Method for the emulsion radical polymerisation of a monomer mixture of 1,3-butadiene and (METH)acrylate

Abstract

The present invention relates to a method for the cold emulsion radical polymerisation of a monomer mixture of 1,3-butadiene and of a (meth)acrylate containing 1,3-butadiene and an alkyl methacrylate, in which the polymerisation reaction is initiated by a redox system comprising an organic hydroperoxide and an iron(II) salt, in the presence of a mercaptan as a transfer agent. The relative amounts of initiator, transfer agent and monomers result in a macrostructure characterised by a novel trade-off between the number-average molar mass, Mn, and the dispersity, Đ. This trade-off further enhances the rolling resistance performance of a tyre comprising a rubber composition that comprises an elastomer prepared according to the method of the invention.

Description

[0001] Process for the radical emulsion polymerization of a monomer mixture of 1,3-butadiene and (meth)acrylate

[0002] The field of the present invention is that of cold radical emulsion polymerization processes of a monomer mixture of 1,3-butadiene and a (meth)acrylate.

[0003] It is known from patent application WO 2016 / 001052 that the use in a rubber composition for a tire tread of elastomers comprising more than 20 mol% of monomer units of a methacrylic acid ester improves the wet grip performance of the tire while maintaining a good compromise between performance and rolling resistance. The elastomers described in this patent application WO 2016 / 001052 are prepared by hot radical emulsion polymerization (50°C). The hot radical emulsion polymerization of 1,3-butadiene has the disadvantage of producing macrogel and branched chains in the elastomer. However, the presence of macrogel and branched chains in an elastomer is known to reduce the rolling resistance performance of a tire whose tread contains such an elastomer.

[0004] Cold radical emulsion polymerization of a monomer mixture of styrene, 1,3-butadiene, and a carbonate-substituted alkyl methacrylate has been described in WO 2018015646 A1 to provide an elastomer that improves the reinforcing properties of a silica-reinforced rubber composition. The improved reinforcement of the rubber composition is attributed to the presence of the carbonate functions in the elastomer. Since the elastomer was synthesized by cold radical emulsion polymerization, its macrogel content is less than 0.3% and its branched chain content is almost zero. However, the molar mass distribution of the elastomer is relatively broad compared to its number-average molar mass.

[0005] There is therefore a need to provide an elastomer containing butadiene units and methacrylate units which does not have the mentioned disadvantages of the above-mentioned elastomers.

[0006] Continuing the research efforts to solve the problem posed, the Applicant has discovered a new process for the cold radical emulsion polymerization of a monomer mixture of 1,3-butadiene and a (meth)acrylate containing 1,3-butadiene and an alkyl methacrylate. The process, which is available in two alternatives, leads to the synthesis of copolymers containing butadiene units and alkyl methacrylate units and having a macrostructure which allows them to further improve the rolling resistance performance of a tire. The macrostructure is characterized by a new compromise between the number-average molar mass, Mn, and the dispersity, £).

[0007] Thus the invention is a process for the cold radical emulsion polymerization of a monomer mixture of 1,3-butadiene and a (meth)acrylate containing 1,3-butadiene and an alkyl methacrylate, in which process the polymerization reaction is initiated by a redox system comprising an organic hydroperoxide and an iron (II) salt in the presence of a mercaptan as transfer agent, the amounts of organic hydroperoxide and mercaptan fulfilling conditions a) or conditions b): conditions a) being defined by a ratio between the number of moles of organic hydroperoxide and the number of moles of monomers which is less than 10 -4 and a ratio between the number of moles of mercaptan and the number of moles of organic hydroperoxide which is greater than 2, conditions b) being defined by a ratio between the number of moles of organic hydroperoxide and the number of moles of monomers which is greater than or equal to 9xl0 -4and a ratio between the number of moles of mercaptan and the number of moles of organic hydroperoxide which is less than 0.5, the polymerization reaction being carried out at a temperature between 0° and 10°C.

[0008] Detailed description

[0009] Any interval of values ​​designated by the expression "between a and b" represents the domain of values ​​greater than "a" and 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 ​​from "a" to "b" (i.e., including the strict limits "a" and "b").

[0010] The compounds mentioned in the description 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. Similarly, the compounds mentioned may also come from the recycling of materials already in use, 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.

