, in order to maintain an optimization of the conversion, the molar ratio (R 1 < R 2 < Mg / neodymium) in said catalytic system advantageously has a value of at least 1 / 1, or even preferably at least 2 / 1. Also, in order to maintain an optimization of the conversion, the molar ratio (R 1 < R 2 < Mg / neodymium) in said catalytic system advantageously has a value of at most 12 / 1. Thus, according to advantageous embodiments of the invention, the molar ratio (R 1 < R 2 < Mg / neodymium) in said catalytic system has a value of at least 1 / 1, preferably at least 2 / 1, and at most 12 / 1, preferably at most 10 / 1, more preferably strictly less than 10 / 1.

[0035] The catalytic system according to the invention is also based on at least one allylic derivative of an alkali metal corresponding to the formula RM, in which M is an alkali metal R is an allylic group.

[0036] According to embodiments of the invention, R may be a radical derived from a mono-alkene or an allylic diene having lost a hydrogen atom and comprising at least 4 carbon atoms and preferably at most 10 carbon atoms. According to these embodiments, R is preferably a butadienyl radical.

[0037] According to other embodiments of the invention, R may be an oligodienyl radical. These embodiments are preferred in particular with a view to optimizing the operating conditions for the polymerization of dienes with a good conversion rate. The term oligodienyl radical is understood to mean in particular a radical derived from a polymer of at least one diene monomer having a number-average molar mass of at most 6,000 g / mol. The target number-average molar mass is determined in a well-known manner from an equation which links the target Mn, the mass of monomer converted into polymer in grams and the number of moles of initiator and which is expressed as: Target Mn = mass of converted monomer / moles of initiator.

[0038] The oligodienyl radical is the radical derived from a polymer by the loss of a hydrogen atom. Such a polymer may be a homopolymer of a diene monomer, particularly a conjugated diene monomer, in particular any homopolymer obtained by polymerization of a conjugated diene monomer having from 4 to 12 carbon atoms. Suitable conjugated dienes include butadiene, isoprene, 2,3-di(C 1 to C 3 alkyl) 1,3-butadienes and mixtures thereof, preferably butadiene. Such a polymer may also be a copolymer obtained by copolymerization of one or more diene monomers with each other or with one or more ethylenically unsaturated monomers. Suitable ethylenically unsaturated monomers include aromatic vinyl compounds having from 8 to 20 carbon atoms, more particularly styrene. According to the invention, the average number of units of these oligomers can vary from 2 to 100.

[0039] Thus, in certain embodiments of the invention, the allylic group R is an oligodienyl radical chosen from oligobutadienyl and oligoisoprenyl groups, the average number of units of these oligomers being able to vary from 2 to 100. Preferably, according to these embodiments, the allylic group R is an oligobutadienyl group preferably comprising an average number of butadiene units varying from 2 to 100, more preferably from 5 to 100.

[0040] In the formula RM, the alkali metal M is K, Li or Na. It is preferably Li.

[0041] According to particular embodiments of the invention, the allylic derivative of an alkali metal of formula RM is an oligodienyllithium, preferably an oligobutadienyllithium preferably comprising an average number of butadiene units varying from 2 to 100, more preferably from 5 to 100.

[0042] The allylic derivative of an alkali metal, when it is an oligodienyl of an alkali metal, can be obtained in a known manner by the anionic polymerization of at least one diene monomer initiated by an organic compound of an alkali metal M.

[0043] According to embodiments of the invention, the molar ratio (allylic derivative of an alkali metal / neodymium) in said catalytic system advantageously has a value ranging from 1 / 2 to 20 / 1, preferably ranging from 1 / 1 to 10 / 1, more preferably still strictly greater than 1 / 1 and strictly less than 10 / 1.

