TERPOLYMER OF ETHYLENE AND 1,3-DIENE

Description

[0001] The field of the invention is that of copolymers of conjugated dienes and ethylene, rich in ethylene units and usable as elastomers in a rubber composition for tires.

[0002] The most widely used diene elastomers in tire manufacturing are polybutadiene, polyisoprene (particularly natural rubber), and 1,3-butadiene-styrene copolymers. These elastomers share a high molar proportion of diene units, generally well over 50%, which can make them susceptible to oxidation, especially under the influence of ozone.

[0003] The Applicant described elastomers that, conversely, are relatively low in diene units, notably to reduce their susceptibility to oxidation. These elastomers are described, for example, in document WO 2007054223. They are copolymers of 1,3-butadiene and ethylene containing more than 50 mole percent ethylene units. These elastomers are classified as ethylene-rich diene elastomers.

[0004] High-ethylene 1,3-butadiene copolymers are crystalline, and their crystallinity increases with the ethylene content. The presence of crystalline components in the copolymer can be problematic when used in a rubber compound. Since melting of the crystalline components of the copolymer reduces its stiffness, a rubber compound containing such a copolymer will also experience a decrease in stiffness when heated to temperatures equal to or exceeding the melting point of the crystalline components. This temperature dependence of stiffness can therefore lead to fluctuations in the properties of the rubber compound, making it less suitable for certain applications requiring greater temperature stability. It is advantageous to have diene polymers rich in ethylene units with reduced or even eliminated crystallinity.

[0005] In document WO 2007054224, the Applicant described ethylene-rich diene copolymers with reduced crystallinity. These copolymers are 1,3-butadiene-ethylene copolymers that also contain saturated 6-member hydrocarbon cyclic units. However, these copolymers, when incorporated into a rubber composition, can impart excessive rigidity to the rubber composition. This high rigidity is attributed to the equally high rigidity of the elastomer. High rigidity in a rubber composition can be problematic because it can also render the rubber composition unsuitable for certain applications.

[0006] In pursuit of its goal of synthesizing ethylene-rich diene elastomers, the Applicant has discovered a new polymer that solves the aforementioned problems, notably by presenting an improved compromise between ethylene content, stiffness and crystallinity for use in tires.

[0007] Thus, a first object of the invention is a terpolymer, preferably an elastomer, of ethylene, of a first 1,3-diene having 4 to 6 carbon atoms and of a second 1,3-diene of formula (I), which terpolymer contains more than 50% by mole of ethylene units and at least 1% by mole of units of the second 1,3-diene, CH2=CR-CH=CH2(I) the symbol R representing a hydrocarbon chain having 3 to 20 carbon atoms.

[0008] A second object of the invention is a process for preparing the terpolymer according to the invention.

[0009] The invention also relates to a rubber composition comprising a terpolymer according to the invention, a reinforcing filler and a crosslinking system, which terpolymer is an elastomer.

[0010] The invention also relates to a tire which comprises a rubber composition according to the invention. Detailed description

[0011] Any range of values ​​designated by the expression "between a and b" represents the range of values ​​greater than "a" and less than "b" (i.e., excluding the bounds a and b), while any range of values ​​designated by the expression "from a to b" means the range of values ​​from "a" to "b" (i.e., including the strict bounds a and b). The abbreviation "pce" means parts by weight per hundred parts by weight of elastomer (of the total elastomers if multiple elastomers are present).

[0012] The expression "based on" used to define the constituents of a catalytic system or composition refers to the mixture of these constituents, or the product of the reaction of some or all of these constituents with each other.

[0013] Unless otherwise stated, the rates of units resulting from the insertion of a monomer into a copolymer are expressed as a mole percentage relative to the total monomer units of the copolymer.

[0014] The compounds mentioned in the description may be of fossil origin or bio-based. In the latter case, they may be partially or entirely derived from biomass or obtained from renewable raw materials derived from biomass. These include, in particular, elastomers, plasticizers, fillers, etc.

[0015] The essential characteristic of the first 1,3-diene is that it contains 4 to 6 carbon atoms. The first 1,3-diene is either a single compound, meaning a single 1,3-diene with 4 to 6 carbon atoms, or a mixture of different 1,3-dienes with 4 to 6 carbon atoms. Examples of 1,3-dienes with 4 to 6 carbon atoms include 1,3-butadiene and isoprene. The first 1,3-diene is preferentially 1,3-butadiene.

[0016] The second 1,3-diene has the essential characteristic of conforming to formula (I) in which the symbol R represents a hydrocarbon chain having 3 to 20 carbon atoms. CH₂=CR-CH=CH₂ (I)

[0017] The second 1,3-diene is a single compound, that is, a single (in English "one") 1,3-diene of formula (I) or is a mixture of 1,3-dienes of formula (I), the 1,3-dienes in the mixture being differentiated from one another by the group represented by the symbol R.

[0018] Preferably, the symbol R represents a hydrocarbon chain having 6 to 16 carbon atoms. The hydrocarbon chain represented by the symbol R may be saturated or unsaturated. Preferably, the symbol R represents an aliphatic chain, in which case, in formula (I) of 1,3-diene, the hydrocarbon chain represented by the symbol R is an aliphatic hydrocarbon chain. It may be a linear or branched chain, in which case the symbol R represents a linear or branched chain. Preferably, the hydrocarbon chain is acyclic, in which case the symbol R represents an acyclic chain. Better still, the symbol R represents an unsaturated, branched, acyclic hydrocarbon chain. The hydrocarbon chain represented by the symbol R is advantageously an unsaturated, branched, acyclic chain containing from 3 to 20 carbon atoms, particularly from 6 to 16 carbon atoms. Most advantageously, the 1,3-diene is myrcene or β-farnesene.According to a preferred embodiment of the invention, 1,3-diene is myrcene.

[0019] According to another preferred embodiment of the invention, 1,3-diene is β-farnesene.

[0020] The terpolymer according to the invention is a terpolymer of ethylene, a first 1,3-diene, and a second 1,3-diene, which implies that the monomer units of the terpolymer are units resulting from the polymerization of ethylene, the first 1,3-diene, and the second 1,3-diene. The copolymer therefore comprises ethylene units, units of the first 1,3-diene, and units of the second 1,3-diene.

[0021] Since the second 1,3-diene is a substituted 1,3-diene, its polymerization can give rise to units of configuration 1,2 represented by formula (1), of configuration 3,4 represented by formula (2) and of configuration 1,4 whose trans form is represented below by formula (3).