[0011] In the present invention, the term "tyre" means a pneumatic or non-pneumatic tire. A pneumatic tire usually comprises two beads intended to come into contact with a rim, a crown composed of at least one crown reinforcement and a tread, two sidewalls, the tire being reinforced by a carcass reinforcement anchored in the two beads. A non-pneumatic tire, for its part, usually comprises a base, designed for example for mounting on a rigid rim, a crown reinforcement, ensuring the connection with a tread and a deformable structure, such as spokes, ribs or cells, this structure being arranged between the base and the crown. Such non-pneumatic tires do not necessarily comprise a sidewall. Non-pneumatic tires are described for example in documents WO 03 / 018332 and FR2898077.According to any one of the embodiments of the invention, the tire according to the invention is preferably a pneumatic bandage.

[0012] All percentages are by mass unless otherwise stated.

[0013] The molar percentages relating to the composition of the copolymer in accordance with the invention are calculated relative to the totality of the monomer units of the copolymer. As is known, the term (meth)acrylate designates either an acrylate or a methacrylate.

[0014] Since the monomer mixture to be polymerized contains 1,3-butadiene and a (meth)acrylate including an alkyl methacrylate, the copolymers prepared according to the process according to the invention are copolymers of 1,3-butadiene and an alkyl methacrylate or copolymers of 1,3-butadiene, an alkyl methacrylate and a (meth)acrylate other than an alkyl methacrylate.

[0015] The process according to the invention is a cold radical emulsion polymerization process, because the production of free radicals useful for initiating the polymerization reaction is done without thermal activation thanks to the presence of the redox system composed of the Fe(ll) salt and the organic hydroperoxide.

[0016] The use of a redox system in a radical emulsion polymerization is well known for being able to polymerize cold, typically around 5°C. In the process according to the invention, the polymerization reaction is carried out at a temperature between 0° and 10°C. The radical cold emulsion polymerization of 1,3-butadiene is known to produce copolymers with a macrogel content of less than 0.3% by mass of the copolymer mass and without branched chains. The radical cold emulsion polymerization of 1,3-butadiene is also known to promote the insertion of 1,3-butadiene in the form of 1,4-trans butadiene units which then represent in the copolymer at least 70% by mole of the butadiene units of the copolymer, compared to less than 65% for a polymerization at 50°C. The copolymer according to the invention typically contains 1,4-trans butadiene units which represent more than 70 mol% of the butadiene units of the copolymer.1,4-trans butadiene units refer to butadiene units that are inserted into the copolymer chain in the 1,4-trans form.

[0017] The polymerization reaction is therefore initiated by free radicals resulting from the transfer of electrons from Fe(ll) to the organic hydroperoxide. The organic hydroperoxide is therefore considered to be the initiator of the polymerization reaction. Suitable organic hydroperoxides are, for example, tert-butyl hydroperoxide, cumene hydroperoxide, p-menthane hydroperoxide. Preferably, the organic hydroperoxide is cumene hydroperoxide.

[0018] The Fe(ll) salt may be, for example, ferrous sulfate, iron(ll) pyrophosphate or their hydrates. The iron(ll) salt is preferably ferrous sulfate or a ferrous sulfate hydrate, more preferably ferrous sulfate heptahydrate, FeSO4-7H2O. The iron(ll) salt is generally used in an amount corresponding to the stoichiometry of the redox reaction, i.e., an Fe(ll) / initiator ratio between the number of moles of Fe(ll) and the number of moles of initiator equal to 1, or close to the stoichiometry, i.e., an Fe(ll) / initiator ratio ranging from 0.9 to 1.0.

[0019] Also known to control the gel rate and the macrostructure of the copolymer, a transfer agent is introduced into the polymerization medium and the conversion of the monomers is typically limited to a conversion of less than 75%, preferably less than 60%. As transfer agents, mention may be made of mercaptans with a chain length of 10 to 14 carbon atoms such as n-dodecyl mercaptan, tert-dodecyl mercaptan. Preferably, the mercaptan is n-dodecyl mercaptan or tert-dodecyl mercaptan.