[0044] According to any embodiment of the invention, the catalytic system preferably comprises a hydrocarbon solvent. The catalytic system may be in the form of a solution when in the presence of a hydrocarbon solvent. The hydrocarbon solvent may be a low molecular weight aliphatic hydrocarbon solvent, such as for example cyclohexane, methylcyclohexane, n-heptane, or a mixture of these solvents, or in an aromatic solvent such as toluene. It should be noted that non-aromatic solvents are particularly preferred. The hydrocarbon solvent is preferably aliphatic, more preferably methylcyclohexane.

[0045] Those skilled in the art will understand that the embodiments of the invention mentioned above can be combined with each other in all their particular and preferred aspects, provided that they are compatible. In particular, the characteristics concerning the nature of the components of the catalytic system and the molar ratios (R 1 < R 2 < Mg / neodymium) and (allylic derivative of an alkali metal / neodymium) are combinable.

[0046] Thus, according to certain particular embodiments of the catalytic system of the invention, at least one of the following characteristics is respected, at least two, at least three, at least four, at least five and preferably all: the allylic derivative of an alkali metal is an allylic derivative of lithium, the allylic radical is an oligodienyl radical, preferably an oligobutadienyl, having from 2 to 100 units derived from diene, the neodymium salt is a diester of phosphoric acid of neodymium, preferably tris [bis (2-ethylhexyl) phosphate] of neodymium, the organic magnesium compound is a dialkylmagnesium, each of the alkyl radicals being C 1 -C 10 , substituted or not, preferably n-butylethylmagnesium or n-butyloctylmagnesium, the molar ratio (R 1 < R 2 < Mg / neodymium) has a value of at most 15 / 1, preferably at most 12 / 1, preferably at most 10 / 1, more preferably strictly less than 10 / 1, the molar ratio (allylic derivative of a alkali metal / neodymium) has a value ranging from 1 / 2 to 20 / 1, advantageously from 1 / 1 to 10 / 1, more preferably still strictly greater than 1 / 1 and strictly less than 10 / 1.

[0047] More particularly still according to these embodiments, at least one of the following characteristics is respected, at least two, at least three, at least four and preferably all: the allylic derivative of an alkali metal is an oligobutadienyllithium, having 2 to 100 units derived from butadiene, the neodymium salt is tris [bis (2-ethylhexyl) phosphate] of neodymium, the organic magnesium compound is n-butylethylmagnesium or n-butyloctylmagnesium, the molar ratio (R 1 < R 2 < Mg / neodymium) has a value of at most 12 / 1, preferably at most 10 / 1, more preferably strictly less than 10 / 1, the molar ratio (allylic derivative of an alkali metal / neodymium) has a value ranging from 1 / 1 to 10 / 1, more preferably still strictly greater than 1 / 1 and strictly less than 10 / 1. A process for preparing a catalytic system as defined above is also the subject of the present invention.

[0048] The process for preparing the catalytic system comprises simultaneously or sequentially contacting the salt of an organic neodymium phosphoric acid, the organic magnesium compound and the allylic derivative of an alkali metal.

[0049] The contact of the different components, in pairs or threes, can be carried out at a temperature between 15 and 100°C, and for 3 to 120 minutes.

[0050] The organic salt of neodymium phosphoric acid can be in the form of a powder, a solution in an inert hydrocarbon solvent, a suspension in an inert hydrocarbon solvent, or a gel in an inert hydrocarbon solvent.

[0051] According to embodiments of the method according to the invention, the allylic alkali metal derivative and the organic magnesium compound are brought into contact prior to their addition to the organic neodymium salt in suspension, in solution or in gel form. According to other embodiments of the method according to the invention, the allylic alkali metal derivative, the organic magnesium compound and the organic neodymium salt, in suspension, in solution or in gel form, are brought into contact concomitantly.

[0052] According to a variant of the process according to the invention, the catalytic system is preformed prior to its introduction into the polymerization medium, i.e. by bringing the various catalytic constituents into contact, followed by aging of the catalytic constituents before use in polymerization, optionally in the presence of a preforming conjugated diene. Before being used, for example, in polymerization, the catalytic system thus obtained in solution can be stored under an inert atmosphere, for example under nitrogen or argon, in particular at a temperature ranging from -20°C to room temperature (23°C).