[0022] As is well known, the first 1,3-diene can give rise to 1,3-diene units which are units of configuration 1,2 or 3,4 as is the case for example of isoprene and units of configuration 1,4.

[0023] Also well known, the ethylene unit is a motif unit -(CH2-CH2)-.

[0024] The terpolymer according to the invention is advantageously a statistical terpolymer according to any one of the embodiments of the invention. Most advantageously, the terpolymer is an atactic polymer according to any one of the embodiments of the invention.

[0025] According to the invention, the terpolymer contains more than 50 mole percent of ethylene units. Preferably, the terpolymer contains more than 60 mole percent of ethylene units. More preferably, it contains at least 70 mole percent of ethylene units.

[0026] The terpolymer preferably contains at most 90% by mole of ethylene units, more preferably at most 85% by mole of ethylene units.

[0027] According to the invention, the terpolymer contains at least 1 mol% of units of the second 1,3-diene. Preferably, the terpolymer contains at most 20 mol% of units of the second 1,3-diene. More preferably, the terpolymer contains at most 10 mol% of units of the second 1,3-diene.

[0028] According to one embodiment of the invention, the terpolymer contains more than 60% to 90% by mole of ethylene units and from 1% to 20%, preferably from 1% to 10% by mole of units of the second 1,3-diene. According to this embodiment of the invention, the terpolymer preferably contains less than 30% by mole of units of the first 1,3-diene or preferably contains less than 20% by mole of units of the first 1,3-diene.

[0029] According to another embodiment of the invention, the terpolymer contains 70% to 90% by mole of ethylene units and 1% to 20%, preferably 1% to 10% by mole, of units of the second 1,3-diene. According to this embodiment of the invention, the terpolymer preferably contains less than 20% by mole of units of the first 1,3-diene.

[0030] According to yet another embodiment of the invention, the terpolymer contains more than 60% to 85% by mole of ethylene units and from 1% to 20%, preferably from 1% to 10% by mole of units of the second 1,3-diene. According to this embodiment of the invention, the terpolymer preferably contains less than 30% by mole of units of the first 1,3-diene or preferably contains less than 20% by mole of units of the first 1,3-diene.

[0031] According to yet another embodiment of the invention, the terpolymer contains 70% to 85% by mole of ethylene units and 1% to 20%, preferably 1% to 10% by mole, of units of the second 1,3-diene. According to this embodiment of the invention, the terpolymer preferably contains less than 20% by mole of units of the first 1,3-diene.

[0032] According to any one of the embodiments of the invention, the terpolymer preferably contains less than 80% by mole of ethylene units, more preferably at most 75% by mole of ethylene units.

[0033] According to a particular embodiment of the invention, especially when the first 1,3-diene is 1,3-butadiene or a mixture of 1,3-dienes, one of which is 1,3-butadiene, the terpolymer further contains 1,2-cyclohexanediyl repeating units. The presence of these cyclic structures in the terpolymer results from a very specific insertion of ethylene and 1,3-butadiene during polymerization. The content of 1,2-cyclohexanediyl repeating units in the terpolymer varies according to the respective contents of ethylene and 1,3-butadiene in the terpolymer. Preferably, the terpolymer contains less than 15 mole percent of 1,2-cyclohexanediyl repeating units.

[0034] Preferably, the terpolymer according to the invention has a glass transition temperature below -35°C, preferably between -70°C and -35°C.

[0035] The terpolymer can be prepared by a process, another object of the invention, which comprises the polymerization of a mixture of ethylene, the first 1,3-diene and the second 1,3-diene in the presence of a catalytic system based at least on a metallocene of formula (II) and an organomagnesium compound of formula (III) P(Cp 1< Cp 2< ) Nd(BH 4 ) (1+y)- L y -N x (II) MgR 1< R 2< (III) Cp1 and Cp2, identical or different, being chosen from the group consisting of the cyclopentadienyl group of formula C5H4, the unsubstituted fluorenyl group of formula C13H8, and the substituted fluorenyl groups, P being a bridging group between the two groups Cp1 and Cp2 and representing a ZR3R4 group, Z representing a silicon or carbon atom, R3 and R4, identical or different, each representing an alkyl group comprising from 1 to 20 carbon atoms, preferably a methyl group, y, an integer, being equal to or greater than 0, x, an integer or not, being equal to or greater than 0, L representing an alkali metal chosen from the group consisting of lithium, sodium, and potassium, N representing a molecule of an ether, preferably diethyl ether or tetrahydrofuran, R1 and R2, identical or different, representing a carbon group.

[0036] Examples of substituted fluorenyl groups include those substituted by alkyl radicals with 1 to 6 carbon atoms or by aryl radicals with 6 to 12 carbon atoms. The choice of radicals is also influenced by the availability of the corresponding molecules, namely the substituted fluorenes, because these are either commercially available or easily synthesized.

[0037] Examples of substituted fluorenyl groups include 2,7-ditertiobutyl-fluorenyl and 3,6-ditertiobutyl-fluorenyl. Positions 2, 3, 6, and 7 respectively designate the positions of the carbon atoms in the rings, as shown in the diagram below, with position 9 corresponding to the carbon atom to which the P-bridge is attached.

[0038] The catalytic system can be prepared conventionally by a process analogous to that described in patent applications WO 2007054224 or WO 2007054223. For example, the organomagnesium compound and the metallocene are typically reacted in a hydrocarbon solvent at a temperature ranging from 20 to 80°C for a duration of 5 to 60 minutes. The catalytic system is generally prepared in a hydrocarbon solvent, either aliphatic such as methylcyclohexane or aromatic such as toluene. Generally, after its synthesis, the catalytic system is used as is in the process for synthesizing the polymer according to the invention.

[0039] Alternatively, the catalytic system can be prepared by a process analogous to that described in patent application WO 2017093654 A1 or in patent application WO 2018020122 A1. According to this alternative, the catalytic system further contains a preforming monomer selected from a conjugated diene, ethylene or a mixture of ethylene and a conjugated diene, in which case the catalytic system is based at least on the metallocene, the organomagnesium and the preforming monomer. For example, the organomagnesium and metallocene are typically reacted in a hydrocarbon solvent at a temperature of 20 to 80°C for 10 to 20 minutes to obtain a first reaction product, then with this first reaction product the preforming monomer chosen from a conjugated diene, ethylene or a mixture of ethylene and a conjugated diene is reacted at a temperature of 40 to 90°C for 1h to 12h.The catalytic system thus obtained can be used immediately in the process according to the invention or stored under an inert atmosphere before its use in the process according to the invention.