[0020] The process can be implemented according to two alternatives for obtaining copolymers having the macrostructure defined by their number-average molar mass and the ratio between their number-average molar mass and their dispersity. According to the first alternative which corresponds to conditions a) the ratio between the number of moles of organic hydroperoxide and the number of moles of monomers is less than 10 -4and the ratio of the number of moles of mercaptan to the number of moles of organic hydroperoxide is greater than 2.

[0021] According to the second alternative which corresponds to conditions b), the ratio between the number of moles of organic hydroperoxide and the number of moles of monomers is greater than or equal to 9xl0 -4 and the ratio between the number of moles of mercaptan and the number of moles of organic hydroperoxide is less than 0.5.

[0022] Each of these conditions allows the production of copolymers having a specific macrostructure which is defined both by the number-average molar mass of the copolymer and a ratio between the number-average molar mass and the dispersity of the copolymer. The copolymers prepared according to the process according to the invention have a number-average molar mass greater than 350,000 g / mol and less than 500,000 g / mol and a dispersity such that the ratio between the number-average molar mass and the dispersity is greater than 150,000 g / mol. These characteristics on the macrostructure define a compromise between the number-average molar mass and the dispersity which makes it possible to reduce the hysteresis of a rubber composition.The reduction in the hysteresis of the rubber composition results in a reduction in the rolling resistance of a tire whose tread consists entirely or partly of a rubber composition comprising an elastomer prepared by the process according to the invention. The rolling resistance performance of a tire containing a rubber composition comprising an elastomer prepared according to the process according to the invention will be reduced as the amount of the elastomer in the elastomer matrix of the rubber composition increases.

[0023] Under conditions a), preferably the ratio between the number of moles of organic hydroperoxide and the number of moles of monomers is preferably greater than 5xl0 -5 and the ratio between the number of moles of mercaptan and the number of moles of organic hydroperoxide is preferably less than 4.

[0024] Under conditions b), preferably the ratio between the number of moles of organic hydroperoxide and the number of moles of monomers is preferably less than 5xl0 -3 and the ratio between the number of moles of mercaptan and the number of moles of organic hydroperoxide is preferably greater than 0.1.

[0025] The monomer mixture preferably contains 30% to 70% mol of 1,3-butadiene, 0 to 5% mol of glycerol carbonate (meth)acrylate and at least 30% mol of an alkyl methacrylate. According to a first embodiment of the invention, the monomer mixture is a mixture of 30% to 70% mol of 1,3-butadiene, 0 to 5% mol of glycerol carbonate (meth)acrylate and at least 30% mol of an alkyl methacrylate.

[0026] According to a second embodiment of the invention, the monomer mixture is a mixture of 30% to 70% mol of 1,3-butadiene and 70% to 30% mol of an alkyl methacrylate.

[0027] According to a third embodiment of the invention, the monomer mixture is a mixture of 30% to 70% mol of 1,3-butadiene, 1 to 5% mol of glycerol carbonate (meth)acrylate and at least 30% mol of an alkyl methacrylate.

[0028] The constituent monomer units of the copolymer prepared according to the first embodiment are butadiene units, alkyl methacrylate units and glycerol carbonate (meth)acrylate units when the monomer mixture contains more than 0 mol% of glycerol carbonate (meth)acrylate; the constituent monomer units of the copolymer prepared according to the second embodiment are butadiene units and alkyl methacrylate units; the constituent monomer units of the copolymer prepared according to the third embodiment are butadiene units, alkyl methacrylate units and glycerol carbonate (meth)acrylate units.

[0029] The molar compositions of the monomer mixture defined in the first embodiment as well as in the second and third embodiments are favorable for obtaining a copolymer having elastomeric properties giving a tire good wet grip performance when used in a rubber composition of a tire tread.

[0030] In the process according to the invention, whether it is carried out according to the first alternative or the second alternative, preferably the alkyl of the alkyl methacrylate contains from 2 to 10 carbon atoms, more preferably the alkyl of the alkyl methacrylate contains from 4 to 10 carbon atoms, even more preferably the alkyl of the alkyl methacrylate is n-butyl.