[0053] According to another variant of the process according to the invention, the catalytic system is formed in situ,that is, all the catalytic constituents, namely the neodymium salt, the allylic derivative of the alkali metal and the organic magnesium compound are introduced simultaneously or sequentially into the reactor just before polymerization in the polymerization medium.

[0054] The catalytic system according to the invention can be advantageously used in a process for synthesizing a diene polymer, which process is also the subject of the present invention. Indeed, it turns out that the catalytic system according to the invention, used in such a process, promotes the 1,4-trans insertion of conjugated diene monomers, thus making it possible to obtain diene polymers having a high 1,4-trans chaining rate, while limiting the 1,2- insertion of the monomers. The synthesis process using a catalytic system according to the invention makes it possible in particular to obtain butadiene polymers having a high 1,4-trans chaining rate greater than or equal to 65% by weight relative to the diene part of the polymer, as well as a low 1,2- chaining rate less than or equal to 10% by weight relative to the diene part of the polymer.

[0055] The invention therefore also relates to a process for synthesizing a diene polymer comprising a step of polymerizing at least one conjugated diene monomer in the presence of a catalytic system as described above.

[0056] According to the invention, the conjugated diene monomer to be polymerized is in particular a 1,3-diene monomer. As 1,3-diene monomer in accordance with the invention, mention may in particular be made of a 1,3-diene monomer having 4 to 15 carbon atoms.

[0057] Suitable 1,3-diene monomers include butadiene, isoprene, 2,3-di(C 1 to C 5 alkyl)-1,3-butadiene, such as, for example, 2,3-dimethyl-1,3-butadiene, 2,3-diethyl-1,3-butadiene, 2-methyl-3-ethyl-1,3-butadiene, 2-methyl-3-isopropyl-1,3-butadiene, phenyl-1,3-butadiene, 1,3-pentadiene, or any other 1,3-diene monomer having 4 to 12 carbon atoms.

[0058] Any linear terpene, such as a linear monoterpene (C 10 H 16 ), such as myrcene, a linear sesquiterpene (C 15 H 24 ), such as farnesene, etc., is also suitable as a 1,3-diene monomer.

[0059] According to certain embodiments of the invention, the conjugated diene monomer is 1,3-butadiene or isoprene, advantageously 1,3-butadiene.

[0060] According to certain embodiments of the invention, the polymerization step of the process is a step of homopolymerization of a conjugated diene monomer in the presence of a catalytic system as described above.

[0061] According to certain embodiments of the invention, the polymerization step of the process is a step of copolymerization of at least one conjugated diene monomer in the presence of a catalytic system as described above. The conjugated diene monomer is then copolymerized with at least one other monomer.

[0062] As another monomer, a conjugated diene monomer having 4 to 15 carbon atoms as defined above, different from the first conjugated diene monomer, is particularly suitable. Suitable conjugated diene monomers include 1,3-butadiene, 2-methyl-1,3-butadiene (or isoprene), 2,3-di(C1-C5 alkyl)-1,3-butadiene, such as, for example, 2,3-dimethyl-1,3-butadiene, 2,3-diethyl-1,3-butadiene, 2-methyl-3-ethyl-1,3-butadiene, 2-methyl-3-isopropyl-1,3-butadiene, phenyl-1,3-butadiene, 1,3-pentadiene, 2,4-hexadiene, or any other conjugated diene having between 4 and 12 carbon atoms. As conjugated diene monomers, we can also cite non-cyclic terpene conjugated diene monomers, more particularly farnesene or myrcene.

[0063] A vinyl aromatic compound is also suitable as another monomer. As a vinyl aromatic monomer, mention may be made of a vinyl aromatic monomer having from 8 to 20 carbon atoms, such as, for example, styrene, ortho-, meta-, para-methylstyrene, 2,4,6-trimethylstyrene, divinylbenzene and vinylnaphthalene, preferably the vinyl aromatic monomer is styrene.