[0040] The metallocene used to prepare the catalytic system can be in the form of crystalline or non-crystalline powder, or as single crystals. The metallocene can be monomeric or dimeric, depending on the method of preparation, as described in patent applications WO 2007054224 or WO 2007054223. The metallocene can be prepared conventionally by a process analogous to that described in patent applications WO 2007054224 or WO 2007054223, specifically by reacting, under inert and anhydrous conditions, the salt of an alkali metal of the ligand with a rare-earth borohydride in a suitable solvent, such as an ether like diethyl ether or tetrahydrofuran, or any other solvent known to those skilled in the art. After reaction, the metallocene is separated from the reaction byproducts by techniques known to those skilled in the art, such as filtration or precipitation in a second solvent.The metallocene is ultimately dried and insulated in solid form.

[0041] As with any synthesis carried out in the presence of organometallic compounds, the synthesis of the metallocene and that of the catalytic system take place under anhydrous conditions in an inert atmosphere. Typically, the reactions are conducted using solvents and anhydrous compounds under anhydrous nitrogen or argon.

[0042] The organomagnesium compound useful for the purposes of the invention has the formula MgR1<R2<, in which R1< and R2<, whether identical or different, represent a carbon group. A carbon group is understood to be a group containing one or more carbon atoms. Preferably, R1< and R2< contain 2 to 10 carbon atoms. More preferably, R1< and R2< each represent an alkyl group. The organomagnesium compound is advantageously a dialkylmagnesium compound, preferably butylethylmagnesium or butyloctylmagnesium, and even better, butyloctylmagnesium.

[0043] According to any one of the embodiments of the invention, the molar ratio of the organomagnesium to the Nd metal constituting the metallocene is preferably in the range of 1 to 100, more preferably greater than or equal to 1 and less than 10. The range of values ​​from 1 to less than 10 is in particular more favorable for obtaining polymers with high molar masses.

[0044] When the terpolymer is a polymer comprising 1,2-cyclohexanediyl motif units, it is prepared according to the process mentioned in this application using a metallocene of formula (II) in which Cp1 and Cp2, identical or different, are selected from the group consisting of substituted fluorenyl groups and the unsubstituted fluorenyl group of formula C13H8. For this variant, the following metallocenes of formula in which the symbol Flu represents the fluorenyl group of formula C13H8 are particularly suitable: [{Me2SiFlu2Nd(µ-BH4)2Li(THF)}2]; [Me2SiFlu2Nd(µ-BH4)2Li(THF)]; [Me2SiFlu2Nd(µ-BH4)(THF)]; [{Me 2 SiFlu 2 Nd(µ-BH 4 )(THF)} 2 ]; [Me 2 SiFlu 2 Nd(µ-BH 4 )].

[0045] A person skilled in the art also adapts the polymerization conditions and the concentrations of each of the reactants (components of the catalytic system, monomers) according to the equipment (tools, reactors) used to carry out the polymerization and the various chemical reactions. As is known to those skilled in the art, polymerization, as well as the handling of the monomers, the catalytic system, and the polymerization solvent(s), is performed under anhydrous conditions and in an inert atmosphere. Polymerization solvents are typically hydrocarbon, aliphatic, or aromatic solvents.

[0046] Polymerization is preferably carried out in solution, either continuously or batchwise. The polymerization solvent can be a hydrocarbon, aromatic, or aliphatic solvent. Examples of polymerization solvents include toluene and methylcyclohexane. Monomers can be introduced into the reactor containing the polymerization solvent and the catalytic system, or conversely, the catalytic system can be introduced into the reactor containing the polymerization solvent and the monomers. Copolymerization is typically carried out under anhydrous conditions and in the absence of oxygen, possibly with the addition of an inert gas. The polymerization temperature generally ranges from 30 to 150°C, preferably from 30 to 120°C. Preferably, copolymerization is carried out at a constant ethylene pressure.

[0047] During the polymerization of ethylene and 1,3-dienes in a polymerization reactor, a continuous addition of ethylene and 1,3-dienes can be carried out in the polymerization reactor, in which case the polymerization reactor is a fed reactor. This embodiment is particularly suitable for the synthesis of statistical terpolymers.

[0048] Polymerization can be stopped by cooling the polymerization medium or by adding an alcohol. The polymer can be recovered using conventional techniques known to those skilled in the art, such as precipitation, evaporation of the solvent under reduced pressure, or steam stripping.

[0049] Alternatively, instead of adding an alcohol, a functionalizing agent can be added, in which case a polymer bearing a functional group such as an amine, a silanol, or an alkoxysilane group is obtained. According to a particular embodiment of the invention, the terpolymer bears an amine, alkoxysilane, or silanol group.

[0050] According to a first variant in which the function carried by the polymer is an amine function, the functionalizing agent is preferably a compound of formula (IV), Si(Fc 1< ) 3-g (Rc 2< ) g (Rca) (IV) the symbols Fc 1< , identical or different, representing an alkoxy group, the symbols Rc 2< , identical or different, representing a hydrogen atom or a hydrocarbon chain, the symbol Rca representing a hydrocarbon chain substituted by an amine function, g being an integer from 0 to 1.

[0051] The alkoxy group represented by the symbol Fc 1< in formula (IV) is preferably methoxy or ethoxy.

[0052] The amine function designated by the symbol Rca in formula (IV), namely the amine function of the functionalizing agent, is a protected primary amine, a protected secondary amine, or a tertiary amine. Examples of protecting groups for the primary and secondary amine functions include silyl groups, for example, trimethylsilyl and tert-butyldimethylsilyl groups. Preferably, the amine function of the functionalizing agent is a tertiary amine. Advantageously, the amine function of the functionalizing agent is a tertiary amine of formula -N(RB)₂, in which each RB represents an alkyl group, preferably a methyl or an ethyl group.