[0031] To stabilize the aqueous emulsion, surfactants are also used in a known manner. The surfactant(s) that can be used in the process according to the invention may be chosen from nonionic, anionic or cationic surfactants, preferably from anionic or cationic surfactants. As anionic surfactants, mention may in particular be made of all anionic surfactants comprising at least one alkyl group having from 6 to 40 carbon atoms (hereinafter referred to as C6-C40 alkyl) or at least one aromatic ring substituted by a C6-C40 alkyl group, and at least one anionic group chosen from sulfates, sulfonates, phosphates, phosphonates, and carboxylates.Preferably, the anionic surfactant(s) comprising at least one C6-C40 alkyl group or at least one aromatic ring substituted by a C6-C40 alkyl group are chosen from sodium stearate, sodium lauryl sulfate, sodium lauryl ether sulfate, dehydrogenated resin acids and their alkali metal salts and sodium dodecylbenzene sulfonates and mixtures of these compounds. As cationic surfactants, mention may in particular be made of all cationic surfactants comprising at least one C6-C40 alkyl group or at least one aromatic ring substituted by a C6-C40 alkyl group, and at least one cationic group chosen from ammoniums and pyridiums. Preferably, the cationic surfactant(s) are chosen from alkyltrimethylammonium salts such as trimethyldecylammonium chloride or bromide and benzalkonium salts and mixtures of these compounds.Preferably, the said surfactant(s) usable in the process according to the invention are chosen from trimethyldecylammonium chloride, sodium dodecyl sulfate and sodium stearate. The surfactant is typically used at a concentration which is higher than its critical micelle concentration (cmc), typically 2 to 10 times its cmc.

[0032] Also known to avoid possible degradation of the initiator by stabilizing the pH of the emulsion, a buffer is used. Examples include phosphate buffers such as tetrasodium pyrophosphate.

[0033] The monomers to be polymerized, in this case the monomer mixture of 1,3-butadiene and a (meth)acrylate which contains 1,3-butadiene and an alkyl methacrylate, are introduced into a reactor which contains an aqueous phase containing the surfactant. The total concentration of monomers introduced is preferably between 5% and 35% by weight relative to the total weight of monomers introduced and water.

[0034] As is known, the polymerization is typically carried out in an oxygen-free reactor. The polymerization can be carried out continuously or discontinuously (called a "batch" process), possibly a semi-fed "batch" process when monomers are fed during the polymerization reaction, in particular throughout the polymerization reaction. The continuous process and the "batch" process are particularly preferred for obtaining a random copolymer without composition drift, the composition drift resulting in the production of a copolymer having a composition gradient.

[0035] In order to stop the polymerization reaction at the correct conversion, in a known manner, a reducing agent, the stopper, is typically introduced. For example, mention may be made of the family of phenols such as hydroquinone, resorcinol, hydroxylamines such as N, N-diethylhydroxylamine. Preferably, the stopper is generally used in excess relative to the initiator introduced into the polymerization medium, typically the ratio between the number of moles of stopper and the number of moles of initiator introduced into the polymerization medium being greater than 1 and less than 10, preferably ranging from 2 to 5.

[0036] Once synthesized, the copolymer can be in either latex or solid form, for example to be mixed with a reinforcing filler to prepare a masterbatch.

[0037] To recover the copolymer in latex phase and without residual monomers, it is preferable to carry out a step of devolatilization of the residual monomers present in the latex, generally by steam distillation (stripping). To recover the copolymer in solid form, it is necessary to add a step of destabilization of the emulsion with or without a prior stripping step and a step of drying the coagulum. The destabilization of the emulsion can be carried out by the addition of a third body, for example an inorganic salt such as calcium, magnesium, potassium or sodium chloride, sodium, magnesium or sodium sulfate or an organic salt such as magnesium or calcium acetate. The coagulation of the latex can also be carried out by the addition of a solvent chosen from ketones and alcohols and in particular acetone, methanol, isopropanol, n-butanol and ethanol.The coagulum is then usually washed with water.

[0038] The process may further comprise a step of drying the copolymer, once coagulated. The copolymer may be dried under vacuum or at atmospheric pressure under nitrogen sparging. Drying temperatures may vary from room temperature (25°C) to 130°C, preferably from room temperature to 100°C, and even more preferably from room temperature to 70°C. Drying times are typically between 10 h and 72 h, preferably between 16 h and 50 h.