[0064] The polymerization step can be carried out in a known manner, continuously or discontinuously, in bulk or in solution, generally and in a known manner at a temperature of at least 40°C and preferably at most 120°C, more preferably at most 100°C.

[0065] The polymerization step can be carried out in an organic solvent conventionally used for the polymerization of diene monomers. According to the invention, the term organic solvent means an inert hydrocarbon solvent which can be, for example, an aliphatic or alicyclic hydrocarbon such as pentane, hexane, heptane, isooctane, cyclohexane, methylcyclohexane, or an aromatic hydrocarbon such as benzene, toluene, xylene, or mixtures of these solvents. It should be noted that non-aromatic solvents are particularly preferred.

[0066] The catalytic system as described above according to all its embodiments is introduced into a reactor and used for polymerization in a reaction medium comprising the monomers to be polymerized and, where appropriate, the polymerization solvent.

[0067] According to certain embodiments of the invention, the catalytic system is used in proportions varying from 20 to 2000 µmol of rare earth metal per 100g of monomers, preferably from 20 to 1000 µmol, even more preferably from 50 to 1000 µmol.

[0068] According to certain embodiments of the invention, the polymerization step of the process is preceded by the addition of a predetermined amount of an organic magnesium compound of formula R 3 < R 4 < Mg, directly into the reaction medium, independently of the introduction of the catalytic system used for the polymerization reaction and in a manner offset from the catalytic system. That is to say, the addition is made in the polymerization medium not at the same time and, consequently, either before, or after, or partly before and partly after, preferably before, relative to the introduction of the catalytic system used to catalyze the polymerization reaction. The addition is preferably made before the introduction of the monomers.

[0069] It should be noted that the addition of the organic magnesium compound of formula R 3 < R 4 < Mg before the preformed catalytic system makes it possible in particular to minimize the dispersion of the characteristics of the elastomer obtained, in particular the molecular masses.

[0070] Advantageously, the quantity of organic magnesium compound of formula R 3 < R 4 < Mg is such that the molar ratio (organic magnesium compounds / neodymium) is preferably at most 15 / 1, more preferably at most 12 / 1, still preferably at most 10 / 1, or even strictly less than 10 / 1. Here, the term molar quantity of organic magnesium compounds should be understood to mean the total molar quantity of magnesium compounds, i.e. the sum of the molar quantity of magnesium compound R 3 < R 4 < Mg and the molar quantity of magnesium compound R 1 < R 2 < Mg of the catalytic system.

[0071] The organic magnesium compound of formula R 3 < R 4 < Mg may be identical to or different from the organic magnesium compound of formula R 1 < R 2 < Mg of the catalytic system.

[0072] In the formula R 3< R 4< Mg, each of R 3< and R 4< represents, independently of one another, a C 1 -C 10 aliphatic radical, substituted or not, a C 6 -C 20 aromatic radical, substituted or not. The aliphatic and aromatic radicals are defined above in relation to R 1< and R 2< . More particularly, each of R 3< and R 4< is a non-cyclic C 1 -C 10 alkyl radical chosen from a methyl, ethyl radical, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl and decyl radicals.

[0073] According to these embodiments of the invention, the organic magnesium compound of formula R 3 < R 4 < Mg is preferably n-butylethylmagnesium or n-butyloctylmagnesium.