[0053] As functionalizing agents for preparing a polymer bearing an amine function according to the first variant, the following compounds may be cited: (N,N-dimethyl-3-aminopropyl)methyldimethoxysilane, (N,N-dimethyl-3-aminopropyl)methyldiethoxysilane, (N,N-dimethyl-3-aminopropyl)ethyldimethoxysilane, (N,N-dimethyl-3-aminopropyl)ethyldiethoxysilane, (N,N-dimethylaminopropyl)trimethoxysilane, (N,N-dimethylaminopropyl)triethoxysilane and (N-(3-trimethoxysilyl)propyl)-N-(trimethylsilyl)silanamine, (N-(3-triethoxysilyl)propyl)-N-(trimethylsilyl)silanamine, preferably (N,N-dimethyl-3-aminopropyl)methyldimethoxysilane, (N,N-dimethyl-3-aminopropyl)ethyldimethoxysilane, (N,N-dimethylaminopropyl)trimethoxysilane and (N-(3-trimethoxysilyl)-propyl)-N-(trimethylsilyl)silanamine, more preferentially (N,N-dimethylaminopropyl)trimethoxysilane and (N-(3-trimethoxysilyl)propyl)-N-(trimethylsilyl)-silanamine.

[0054] According to a second variant in which the function carried by the polymer is a silanol or alkoxysilane function, the functionalizing agent is preferably a compound of formula (V), Si(Fc 1< ) 4-g (Rc 2< ) g (V) the symbols Fc 1< , identical or different, representing an alkoxy group or a halogen atom, the symbols Rc 2< , identical or different, representing a hydrogen atom, a hydrocarbon chain or a hydrocarbon chain substituted by a chemical function Fc 2< , g being an integer from 0 to 2.

[0055] When the symbol Fc 1< represents an alkoxy group in formula (V), the alkoxy group is preferably methoxy or ethoxy. When the symbol Fc 1< represents a halogen atom in formula (V), the halogen atom is preferably chlorine.

[0056] Among the hydrocarbon chains represented by the symbols Rc 2< in the formulas (V), we can mention alkyls, preferably alkyls having at most 6 carbon atoms, more preferably methyl or ethyl, better methyl.

[0057] Among the hydrocarbon chains substituted by a chemical function Fc 2< which are represented by the symbols Rc 2< in the formulas (V), we can cite the alkanediyl chains, preferably those having at most 6 carbon atoms, more preferably the 1,3-propanediyl group, the alkanediyl group bearing a substituent, the chemical function Fc 2<, in other words, one valence of the alkanediyl chain for the function Fc 2<, the other valence for the silicon atom of the methoxysilane function.

[0058] In formulas (V), a chemical function is defined as a group that is different from a saturated hydrocarbon group and that can participate in chemical reactions. Those skilled in the art understand that the chemical function Fc 2< in formula (V) is a group that is chemically inert with respect to the chemical species present in the polymerization medium. The chemical function Fc 2< in formula (V) may be in a protected form, such as a primary amine, secondary amine, or thiol. Examples of chemical functions Fc 2< include ether, thioether, protected primary amine, protected secondary amine, tertiary amine, protected thiol, and silyl. Preferably, the chemical function Fc 2< in formula (V) is a protected primary amine, a protected secondary amine, a protected tertiary amine, or a protected thiol.Examples of protecting groups for primary amine, secondary amine and thiol functions include silyl groups, for example trimethylsilyl and tert-butyldimethylsilyl groups.

[0059] As functionalizing agents for preparing a polymer bearing a silanol or alkoxysilane function according to the second variant, the following compounds can be cited: dimethoxydimethylsilane, diethoxydimethylsilane, dimethoxydiethylsilane, diethoxydiethylsilane, (N,N-dimethyl-3-aminopropyl)methyldimethoxysilane, (N,N-dimethyl-3-aminopropyl)methyldiethoxysilane, (N,N-dimethyl-3-aminopropyl)ethyldimethoxysilane, (N,N-dimethyl-3-aminopropyl)ethyl-diethoxysilane, 3-methoxy-3,8,8,9,9-pentamethyl-2-oxa-7-thia-3,8-disiladecane, trimethoxy-methylsilane, triethoxymethylsilane, trimethoxyethylsilane, triethoxyethylsilane, (N,N-dimethylaminopropyl)trimethoxysilane, (N,N-dimethylaminopropyl)triethoxysilane, (N-(3-trimethoxysilyl)propyl)-N-(trimethylsilyl)silanamine, (N-(3-triethoxysilyl)propyl)-N-(trimethylsilyl)silanamine and 3,3-dimethoxy-8,8,9,9-tetramethyl-2-oxa-7-thia-3,8-disiladecane, preferably dimethoxydimethylsilane, dimethoxydiethylsilane, (N,N-dimethyl-3-aminopropyl)methyldimethoxysilane, (N,N-dimethyl-3-aminopropyl)ethyldimethoxysilane, 3-methoxy-3,8,8,9,9-pentamethyl-2-oxa-7-thia-3,8-disiladecane, trimethoxymethylsilane, trimethoxyethylsilane, (N,N-dimethylaminopropyl)trimethoxysilane, (N-(3-trimethoxysilyl)-propyl)-N-(trimethylsilyl)silanamine and 3,3-dimethoxy-8,8,9,9-tetramethyl-2-oxa-7-thia-3,8-disiladecane, more preferably trimethoxymethylsilane, trimethoxyethylsilane, (N,N-dimethylaminopropyl)trimethoxysilane, (N-(3-trimethoxysilyl)propyl)-N-(trimethylsilyl)-silanamine and 3,3-dimethoxy-8,8,9,9-tetramethyl-2-oxa-7-thia-3,8-disiladecane.

[0060] Whether using the first or second variant, the functionalizing agent is typically added to the polymerization medium. It is typically added at a monomer conversion rate chosen by those skilled in the art, according to the desired copolymer macrostructure. Since the polymerization step is generally carried out under ethylene pressure, degassing of the polymerization reactor may be performed before adding the functionalizing agent. The functionalizing agent is added under inert and anhydrous conditions to the polymerization medium, which is maintained at the polymerization temperature. Typically, 0.25 to 10 moles of functionalizing agent are used per 1 mole of cocatalyst, preferably 2 to 4 moles of functionalizing agent per 1 mole of cocatalyst.The functionalizing agent is brought into contact with the polymerization medium for a sufficient time to allow the functionalization reaction to occur. This contact time is judiciously chosen by a person skilled in the art, depending on the concentration and temperature of the reaction medium. Typically, the functionalization reaction is carried out under stirring, at a temperature ranging from 17 to 80°C, for 0.01 to 24 hours.