[0039] The above-mentioned characteristics of the present invention, as well as others, will be better understood upon reading the following description of the exemplary embodiments of the invention, given for illustrative and non-limiting purposes.

[0040] Examples

[0041] Determination of the glass transition temperature:

[0042] The glass transition temperatures Tg and glass transition widths AT of the copolymers are measured using a differential scanning calorimeter according to ASTM D3418-08.

[0043] Size exclusion chromatography (SEC):

[0044] Size exclusion chromatography (SEC) separates macromolecules in solution according to their size through columns filled with a porous gel. The macromolecules are separated according to their hydrodynamic volume, with the largest being eluted first. Although not an absolute method, SEC allows us to understand the molar mass distribution of a polymer. From commercial standard products, the various number-average (Mn) and weight-average (Mw) molar masses can be determined and the polydispersity index or dispersity (£) = Mw / Mn or also noted Ip) calculated via a so-called MOORE calibration. There is no special treatment of the polymer sample before analysis. It is simply solubilized in a tetrahydrofuran solution at a concentration of approximately 1 g / L. Then, the solution is filtered through a 0.45 pm porosity filter before injection.

[0045] The apparatus used is a "WATERS alliance e2695" chromatograph. The elution solvent is tetrahydrofuran. The flow rate is 1 mL / min, the system temperature is 35°C and the analysis time is 35 min. A set of three MIXED-B-LS columns from Agilent is used in series. The injected volume of the polymer sample solution is 100 pL. The detector is a "WATERS 2410" differential refractometer and the chromatographic data processing software is the "WATERS EMPOWER" system. The calculated average molar masses are relative to a calibration curve produced from commercial standard polystyrenes "PSS READY CAL-KIT".

[0046] The dynamic properties are measured on a viscoanalyzer (Metravib VA4000), according to the ASTM D 5992-96 standard. The response of a sample of vulcanized composition (cylindrical specimen 4 mm thick and 400 mm2 in cross-section), subjected to sinusoidal stress in alternating simple shear, at a frequency of 10 Hz, under normal temperature conditions (23°C) according to the ASTM D 1349-99 standard, is recorded.

[0047] A strain amplitude sweep is carried out from 0.1% to 100% (forward cycle), then from 100% to 0.1% (return cycle). The result used is the loss factor measured on the return cycle, tanô at 10% strain.

[0048] Synthesis of copolymers:

[0049] The copolymers of Examples 1 to 30 are prepared according to the following protocol:

[0050] Preparing the following charges in advance:

[0051] We weigh approximately exactly the quantity of solutes to prepare the solutions at an exactly known concentration which is close to a target concentration whose value is given below and introduced by the term "approximately".

[0052] - Suspension in water of Na2FeP2O7 (iron complex) at approximately 0.015 mol / L: FeSO4, 7H2O and Na4P2O7 are diluted in bubbled water, then the mixture is heated to 60°C for 45 minutes while stirring regularly

[0053] - Preparation of a solution of cumene hydroperoxide (initiator) in alkyl methacrylate at approximately 0.04 mol / L

[0054] - Preparation of a solution of tert-dodecylmercaptan (RSH) in alkyl methacrylate at 0.1 mol / L

[0055] - Preparation of a solution of N,N-diethylhydroxylamine (stopper) in water at approximately 0.05 mol / L in water.

[0056] Sodium dodecyl sulfate (SDS), iron sulfate, cumene hydroperoxide, sodium pyrophosphate (Sodium pyrophosphate tetrabasic), potassium persulfate, terdodecyl mercaptan, N,N-diethylhydroxylamine, n-butyl methacrylate (BuMA), 2-ethylhexyl methacrylate (EHMA) are commercially available from Aldrich. 4-(Hydroxymethyl)-1,3-dioxolan-2-one methacrylate (CCMA) is from Specific Polymers. 1,3-butadiene (btd) and alkyl methacrylates are purified by passage through an alumina guard and nitrogen sparging.