[0074] According to certain particular embodiments of the method for synthesizing a diene polymer according to the invention, at least one of the following characteristics is respected, at least two, at least three, at least four, at least five and preferably all: the conjugated diene monomer to be polymerized is a 1,3-diene monomer having 4 to 15 carbon atoms, preferably 1,3-butadiene, the organic neodymium phosphoric acid salt of the catalyst system is a neodymium phosphoric acid diester, preferably neodymium tris[bis(2-ethylhexyl)phosphate], the allylic derivative of an alkali metal is an oligodienyllithium, preferably an oligobutadienyllithium, having an average number of diene-derived units of 2 to 100, the organic magnesium compound of the catalyst system is a dialkylmagnesium, each of the alkyl radicals being C1-C10, substituted or not, preferably n-butylethylmagnesium or n-butyloctylmagnesium, the molar ratio (R1<R2<Mg / neodymium) has a value of at most 15 / 1, preferably at most 12 / 1, more preferably at most 10 / 1, or even strictly less than 10 / 1, the molar ratio (allylic derivative of an alkali metal / neodymium) has a value ranging from 1 / 2 to 20 / 1,advantageously from 1 / 1 to 10 / 1, more preferably strictly greater than 1 / 1 and strictly less than 10 / 1.

[0075] To these characteristics of the process for synthesizing a diene polymer according to the invention can be added the addition of a predetermined quantity of an organic magnesium compound of formula R 3 < R 4 < Mg as described above.

[0076] Thus, in certain particular embodiments of the method for synthesizing a diene polymer according to the invention, at least one of the following characteristics is respected, at least two, at least three, at least four, at least five, at least six and preferably all: the conjugated diene monomer is 1,3-butadiene, the organic neodymium phosphoric acid salt of the catalytic system is a neodymium phosphoric acid diester, preferably neodymium tris[bis(2-ethylhexyl)phosphate], the allylic derivative of an alkali metal is an oligodienyllithium, preferably an oligobutadienyllithium, having an average number of diene-derived units of 2 to 100, the organic magnesium compound of the catalytic system is a dialkylmagnesium, each of the alkyl radicals being C1-C10, substituted or not, preferably n-butylethylmagnesium or n-butyloctylmagnesium, in the catalytic system the molar ratio (R1<R2<Mg / neodymium) has a value of at most 15 / 1, advantageously at most 12 / 1, more preferably of at most 10 / 1, or even strictly less than 10 / 1, in the catalytic system the molar ratio (allylic derivative of an alkali metal / neodymium) has a value ranging from 1 / 2 to 20 / 1, advantageously from 1 / 1 to 10 / 1,more preferably strictly greater than 1 / 1 and strictly less than 10 / 1 an organic magnesium compound R 3< R 4< Mg, each of R 3< and R 4< represents, independently of one another, a C 1 -C 10 aliphatic radical, substituted or not, a C 6 -C 20 aromatic radical, substituted or not, preferably n-butylethylmagnesium or n-butyloctylmagnesium, is added to the polymerization medium independently and in a manner offset from the catalytic system at a rate such that the molar ratio (organic magnesium compounds / neodymium) less than 15 / 1, advantageously at most 12 / 1, more preferably at most 10 / 1, or even strictly less than 10 / 1, the molar quantity of organic magnesium compounds being the sum of the molar quantity of R 3< R 4< Mg and the molar quantity of R 1< R 2< Mg. ,

[0077] At the end of the polymerization step, the process for synthesizing a diene polymer according to the invention can be continued in a manner known per se. Thus, in certain embodiments, the polymerization can be stopped, possibly after a post-polymerization modification step of the diene polymer.

[0078] The process then continues in a manner known per se by separating and recovering the prepared diene polymer. The unreacted monomers can be removed using methods known to those skilled in the art, as can the solvent.

[0079] The above-mentioned characteristics of the present invention, as well as others, will be better understood upon reading the following description of several exemplary embodiments of the invention, given for information purposes. Examples Measures and tests used Near infrared spectroscopy (NIR):

[0080] The microstructure of elastomers is characterized by the near infrared spectroscopy (NIR) technique.