[0061] When the functionalizing agent carries a protected function as described above, the polymer functionalization step can be followed by a hydrolysis reaction to form a copolymer carrying a deprotected function, such as a primary amine, a secondary amine, or a thiol function.

[0062] A hydrolysis reaction can also follow the polymer functionalization reaction when the functionalization reaction leads to the formation of a polymer bearing an alkoxysilane functional group. Hydrolysis of the alkoxysilane-bearing polymer leads to the preparation of a silanol-bearing polymer.

[0063] The terpolymer according to the invention, described in any one of its embodiments, including preferred variants thereof, exhibits both lower stiffness and a comparable, or even lower, degree of crystallinity than an ethylene-1,3-butadiene copolymer with the same ethylene content. Substituting the terpolymer according to the invention for an ethylene-1,3-butadiene copolymer with the same ethylene content in a rubber composition results in a rubber composition with lower stiffness. The terpolymer according to the invention is advantageously an elastomer. It is particularly intended for use in rubber compositions, especially for tires.

[0064] The rubber composition, another object of the invention, is characterized by comprising the elastomer according to the invention, a reinforcing filler and a crosslinking system.

[0065] The rubber compound may include any type of reinforcing filler, known for its ability to strengthen a rubber compound suitable for tire manufacturing. Examples include an organic filler such as carbon black, an inorganic reinforcing filler such as silica combined with a known coupling agent, or a mixture of both types of filler. Such a reinforcing filler typically consists of nanoparticles with an average size (by mass) of less than one micrometer, generally less than 500 nm, most often between 20 and 200 nm, and particularly and preferentially between 20 and 150 nm. The reinforcing filler content is adjusted by a person skilled in the art according to the intended use of the rubber compound.

[0066] The crosslinking system can be based on sulfur, sulfur donors, peroxides, bismaleimides, or mixtures thereof. The crosslinking system is preferably a vulcanization system, that is, a system based on sulfur (or a sulfur-donating agent) and a primary vulcanization accelerator. Various known secondary accelerators or vulcanization activators, such as zinc oxide, stearic acid or equivalent compounds, guanidine derivatives (particularly diphenylguanidine), or known vulcanization retarders, can be added to this basic vulcanization system.

[0067] The rubber composition may also contain other additives known to be used in tire rubber compositions, such as plasticizers, anti-ozonants, and antioxidants.

[0068] The rubber composition according to the invention is typically manufactured in suitable mixers, using two successive preparation phases well known to those skilled in the art: a first thermomechanical working or mixing phase (the so-called "non-productive" phase) at high temperature, up to a maximum temperature between 130°C and 200°C, followed by a second mechanical working phase (the so-called "productive" phase) down to a lower temperature, typically below 110°C, for example between 40°C and 100°C, a finishing phase during which the crosslinking system is incorporated.

[0069] The rubber composition according to the invention, which can be either in the raw state (before crosslinking or vulcanization) or in the cooked state (after crosslinking or vulcanization), can be used in a semi-finished article for pneumatics.

[0070] The aforementioned features of the present invention, as well as others, will be better understood upon reading the following description of several examples of embodiments of the invention, given by way of illustration and not limitation. Example 1) Determination of the macrostructure of polymers by size exclusion chromatography (SEC): a) Principle of measurement:

[0071] Size exclusion chromatography (SEC) separates macromolecules in solution according to their size using columns filled with a porous gel. The macromolecules are separated according to their hydrodynamic volume, with the largest being eluted first.

[0072] Combined with 3 detectors (3D), a refractometer, a viscometer, and a 90° light scattering detector, SEC allows for the determination of the absolute molar mass distribution of a polymer. The various absolute molar masses, average by number (Mn), by weight (Mw), and the dispersity ( Ð (Mw / Mn) can also be calculated. b) Polymer preparation:

[0073] Each sample is solubilized in tetrahydrofuran at a concentration of approximately 1 g / L. The solution is then filtered through a 0.45µm porosity filter before injection. c) SEC 3D Analysis:

[0074] To determine the number-average molar mass (Mn), and where applicable the weight-average molar mass (Mw) and the polydispersity index (Ip) of the polymers, the method below is used.

[0075] The number-average molar mass (Mn), weight-average molar mass (Mw), and polydispersity index of the polymer (hereafter referred to as the sample) are determined in absolute terms by triple-detection size exclusion chromatography (SEC). Triple-detection size exclusion chromatography has the advantage of directly measuring average molar masses without calibration.

[0076] The refractive index increment (dn / dc) of the sample solution is measured online using the peak area detected by the refractometer (RI) of the liquid chromatography equipment. To apply this method, it is essential to ensure that 100% of the sample mass is injected and eluted through the column. The RI peak area depends on the sample concentration, the RI detector constant, and the dn / dc value. To determine the average molar masses, a previously prepared and filtered 1 g / L solution is injected into the chromatographic system. The equipment used is a WATERS Alliance chromatographic system. The elution solvent is tetrahydrofuran containing 250 ppm of BHT (2,6-diter-butyl 4-hydroxytoluene), the flow rate is 1 mL.min-1, the system temperature is 35°C and the analysis time is 60 min.The columns used are a set of three AGILENT columns, commercially known as "PL GEL MIXED B LS". The injected volume of the sample solution is 100 µL. The detection system consists of a Wyatt differential viscometer, commercially known as "VISCOSTAR II", a Wyatt differential refractometer, commercially known as "OPTILAB T-REX" with a wavelength of 658 nm, and a Wyatt multi-angle static light scattering detector, commercially known as "DAWN HELEOS 8+", with a wavelength of 658 nm.

[0077] For the calculation of the number-average molar masses and the polydispersity index, the value of the refractive index increment dn / dc of the sample solution obtained above is incorporated. The software used for processing the chromatographic data is Wyatt's ASTRA system. 2) Determination of the polymer microstructure: a) Determination of the microstructure of Ethylene-Butadiene-Myrcene terpolymers:

[0078] Spectral characterization and microstructure measurements of Ethylene-Butadiene-Myrcene copolymer are performed by Nuclear Magnetic Resonance (NMR) spectroscopy.

[0079] For these measurements, a Bruker Avance III HD 400 MHz spectrometer is used, equipped with a Bruker cryo-BBFO z-grad 5 mm probe. The 1H experiments are recorded using a radiofrequency pulse with a 30° flip angle, with 128 repetitions and a 5-second recycle time. The 1H-13C HSQC (Heteronuclear Single Quantum Coherence) and HMBC (Heteronuclear Multiple-Bond Correlation) correlation NMR experiments are recorded with 128 repetitions and 128 increments. The experiments are performed at 25 °C.