[0057] A stirred reactor is charged according to the following operations: introduce the bubbled water for 45 minutes at 25°C then the sodium dodecyl sulfate (surfactant, TA) under nitrogen at 25°C followed by a nitrogen flush of 10 min inject the RSH solution at 25°C under nitrogen cool the reactor to reach 5°C when the reactor reaches approximately 12°C, inject the rest of the monomer charge with the exception of the 1,3-butadiene, under nitrogen let the reactor cool to 5°C, then inject the Na2FeP2O? solution, then inject the 1,3-butadiene, leave to stir for 10 to 15 minutes while the emulsion forms then inject the initiator, the cumene hydroperoxide solution.

[0058] The end of the addition of the initiator marks the start of polymerization (i.e. t=0 min).

[0059] Stirring is maintained at 5°C. To stop the polymerization reaction, the latex is transferred by residual pressure of the monomers into another reactor containing the aqueous solution N,N-diethylhydroxylamine (stopper). The latex is then coagulated by adding a mixture of acetone and methanol (acetone / methanol: 50 / 50 by volume) at a rate of 3 volumes of acetone / methanol for one volume of reaction medium. The coagulum is dried under partial vacuum and under nitrogen flushing for 48 h at 40°C.

[0060] The synthesis conditions of the copolymers of Examples 1 and 2 are shown in Table 1. In the table, the abbreviation "eq." denotes molar equivalent. Conventionally, the quantity of water and that of the surfactant are given in part by mass per hundred parts of monomer mixture. The quantity of cumene hydroperoxide (initiator) is given in molar percentage relative to the total quantity of starting monomers, the quantities of RSH, iron complex, and stopper are given in molar equivalent relative to the quantity of initiator. The composition of the starting monomer charge is given in molar percentage calculated on the total number of moles of monomers making up the starting monomer charge, the starting monomer charge constituting the monomer mixture to be polymerized.

[0061] Example 1 is not in accordance with the invention, since the polymerization conditions are those described in document WO 2018015646 A1, the ratio between the number of moles of organic hydroperoxide and the number of moles of monomers being equal to 7.4xl0 -4 and the ratio between the number of moles of mercaptan and the number of moles of organic hydroperoxide being equal to 0.71. Example 2 is an example in accordance with the invention, since the ratio between the number of moles of organic hydroperoxide and the number of moles of monomers being equal to 9xl0 -3 is less than 10 -4 and the ratio between the number of moles of mercaptan and the number of moles of organic hydroperoxide being equal to 2.8 is greater than 2.

[0062] The results of Examples 1 and 2 are shown in Table 1.

[0063] The copolymer of Example 2 has an Mn of the order of 430,000 and an Mn / £ ratio of approximately 166,000 g / mol, whereas the copolymer of Example 1 has an Mn of only 158,000 and an Mn / £ ratio of approximately 41,000 g / mol. It is clearly observed that the copolymer of Example 1 has a molar mass distribution which is broad given its number-average molar mass, compared to the copolymer of Example 2. Table 1:

[0064] The syntheses of the copolymers of Examples 3 to 23 are carried out under the conditions described in Table 2, the quantity of stopper (N,N-diethylhydroxylamine) being 3 equivalents relative to the quantity of initiator.

[0065] In examples 3 to 9 and 15 to 16, the molar composition of the monomer mixture is 1,3-butadiene / BuMA / CCMA: 50 / 50 / 0: the elastomers prepared are copolymers of 1,3-butadiene and BuMA.

[0066] In Examples 10 to 14 and 17 to 23, the molar composition of the monomer mixture is 1,3-butadiene / BuMA / CCMA: 48 / 50 / 2: the elastomers prepared are terpolymers of 1,3-butadiene, BuMA and CCMA.

[0067] In Table 2, S / M is the ratio of the mass of water to the mass of starting monomers, T poiy the polymerization reaction time, Conv. the conversion of the polymerization reaction, the amount of initiator is given as a molar percentage relative to the total amount of starting monomers, the amount of RSH is given as a molar equivalent relative to the amount of initiator, the amount of surfactant (TA) is given by the ratio between the concentration of the surfactant in the polymerization medium and its critical micellar concentration (CMC which is 0.0082 mol / L).