[0081] Near infrared (NIR) spectroscopy is used to quantitatively determine the microstructure of the elastomer (relative distribution of 1,2-, 1,4-trans and 1,4-cis butadiene units). The principle of the method is based on the Beer-Lambert law generalized to a multi-component system. Since the method is indirect, it uses a multivariate calibration [Vilmin, F.; Dussap, C.; Coste, N. Applied Spectroscopy 2006, 60, 619-29] carried out using standard elastomers of composition determined by 13< C NMR. The microstructure is then calculated from the NIR spectrum of an elastomer film approximately 750 µm thick. The spectrum acquisition is carried out in transmission mode between 7800 and 3900 cm -1< with a resolution of 2 cm -1< , using a Fourier transform near-infrared spectrometer equipped with a Peltier-cooled InGaAs detector. Conversion :

[0082] The conversion is calculated via the ratio between the mass of the polymer isolated at the end of the reaction and the mass of butadiene introduced into the reactor. C = m polybutadi è ne m butadi è ne . 100 m butadiene which represents the mass of butadiene introduced into the reactor, M polybutadiene which represents the mass of polybutadiene obtained.

[0083] The following examples aim to show the great interest of the catalytic system according to the invention for the polymerization of 1,3-diene monomer, in particular by promoting 1,4-trans chains while allowing a high conversion. The following examples also aim to show the significant and surprising nature of this technical effect, more particularly in comparison with a catalytic system not comprising an allylic derivative of an alkali metal but instead an alkyllithium compound used in the state of the art. Process for the synthesis of catalytic systems :

[0084] Two solutions are prepared separately: Solution 1 : In a sealed steinie bottle inerted under nitrogen flushing are introduced the degassed methylcyclohexane solvent and powdered neodymium tris [bis (2-ethylhexyl) phosphate] (the concentration of Nd in the catalytic system is 0.009 mol / L). The bottle is stirred at room temperature for 16 hours. Solution 2 : In a sealed steinie bottle inerted under nitrogen flushing are introduced the degassed methylcyclohexane solvent and butadiene. Butyllithium is injected, the reaction medium is heated to 60°C for 60 minutes and the butadiene is polymerized to form an oligobutadienyllithium of Mn 2000 g / mol targeted (PBLi). Then butylethylmagnesium in solution in methylcyclohexane is added to this solution (Mg / Nd=5). The catalytic system is used directly (catalytic system C1) or placed at room temperature under stirring for a contact time of 30 minutes (catalytic system C2).

[0085] All of solution 2 is injected into solution 1. The reaction medium is then heated to 30°C with stirring for 60 minutes.

[0086] The characteristics of the catalytic systems are recalled in Table 1: Table 1 Catalytic system PBLi / Nd Mg / Nd Contact time PBLi-R 1 R 2 Mg (minutes) C0 0 5 0 C1 5 5 0 C2 5 5 30 C3 5* 5 30 C4 1 5 30 C5 10 5 30 * BuLi Process for the synthesis of butadiene polymers : Example of synthesis of polybutadiene with the catalytic systems in accordance with the invention

[0087] In a sealed steinie bottle inerted under nitrogen flushing are introduced the degassed methylcyclohexane solvent, butylethylmagnesium (according to the rate indicated in the table) in solution in methylcyclohexane then butadiene (9.5g). The solvent / butadiene mass ratio is 6.9 / 1 for all tests. The quantities of the reagents are mentioned in the table below. The CX catalytic system is then introduced into the reaction medium in a quantity of 400 µmol Nd / 100g monomer. The reaction medium is heated to the indicated temperature. After a few hours, the reaction is stopped using methanol (1 mL) and a conversion measurement is carried out. The solution is then anti-oxidized and left to dry in an oven. Table 2 Examples Catalytic system Polymerization temperature Added Mg* / Nd Polymerization time (min) Conv (%) %Trans %cis %1,2 1 C0 90 0.5 150 7 nd nd nd 2 C1 90 0.5 75 85 69 24 7 3 C1 90 0.5 120 86 69 24 7 4 C1 75 0.5 80 64 80 15 5 5 C1 75 0.5 145 77 80 15 5 6 C1 50 0.5 240 59 86 11 3 7 C1 50 0.5 360 72 88 9 3 8 C2 90 0.5 75 82 70 23 7 9 C2 90 0.5 120 86 70 23 7 10 C3 90 0.5 150 47 64 28 8 11 C4 90 0.5 150 13 nd nd nd 12 C5 90 0.5 150 95 61 28 11 13 C2 90 5 150 42 57 27 16 14 C2 90 10 150 31 nd nd nd *"Added Mg" being the organic magnesium compound of formula R 3< R 4< Mg added here before the catalytic system "nd": the quantity of product obtained is not sufficient to carry out the measurement of the microstructure