[0080] 25 mg of sample are solubilized in 1 mL of deuterated orthodichlorobenzene (ODCB).

[0081] The axes of the chemical shifts 1H and 13C are calibrated with respect to the protonated impurity of the solvent at δ1H = 7.2 ppm (for the most deshielded signal) and δ13C = 127 ppm (for the least deshielded signal).

[0082] The possible monomer units in the terpolymer are -CH2-CH(CH=CH2)-, -CH2-CH=CH-CH2-, -CH2-CH2-, the 1,2-cyclohexanediyl motif and the following structures, R1 and R2 representing the polymer chain:

[0083] The 1,2-cyclohexanediyl motif has the following structure:

[0084] The signals of the myrcene A insertion forms were observed on the different recorded spectra. According to S. Georges et al.(S.Georges, M.Bria, P. Zinck and M. Visseaux. Polymer 55 (2014) 3869-3878), the signal of the -CH= group no. 8" characteristic of the C form has chemical shifts 1< H and 13< C identical to the -CH= group no. 3.

[0085] The chemical shifts of the characteristic signals of the polymer are presented in Table 1 (Assignment of the 1< H and 13< C signals of Ethylene-Butadiene-Myrcene terpolymers other than those of the 1,3-butadiene units). Table 1 δ1H (ppm) δ13C (ppm) Group 5.19 125.1 3 + 8" 4.86 109.0 7 1.59 and 1.68 247 and 17.6 1 1.3 37.5 - 24.0 CH2 ethylene

[0086] Quantifications were performed from the integration of 1D 1< H NMR spectra using Topspin software.

[0087] The integrated signals for quantifying the different patterns are: Ethylene: Total signals between 0.5 ppm and 3.0 ppm, subtracting the aliphatic contributions from the other terpolymer units. The calculation corresponds to 4 protons from the Ethylene unit. Form A: signal #7 (4.86 ppm) corresponding to 2 protons.

[0088] The proportion of form C is not directly accessible but can be calculated from signal #3+8" by subtracting the contribution of form A. PB1-4: Signal between 5.71 ppm and 5.32 ppm corresponds to 2 protons (after removing the PB1-2 contribution). PB1-2: Signal between 5.11 ppm and 4.92 ppm corresponds to 2 protons. Cyclohexane rings: Signal between 1.80 ppm and 1.70 ppm corresponds to 2 protons.

[0089] The quantification of the microstructure is carried out in molar percentage (% molar) as follows: % molaire d ′ un motif = intégrale H <mprescripts / > <none / > 1 d ′ un motif * 100 / Σ intégrale H <mprescripts / > <none / > 1 de chaque motif . b) Determination of the microstructure of ethylene-butadiene-farnesene terpolymers:

[0090] Spectral characterization and microstructure measurements of ethylene-butadiene-farnesene copolymer were performed by Nuclear Magnetic Resonance (NMR) spectroscopy. A Bruker Avance III HD 400 MHz spectrometer, equipped with a Bruker cryo-BBFO z-grad 5 mm probe, was used for these measurements. The 1H experiments were recorded using a radiofrequency pulse with a 30° flip angle, with 128 repetitions and a 5-second recycle time. The 1H-13C HSQC (Heteronuclear Single Quantum Coherence) and HMBC (Heteronuclear Multiple-Bond Correlation) correlation NMR experiments were recorded with 128 repetitions and 128 increments. The experiments were performed at 25 °C. 25 mg of sample are solubilized in 1 mL of deuterated orthodichlorobenzene (ODCB).The axes of chemical shifts 1< H and 13< C are calibrated with respect to the protonated impurity of the solvent at δ 1H = 7.2 ppm (for the most deshielded signal) and δ 13C = 127 ppm (for the least deshielded signal).

[0091] The possible monomer units in the terpolymer are -CH2-CH(CH=CH2)-, -CH2-CH=CH-CH2-, -CH2-CH2-, the 1,2-cyclohexanediyl motif and the following structures, R1 and R2 representing the polymer chain:

[0092] The signals of the insertion form of farnesene A were observed on the different recorded spectra. The signal of the -CH= group #11, characteristic of form C, shows chemical shifts 1 < H and 13 < C identical to the -CH= groups #3 and #7.

[0093] The chemical shifts of the characteristic signals of the polymer are presented in Table 2 (Assignment of the 1< H and 13< C signals of Ethylene-Butadiene-Farnesene terpolymers other than those of the 1,3-butadiene units). Table 2 (ppm) δ13C (ppm) Group 5.25 125.0 7 5.15 125.0 3, 11" 4.87 109.0 14 1.59 and 1.67 24.6 and 17.5 1, 13 1.28 38 - 24.0 CH2 ethylene

[0094] Quantifications were performed from the integration of 1D 1< H NMR spectra using Topspin software.

[0095] The integrated signals for quantifying the different patterns are: Farnesene pattern form A from the signal No. 14 CH 2 = for 2 protons, Farnesene motif form C from the signals No. 3, 11" and No. 7 CH=(Subtracting the contribution of form A), for 2 protons, Farnesene motif form B: from signal #11', specific to this form, for 1 proton. PB1-4: Signal between 5.71 ppm and 5.32 ppm corresponds to 2 protons (removing the contribution of PB1-2). PB1-2: signal between 5.11 ppm and 4.92 ppm corresponds to 2 protons. Cyclohexane rings: signal between 1.80 ppm and 1.70 ppm corresponds to 2 protons. Ethylene motif by integrating all aliphatic signals (from ~0.5 to 3 ppm) and subtracting the contribution of all other aliphatic motifs (PB1-4, PB1-2, EBR ring, farnesene forms A and C).

