This application is a 371 national phase entry of PCT/FR2018/053100, filed on Dec. 4, 2018, which claims benefit of French Patent Application No. 1761660, filed Dec. 5, 2017, the entire contents of which are incorporated herein by reference for all purposes.

BACKGROUND

1. Field of the Invention

The present invention relates to a continuous method for producing a diene elastomer, such as a butadiene homopolymer or copolymer. The invention applies notably to the production of a polybutadiene with a high proportion of cis-1,4 chains having both a Mooney viscosity greater than or equal to 35 and a reduced polydispersity index.

Polymerization is carried out in solution. In solution polymerization, the reaction mixture comprises a solvent or a diluent. The reactor may then be filled with the polymerization solution (and thus lacks a gas phase) or may comprise a liquid phase and a gas phase. In the present invention, the polymerization reactions are carried out in solution in full reactors, i.e. substantially lacking a gas phase.

2. Related Art

For producing butadiene homopolymers or copolymers having a high proportion of cis-1,4 chains, it is known to use catalytic systems based on:a rare earth salt in solution in a hydrocarbon solvent,an alkylating agent of this salt consisting of an alkylaluminium, andan alkylaluminium halide.

The patent documents WO-A-02/38636 and WO-A-03/097708 in the applicants' name teach, for producing polybutadienes, the use of a catalytic system of the preformed type based on at least:a preforming conjugated diene,a salt of one or more rare earth metals of an organic phosphoric acid, in at least one inert, saturated hydrocarbon solvent of the aliphatic or alicyclic type,an alkylating agent consisting of an alkylaluminium of formula AIR3 or HAIR2, anda halogen donor that belongs to the class of the alkylaluminium halides but excluding the alkylaluminium sesquihalides.

The polybutadienes obtained by means of this catalytic system notably have a polydispersity index below 2.5 and a Mooney viscosity ML(1+4) at 100° C. greater than or equal to 40. This combination of characteristics makes these polybutadienes very suitable for use in tyre casing treads.

A continuous method for synthesis of polybutadiene is moreover also described in patent application EP 1 845 118, in the applicants' name.

The patent document WO-A-2010/139449 in the applicants' name describes another catalytic system for polymerization of conjugated dienes based on a rare earth metal salt and an organometallic compound based on a metal belonging to the 2nd or the 13th column of the periodic table.

In the continuous synthesis of polymers of conjugated dienes in full reactors, control of the temperature in the polymerization process with one or more reactors in series is decisive. For a given process, the range of operating conditions (specific flow rates and concentration of monomer(s)) is determined by the polymerization temperatures selected.

Thus, for full reactors without any cooling system, the amount of heat released by the production of polymer cannot exceed the amount of heat absorbed by the reaction mixture when it is heated from its initial temperature to the polymerization temperature. Now, the latter cannot be increased without altering the properties of the reaction mixture and/or of the catalyst.

This limitation must therefore be overcome by using a cooling system inside the reactor, such as a double jacket or internal cooling tube bundles.

The problem resulting from the presence of a double jacket or cooling tube bundles is that this equipment also has limits to its cooling capacity, which depends directly on the exchange surface area or on the temperature of the heat-transfer fluid and the properties of the polymer fluid within the reactor. The concentration of monomer and the conversion are therefore also limited.

To overcome these limitations, the applicants discovered that it was beneficial not to inject all of the monomer already in the first reactor, but to inject a portion thereof directly in a subsequent reactor. This injection makes it possible to provide a significant additional cooling capacity for all of the reactors while improving the conversion efficiency of each of the reactors.

In these conditions, the method according to the invention makes it possible to operate with a higher concentration by weight of conjugated diene monomer to be polymerized.

SUMMARY

The invention relates to a continuous method for producing a diene elastomer, comprising a polymerization reaction of a catalytic system with at least one conjugated diene monomer to be polymerized in solution in a solvent, the catalytic system being based on at least:a rare earth organic salt;an alkylating agent; and, if applicablea halogen donor and/or a preforming conjugated diene monomer
in at least two full polymerization reactors in series, characterized in that a portion of the conjugated diene monomer to be polymerized is introduced into a polymerization reactor subsequent to the first polymerization reactor.

Advantageously, a portion of the solvent is also introduced with the conjugated diene monomer to be polymerized into the subsequent polymerization reactor.

Advantageously, at least 20 wt %, relative to the total weight, of the conjugated diene monomer to be polymerized is introduced into a subsequent polymerization reactor.

The portion of the conjugated diene monomer to be polymerized introduced into a subsequent polymerization reactor is advantageously at a temperature between −10° C. and 35° C., advantageously between 0° C. and 20° C.

In particular, the continuous method comprises the following simultaneous steps:a) introducing, into a first polymerization reactor, the solvent, all of the catalytic system, a portion of the conjugated diene monomer to be polymerized;b) polymerizing said diene monomer in said first polymerization reactor;c) introducing, into a second polymerization reactor, the solution resulting from step b) and the remainder of the conjugated diene monomer to be polymerized;d) polymerizing said diene monomer in said second polymerization reactor;e) if applicable, introducing the solution resulting from step d) into a third polymerization reactor;f) if applicable, polymerizing said diene monomer in said third polymerization reactor.

The polymerization in step b) is advantageously carried out at a temperature greater than or equal to 50° C.

The polymerization in step d) is advantageously carried out at a temperature greater than or equal to 50° C.

The polymerization in step f) is advantageously carried out at a temperature greater than or equal to 50° C.

The conjugated diene monomer to be polymerized and the solvent are advantageously introduced, into the first and/or subsequent polymerization reactor, at a temperature between −10° C. and 35° C., advantageously between 0° C. and 20° C.

In particular, the continuous method comprises the following simultaneous steps:a1) separating the incoming stream F comprising the solvent and all of the conjugated diene monomer to be polymerized into two streams F1 and F2;a2) introducing the catalytic system into a first polymerization reactor of stream F1;b) polymerizing said diene monomer continuously in said first polymerization reactor;c) injecting the solution resulting from step b) and stream F2 into a second reactor;d) polymerizing said diene monomer continuously in said second polymerization reactor;e) if applicable, injecting the solution resulting from step d) into a third reactor;f) if applicable, polymerizing said diene monomer continuously in said third polymerization reactor.

Advantageously, at least stream F2 is at a temperature between −10° C. and 35° C., advantageously between 0° C. and 20° C.

The concentration by weight of conjugated diene monomer to be polymerized is advantageously above 20%, relative to the total weight of conjugated diene monomer to be polymerized+solvent.

The diene monomer to be polymerized is advantageously butadiene.