[0088] On reading the results, it is noted that the catalytic systems in accordance with the invention make it possible to polymerize butadiene by promoting insertion of the monomers in the 1,4-trans position.

[0089] It is noted that polybutadienes are obtained with a high 1,4-trans chain rate, notably greater than 65% by weight.

[0090] The polybutadienes obtained have high trans levels, notably greater than 65% by weight, and a limited 1,2- linkage level, less than 10% by weight, which reduces their crystallinity.

[0091] Furthermore, it is observed that at 90°C, a PBLi / Nd ratio greater than 1 / 1 and less than 10 / 1 allows the production with acceptable conversion of polybutadienes presenting a high 1,4-trans chaining rate and a limited 1,2- chaining rate (tests 9, 11 and 12).

[0092] It is also observed that at 90°C, a total Mg / Nd ratio greater than or equal to 10 / 1 leads to a decrease in conversion and a degradation of the microstructure at a high 1,4-trans chain rate, because this rate decreases in favor of the formation of 1,2- chain (tests 9, 13 and 14). Total Mg is the sum of the molar quantities of the magnesium compounds used during the synthesis of the catalytic system and added before the catalytic system in the polymerization medium (R 3 < R 4 < Mg).

Claims

1. Catalytic system based on at least: • a salt of an organic phosphoric acid of neodymium, • an organic magnesium compound of formula R1R2Mg, where each of R1 and R2 independently represents an aliphatic radical in C1-C10, substituted or not, or an aromatic radical in C6-C20, substituted or not, and • an allylic derivative of an alkali metal of formula RM, where M denotes an alkali metal and R represents an allylic group.

2. Catalytic system according to claim 1, characterized in that the aliphatic radical in the definition of R1 and R2 is an alkyl radical in C1-C10, substituted or not.

3. Catalytic system according to any of the preceding claims, characterized in that the organic magnesium compound of formula R1R2Mg is n-butylethylmagnesium or n-butyl-octylmagnesium.

4. Catalytic system according to any of the preceding claims, characterized in that in the allylic derivative of formula RM, the allylic group defining R is an oligodienyl radical, preferably an oligobutadienyl radical.

5. Catalytic system according to any of the preceding claims, characterized in that in the allylic derivative of formula RM, M denotes lithium.

6. Catalytic system according to any of the preceding claims, characterized in that the salt of an organic phosphoric acid of neodymium is a tris(organophosphate) of neodymium.

7. Catalytic system according to any of the preceding claims, characterized in that the salt of an organic phosphoric acid of neodymium is tris [bis (2-ethylhexyl) phosphate] of neodymium.

8. Catalytic system according to any of the preceding claims, characterized in that the molar ratio (organic magnesium compound / neodymium) has a value of at most 15 / 1, preferably at most 12 / 1, preferably at most 10 / 1, preferably strictly less than 10 / 1.

9. Catalytic system according to any of the preceding claims, characterized in that the molar ratio (allylic derivative of an alkali metal / neodymium) has a value between 1 / 2 and 20 / 1, advantageously ranging from 1 / 1 to 10 / 1, preferably strictly greater than 1 / 1 and strictly less than 10 / 1.