[0068] The results of Examples 3 to 23 are shown in Table 2. Table 2:

[0069] Examples 12 to 14 and 21 to 22 are examples in accordance with the invention, since the synthesis process complies with conditions a) or conditions b), conditions a) being that the ratio between the number of moles of organic hydroperoxide and the number of moles of monomers is less than 10 -4 and the ratio between the number of moles of mercaptan and the number of moles of organic hydroperoxide is greater than 2, conditions b) being that the ratio between the number of moles of organic hydroperoxide and the number of moles of monomers is greater than or equal to 9xl0 -4and the ratio between the number of moles of mercaptan and the number of moles of organic hydroperoxide is less than 0.5. The other examples 3 to 11, 15 to 20 and 23 are not in accordance with the invention. Only the process in accordance with the invention makes it possible to obtain copolymers with the desired macrostructure, i.e. defined by a number-average molar mass greater than 350,000 g / mol and less than 500,000 g / mol and a ratio Mn / £) > 150,000 g / mol.

[0070] Examples 24 to 29:

[0071] The syntheses of the copolymers of Examples 24 to 29 are carried out under the conditions described in Table 3, the quantity of stopper (N,N-diethylhydroxylamine) being 3 equivalents relative to the quantity of initiator. In Table 3, S / M is the ratio between the mass of water and the mass of starting monomers, T poiythe polymerization reaction time, Conv. the conversion of the polymerization reaction, the amount of initiator is given as a molar percentage relative to the total amount of starting monomers, the amount of RSH is given as a molar equivalent relative to the amount of initiator, the amount of TA is given by the ratio between the concentration of the surfactant in the polymerization medium and its critical micelle concentration (CMC which is 0.0082 mol / L). The composition of the starting monomer charge is given as a molar percentage calculated on the total number of moles of monomers making up the starting monomer charge.

[0072] Table 3:

[0073] The number-average molar mass values ​​(Mn), dispersity (£>), molar composition of the elastomers of Examples 24 to 29 are shown in Table 4, as well as the value of the Mn / D ratio. The synthesized copolymers are all elastomeric and statistical (AT less than 10°C). The determination of the molar composition of the elastomers is carried out by 1H NMR analysis. The spectra are acquired on a BRUKER Avance 500 MHz spectrometer equipped with a BBFO z-grad 5 mm "broadband" cryoprobe for soluble samples and a 4 mm z-grad HRMAS probe. 1 H / 13 C. The quantitative 1H NMR experiment uses a single 30° pulse sequence and a 5-second repetition delay between each acquisition. Samples are solubilized or swollen in deuterated chloroform. Chemical shifts are calibrated relative to the protonated impurity of chloroform (6 ppm 1 H at 7.2 ppm).

[0074] Table 4: Preparation of rubber compositions:

[0075] To prepare the rubber compositions, the procedure is as follows: The elastomer, the reinforcing filler and the other additives are successively introduced into an internal mixer (final filling rate approximately 70% by volume), whose initial tank temperature is approximately 100°C. Thermomechanical work is then carried out (non-productive phase) in one step (total mixing time equal to approximately 5 min), until a maximum "fall" temperature ranging from 140 to 165°C is reached depending on the compositions. The mixture thus obtained is recovered, cooled and then the vulcanization system is added to an external mixer to carry out a second phase of mechanical work at approximately 40°C.

[0076] The rubber compositions are given in Table 5. The quantities are expressed in parts per 100 parts by weight of elastomer.

[0077] Table 5:

[0078] (1) Silica “Zeosil 1165 MP” from Rhodia (HDS type)

[0079] (2) Tris(2-ethylhexyl)phosphate

[0080] (3) Polylimonene resin "Dercolyte L120" from DRT company

[0081] (4) TESPT (“Si69” from the company Degussa)

[0082] (5) Diphenylguanidine (“Perkacit” DPG from Flexsys)

[0083] (6) N-(l,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine, from Flexsys (7) Industrial grade zinc oxide from Umicore

[0084] (8) Stearin “Pristerene 4931” from Uniqema company

[0085] (9) N-cyclohexyl-2-benzothiazyl sulfenamide “Santocure CBS” from Flexys

[0086] The compositions thus obtained are then calendered either in the form of plates (thickness of 2 to 3 mm) or thin sheets of rubber for the measurement of their physical or mechanical properties. The calendered compositions are then cooked under pressure for 30 min at 150°C, then characterized.

[0087] The results of the characterizations of the rubber compositions are shown in Table 6.