[0096] The quantification of the microstructure is carried out in molar percentage (% molar) as follows: % molaire d ′ un motif = intégrale H <mprescripts / > <none / > 1 d ′ un motif * 100 / Σ intégrale H <mprescripts / > <none / > 1 de chaque motif 3) Determination of the glass transition temperature of polymers:

[0097] The glass transition temperature is measured using a Differential Scanning Calorimeter according to ASTM D3418 (1999). 4) Determination of polymer stiffness (as is):

[0098] The measurements are performed on an Anton Paar MCR301 rheometer in Shear mode with cylindrical specimens of controlled geometry (thickness between 1.5 mm and 3 mm and diameter between 22 mm and 28 mm). The sample is subjected to sinusoidal shear loading at a fixed temperature (corresponding to the end of the glass transition of the elastomer on a 10 Hz temperature sweep), and over a frequency range from 0.01 Hz to 100 Hz. The stiffness value retained as the stiffness of the rubbery plateau of the sample is the value of the shear modulus G' for the frequency at which the loss modulus G" reaches its minimum, in accordance with the method described by C. Liu, J. He, E. van Ruymbeke, R. Keunings, C. Bailly, Evaluation of different methods for the determination of the plateau modulus and the entanglement molecular weight, Polymer 47 (2006) 4461-4479. 5) Determination of the degree of crystallinity of polymers:

[0099] ISO 11357-3:2011 is used to determine the melting and crystallization temperatures and enthalpies of polymers by differential scanning calorimetry (DSC). The reference enthalpy of polyethylene is 277.1 J / g (from Handbook of Polymers, 4th Edition, J. Brandrup, E.H. Immergut, and E.A. Grulke, 1999). 6) Polymer synthesis:

[0100] In the synthesis of terpolymers according to the invention, the first 1,3-diene used is 1,3-butadiene, the second 1,3-diene is myrcene or β-farnesene. Myrcene is a 1,3-diene of formula (I) in which R is a hydrocarbon group having 6 carbon atoms of formula CH2-CH2-CH=CMe2.

[0101] All reagents are obtained commercially except for the metallocenes [{Me 2 SiFlu 2 Nd(µ-BH 4 ) 2 Li(THF)}] and [Me 2 SiCpFluNd(µ-BH 4 ) 2 Li(THF)] which are prepared according to the operating procedures described in patent applications WO 2007054224 and WO 2007054223.

[0102] Butylloctylmagnesium BOMAG (20% in heptane, C = 0.88 mol L⁻¹) is sourced from Chemtura and stored in a Schlenk tube under an inert atmosphere. The N35 grade ethylene is sourced from Air Liquide and used without prior purification. The myrcene (purity ≥95%) is obtained from Sigma-Aldrich. Example 1 not in accordance with the invention: synthesis of a copolymer of ethylene and 1,3-butadiene

[0103] The polymer is synthesized according to the following procedure: In a 500 mL glass reactor containing 300 mL of toluene, the co-catalyst, butylmagnesium (BOMAG), is added, followed by the metallocene [Me₂SiCpFluNd(µ-BH₄)₂Li(THF)]. The alkylation time is 10 minutes, and the reaction temperature is 20 °C. The respective quantities of the catalytic system components are given in Table 3. Next, the monomers are added in the proportions indicated in Table 3, with ethylene (Eth) and 1,3-butadiene (Bde) in the form of a gaseous mixture. Polymerization is carried out at 80 °C and a constant ethylene pressure of 4 bar.

[0104] The polymerization reaction is stopped by cooling, degassing the reactor, and adding 10 mL of ethanol. An antioxidant is added to the polymer solution. The copolymer is recovered by drying in a vacuum oven until a constant mass is reached. Example 2 not in accordance with the invention: synthesis of a copolymer of ethylene and 1,3-butadiene

[0105] The polymer is synthesized according to the following procedure: In a 500 mL glass reactor containing 300 mL of methylcyclohexane, the co-catalyst, butylmagnesium (BOMAG), is added, followed by the metallocene [Me₂Si(Flu)₂Nd(µ-BH₄)₂Li(THF)]. The alkylation time is 10 minutes, and the reaction temperature is 20 °C. The respective quantities of the catalytic system components are given in Table 3. Next, the monomers are added in the proportions indicated in Table 3, with ethylene (Eth) and 1,3-butadiene (Bde) in the form of a gaseous mixture. Polymerization is carried out at 80 °C and a constant ethylene pressure of 4 bar.

[0106] The polymerization reaction is stopped by cooling, degassing the reactor, and adding 10 mL of ethanol. An antioxidant is added to the polymer solution. The copolymer is recovered by drying in a vacuum oven until a constant mass is reached. Example 3 not in accordance with the invention: synthesis of a copolymer of ethylene and 1,3-butadiene

[0107] The polymer is synthesized according to the following procedure: In a reactor containing methylcyclohexane, the co-catalyst, butylmagnesium (BOMAG), is added, followed by the metallocene [Me₂Si(Flu)₂Nd(µ-BH₄)₂Li(THF)]. The alkylation time is 10 minutes, and the reaction temperature is 20 °C. The respective quantities of the catalytic system components are given in Table 3.

[0108] Next, the monomers are added continuously in the respective quantities indicated in Table 3. Polymerization is carried out at 80°C and a constant pressure of 4 bar. The polymerization reaction is stopped by cooling, degassing the reactor, and adding ethanol. An antioxidant is added to the polymer solution. The copolymer is recovered by drying in a vacuum oven until a constant mass is reached. Examples 4 to 10 according to the invention: terpolymers of ethylene, 1,3-butadiene and myrcene or β-farnesene

[0109] The polymers are synthesized according to the following procedure: In a reactor containing the hydrocarbon solvent: methylcyclohexane (MCH), the co-catalyst, butylmagnesium (BOMAG), is added, followed by the metallocene [Me 2 Si(Flu) 2 Nd(µ-BH 4 ) 2 Li(THF)]. The alkylation time is 10 minutes, the reaction temperature is 20 °C.

[0110] Polymerization is carried out at 80°C and a constant pressure of 4 bar in a 500 mL glass reactor containing 300 mL of polymerization solvent, methylcyclohexane, the catalytic system, and the monomers. Myrcene (Myr) or β-farnesene (Far) is introduced into the reactor in liquid form, and ethylene / 1,3-butadiene is introduced in gaseous form. The polymerization reaction is stopped by cooling and degassing the reactor. The copolymer is recovered by precipitation in methanol and then dried, either directly or through direct drying.

[0111] The respective quantities of the constituents of the catalytic system, the composition of the gas mixture in ethylene and 1,3-butadiene in the feed and the quantity of myrcene or β-farnesene are shown in Table 4. 7) Results:

[0112] The characteristics of the synthesized polymers are shown in Tables 5 and 6.

[0113] The elastomers in Examples 4 through 9 have a much lower degree of crystallinity than the elastomer in Example 1, even though they have a comparable or even higher ethylene unit content. Even the elastomers in Examples 7 and 10, which have an ethylene unit content exceeding 80%, have a much lower degree of crystallinity than the elastomer in Example 1, despite the latter having a significantly lower ethylene unit content.