10. Catalytic system according to any of the preceding claims, characterized in that at least one of the following characteristics is respected, at least two, at least three, at least four, at least five and preferably all: • the allylic derivative of an alkali metal is an allylic derivative of lithium, • the allylic radical is an oligodienyl radical, preferably an oligobutadienyl, having 2 to 100 units derived from diene, • the neodymium salt is a diester of phosphoric acid of neodymium, preferably tris [bis (2-ethylhexyl) phosphate] of neodymium, • the organic magnesium compound is a dialkylmagnesium, each of the alkyl radicals being in C1-C10, substituted or not, preferably n-butylethylmagnesium or n-butyl-octylmagnesium, • the molar ratio (organic magnesium compound / neodymium) has a value of at most 15 / 1, preferably at most 12 / 1, preferably at most 10 / 1, preferably strictly less than 10 / 1, • the molar ratio (allylic derivative of an alkali metal / neodymium) has a value ranging from 1 / 2 to 20 / 1, advantageously from 1 / 1 to 10 / 1, preferably strictly greater than 1 / 1 and strictly less than 10 / 1.

11. Process for preparing a diene polymer comprising the polymerization reaction of at least one conjugated diene monomer in a reactor in the presence of a catalytic system as defined in any of the preceding claims.

12. Process according to claim 11 comprising, staggered to the introduction of the polymerization catalytic system into the reactor, the addition into the reactor of at least one organic magnesium compound of formula R3R4Mg, where each of R3 and R4 independently represents an aliphatic radical in C1-C10, substituted or not, or an aromatic radical in C6-C20, substituted or not.

13. Process according to claim 12 characterized in that the molar ratio (organic magnesium compounds / neodymium) is at most 15 / 1, preferably at most 12 / 1, preferably at most 10 / 1, preferably strictly less than 10 / 1, the molar quantity of organic magnesium compound being the sum of the molar quantities of magnesium compounds R1R2Mg and R3R4Mg.

14. Process according to any of claims 11 to 13 characterized in that the conjugated diene monomer to be polymerized is a 1,3-diene monomer having 4 to 15 carbon atoms, preferably 1,3-butadiene.

15. Process according to any of claims 11 to 14 in which at least one of the following characteristics is respected, at least two, at least three, at least four, at least five, at least six and preferably all: • the conjugated diene monomer is a 1,3-diene monomer having 4 to 15 carbon atoms, preferably 1,3-butadiene, • the neodymium salt of the catalytic system is a diester of phosphoric acid of neodymium, preferably tris [bis (2-ethylhexyl) phosphate] of neodymium, • the allylic derivative of an alkali metal of the catalytic system is an oligodienyllithium, preferably an oligobutadienyllithium, preferably having 2 to 100 units derived from butadiene, • the organic magnesium compound of formula R1R2Mg of the catalytic system is a dialkylmagnesium, each of the alkyl radicals being in C1-C10, substituted or not, preferably n-butylethylmagnesium or n-butyl-octylmagnesium, • in the catalytic system the molar ratio (R1R2Mg / neodymium) has a value of at most 15 / 1, preferably at most 12 / 1, preferably at most 10 / 1, preferably strictly less than 10 / 1, • in the catalytic system the molar ratio (allylic derivative of an alkali metal / neodymium) has a value ranging from 1 / 1 to 10 / 1, preferably strictly greater than 1 / 1 and strictly less than 10 / 1, • an organic magnesium compound R3R4Mg, where each of R3 and R4 independently represents an aliphatic radical in C1-C10, substituted or not, an aromatic radical in C6-C20, substituted or not, preferably n-butylethylmagnesium or n-butyl-octylmagnesium, is added into the reactor independently of the catalytic system, provided that the molar ratio (organic magnesium compounds / neodymium) is at most 15 / 1, preferably at most 12 / 1, preferably at most 10 / 1, preferably strictly less than 10 / 1, the molar quantity of organic magnesium compounds being the sum of the molar quantities of magnesium compounds R1R2Mgand R3R4Mg.