[0088] Table 6:

[0089] Elastomers E4 to E6 have a number-average molar mass greater than 350,000 g / mol and less than 500,000 g / mol and a Mn / £ ratio > 150,000 g / mol and are prepared according to a process according to the invention, unlike elastomers E1 to E3. Elastomers E4 to E6 give the rubber composition lower hysteretic properties compared to elastomers E1 to E3. This result is attributed to the greater chain length homogeneity of elastomers E4 to E6. Indeed, elastomers E4 to E6 have the best compromise between number-average molar mass and dispersity which results in a Mn / £ ratio greater than 150,000 g / mol. In other words, although elastomers E4 to E6 are composed of much longer chains, the constituent chains of elastomers E4 to E6 exhibit a chain length homogeneity comparable to the chains of elastomers E1 to E3.It is observed that the higher the ratio Mn / £), the lower the tanô 10% values, which indicates an improvement in the rolling resistance of a tire whose tread contains an elastomer in accordance with the invention in its rubber composition.

Claims

Claims 1. A process for the radical emulsion polymerization of a monomer mixture of 1,3-butadiene and a (meth)acrylate containing 1,3-butadiene and an alkyl methacrylate, wherein the polymerization reaction is initiated by a redox system comprising an organic hydroperoxide and an iron (II) salt in the presence of a mercaptan as transfer agent, the amounts of organic hydroperoxide and mercaptan fulfilling conditions a) or conditions b): conditions a) being defined by a ratio between the number of moles of organic hydroperoxide and the number of moles of monomers which is less than 10 -4 and a ratio between the number of moles of mercaptan and the number of moles of organic hydroperoxide which is greater than 2, conditions b) being defined by a ratio between the number of moles of organic hydroperoxide and the number of moles of monomers which is greater than or equal to 9xl0 -4and a ratio between the number of moles of mercaptan and the number of moles of organic hydroperoxide which is less than 0.5, the polymerization reaction being carried out at a temperature between 0° and 10°C.

2. Process according to claim 1 in which under conditions a) the ratio between the number of moles of organic hydroperoxide and the number of moles of monomers is greater than 5 x 10 -5 and the ratio of the number of moles of mercaptan to the number of moles of organic hydroperoxide is less than 4.

3. Process according to claim 1 or 2 in which under conditions b) the ratio between the number of moles of organic hydroperoxide and the number of moles of monomers is less than 5 x 10 -3 and the ratio of the number of moles of mercaptan to the number of moles of organic hydroperoxide is greater than 0.

1.

4. A process according to any one of claims 1 to 3 wherein the monomer mixture contains 30% to 70 mol% of 1,3-butadiene, 0 to 5 mol% of glycerol carbonate (meth)acrylate and at least 30 mol% of an alkyl methacrylate.

5. A process according to any one of claims 1 to 4 wherein the monomer mixture is a mixture of 30% to 70 mol% of 1,3-butadiene, 0 to 5 mol% of glycerol carbonate (meth)acrylate and at least 30 mol% of an alkyl methacrylate.

6. A process according to any one of claims 1 to 5 wherein the monomer mixture is a mixture of 30% to 70% mol of 1,3-butadiene and 70% to 30% mol of an alkyl methacrylate.

7. A process according to any one of claims 1 to 5 wherein the monomer mixture is a mixture of 30% to 70 mol% of 1,3-butadiene, 1 to 5 mol% of glycerol carbonate (meth)acrylate and at least 30 mol% of an alkyl methacrylate.

8. A method according to any one of claims 1 to 7 wherein the alkyl of the alkyl methacrylate contains from 2 to 10 carbon atoms.

9. A method according to any one of claims 1 to 8 wherein the alkyl of the alkyl methacrylate contains from 4 to 10 carbon atoms.

10. A method according to any one of claims 1 to 9 wherein the alkyl of the alkyl methacrylate is n-butyl.

11. A process according to any one of claims 1 to 10 wherein the organic hydroperoxide is cumene hydroperoxide.

12. A process according to any one of claims 1 to 11 wherein the iron (II) salt is ferrous sulfate or a ferrous sulfate hydrate.

13. A method according to any one of claims 1 to 12 wherein the mercaptan is n-dodecyl mercaptan or tert-dodecyl mercaptan.

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