[0114] Comparison of Examples 4, 6, and 8 with Example 2 further shows that the insertion of second 1,3-diene units into the polymer at levels according to the invention reduces the polymer's stiffness, even though the ethylene unit levels are comparable. Comparison of Examples 5 and 9 with Example 2 also shows that the insertion of second 1,3-diene units at levels as low as 1% reduces the elastomer's stiffness, even though the elastomers in Examples 5 and 9 have an ethylene unit level as high as that of Example 2. The elastomers in Examples 5 and 9 also have a lower degree of crystallinity than the elastomer in Example 3, which has a similar ethylene unit level.

[0115] The presence in an ethylene-rich diene copolymer of units of a 1,3-diene of formula (I) in the rates according to the invention makes it possible to improve the trade-off between the rate of ethylene units in the polymer, the degree of crystallinity and the rigidity of the polymer. Table 3 Ex Metallocene (mmol / L) Alkylating agent (mmol / L) Composition of gaseous mixture (%mol Eth / Bde) 1 0.32 0.97 80 / 20 2 0.16 0.78 70 / 30 3 0.15 0.75 80 / 20 Table 4 Ex Metallocene (mmol / L) Alkylating agent (mmol / L) Composition of gaseous mixture (%mol Eth / Bde) Volume of Myr (ml) Volume of Far (ml) Mn (g / mol) IP 4 0.00015 0.00047 80 / 20 5 92500 1.7 5 0.00015 0.00047 80 / 20 1 105200 1.7 6 0.00015 0.00047 80 / 20 10 112900 2.0 7 0.00015 0.00047 90 / 10 5 - - 8 0.00015 0.00047 80 / 20 5 103300 1.8 9 0.00015 0.00047 80 / 20 1 94000 1.6 10 0.00015 0.00047 90 / 10 3 - - Table 5 Ex Eth unit (%mol) Unit of Bde (%mol) 1,2-cyclohexanediyl motif (%mol) Tg (°C) Crystallinity (%) Unladen stiffness (MPa) 1 73.5 26.5 -54 31 - 2 71 16 13 -35 0 1 3 76.7 11.4 11.9 -38 2.1 - Table 6 Ex Eth unit (%mol) Unit of Bde (%mol) Myr unit (%mol) Far unit (%mol) 1,2-cyclohexane diyl motif (%mol) Tg (°C) Crystallinity (%) Unladen stiffness (MPa) 4 70 12 7 11 -47 0% 0.8 5 75 12 1 12 -39 0.7% 0.9 6 71 11 9 9 -48 0% 0.7 7 82 5 6 7 -47 10.9% - 8 72 12 5 11 -49 0.6% 0.8 9 74 13 1 12 -39 0% 0.9 10 85 5 3 7 -44 14.9% -

Claims

1. Terpolymer of ethylene, of a first 1,3-diene having from 4 to 6 carbon atoms and of a second 1,3-diene of formula (I), which terpolymer contains more than 50 mol% of ethylene units and at least 1 mol% of units of the second 1,3-diene,         CH2=CR-CH=CH2     (I) the symbol R representing a hydrocarbon chain having from 3 to 20 carbon atoms.

2. Terpolymer according to Claim 1, which terpolymer contains more than 60 mol% of ethylene units, preferentially at least 70 mol% of ethylene units.

3. Terpolymer according to Claim 1 or 2, which terpolymer contains at most 90 mol% of ethylene units, preferably at most 85 mol% of ethylene units.

4. Terpolymer according to any one of Claims 1 to 3, which terpolymer contains at most 20 mol% of unit of the second 1,3-diene, preferentially at most 10 mol% of unit of the second 1,3-diene of formula (I).

5. Terpolymer according to Claims 2, 3 and 4, which terpolymer contains less than 30 mol% of unit of the first 1,3-diene or less than 20 mol% of unit of the first 1,3-diene.

6. Terpolymer according to any one of Claims 1 to 5, which terpolymer contains less than 80 mol% of ethylene unit, preferably at most 75 mol% of ethylene unit.

7. Terpolymer according to any one of Claims 1 to 6, in which the first 1,3-diene is 1,3-butadiene or isoprene.

8. Terpolymer according to any one of Claims 1 to 7, which terpolymer additionally contains units of 1,2-cyclohexanediyl motif and in which the first 1,3-diene is 1,3-butadiene or a mixture of first 1,3-dienes, one of which is 1,3-butadiene.

9. Terpolymer according to any one of Claims 1 to 8, in which the symbol R represents a hydrocarbon chain having from 6 to 16 carbon atoms, preferably an acyclic chain.

10. Terpolymer according to any one of Claims 1 to 9, in which the second 1,3-diene is myrcene or β-farnesene.

11. Terpolymer according to any one of Claims 1 to 10, which terpolymer is a statistical terpolymer.

12. Terpolymer according to any one of Claims 1 to 11, which terpolymer bears an amine, alkoxysilane or silanol functional group.

13. Rubber composition which comprises at least one terpolymer defined according to any one of Claims 1 to 12, a reinforcing filler and a crosslinking system, which terpolymer is an elastomer.

14. Tyre which comprises a rubber composition defined according to Claim 13.

15. Process for preparing the terpolymer according to any one of Claims 1 to 13, which comprises the polymerization of a mixture of ethylene, of the first 1,3-diene and of the second 1,3-diene in the presence of a catalytic system based at least on a metallocene of formula (II) and on an organomagnesium compound         P(Cp1Cp2) Nd(BH4)(1+y)-Ly-Nx     (II)         MgR1R2     (III) Cp1 and Cp2, which are identical or different, being selected from the group consisting of the cyclopentadienyl group of formula C5H4, the unsubstituted fluorenyl group of formula C13H8 and substituted fluorenyl groups, P being a group bridging the two Cp1 and Cp2 groups and representing a ZR3R4 group, Z representing a silicon or carbon atom, R3 and R4, which are identical or different, each representing an alkyl group comprising from 1 to 20 carbon atoms, preferably a methyl, y, which is an integer, being equal to or greater than 0, x, which is or is not an integer, being equal to or greater than 0, L representing an alkali metal selected from the group consisting of lithium, sodium and potassium, N representing a molecule of an ether, preferably diethyl ether or tetrahydrofuran, R1 and R2, which are identical or different, representing a carbon group.