Process for continuous synthesis of a diene elastomer

Abstract

A process for the continuous synthesis of diene elastomers with a high degree of conversion is provided. The process includes simultaneously: a) introducing continuously into a polymerization reactor containing a gas phase and equipped with at least one stirring rotor and a discharge device, at least i. one or more monomers, including at least one conjugated diene monomer, and ii. from 0% to 70% by mass of an organic solvent, calculated relative to the total mass of monomers and of solvent b) continuously polymerizing the monomer(s), c) stirring the polymerization medium via the continuous movement of at least one stirring rotor about a rotary axle, d) continuously discharging the elastomer paste, e) continuously conveying the discharged elastomer paste to a chopping device and chopping it into particles, f) removing solvent from the particles of the elastomer paste, and g) recovering diene elastomer.

Claims

1. A process for the continuous synthesis of a diene elastomer, comprising, simultaneously: a) introducing continuously into a polymerisation reactor containing a gas phase and equipped with at least one stirring rotor and a discharge device, at least i. one or more monomer(s) to be polymerised, including at least one conjugated diene monomer, and ii. from 0% to 70% by mass of an organic solvent, calculated relative to the total mass of monomer(s) and of solvent b) continuously polymerising the monomer(s), c) stirring a polymerisation medium via the continuous movement of at least one stirring rotor about a rotary axle, d) continuously discharging an elastomer paste derived from the polymerisation, e) continuously conveying the discharged elastomer paste to a chopping device and chopping it into particles, f) removing the solvent from the particles of the elastomer paste and g) recovering the diene elastomer of the particles obtained in the preceding step; wherein (1) the process has a degree of conversion of at least 60%, at the limit of the first third of the reaction volume of the polymerisation reactor, and (2) the process has a residence time distribution function in the polymerisation reactor having a standard deviation that is greater than the mean residence time divided by 2√3.

2. The process for the continuous synthesis of a diene elastomer according to claim 1, wherein the degree of conversion is at least 80%, at the limit of the first third of the reaction volume of the polymerisation reactor.

3. The process for the continuous synthesis of a diene elastomer according to claim 1, wherein the standard deviation of the residence time distribution function in the polymerisation reactor is greater than the mean residence time divided by 2.

4. The process according to claim 1, wherein the stirring of the polymerisation medium is performed by the continuous movement of two stirring rotors.

5. The process according to claim 4, wherein the blades are blades of sigma type or Z-shaped blades.

6. The process according to claim 1, wherein, in step e), the elastomer paste discharged is conveyed to a granulator for chopping said elastomer paste.

7. The process according to claim 6 wherein the granulator is an underwater granulator.

8. The process according to claim 1, further comprising, after step d) and before step e), d.sub.1) continuously transferring the elastomer paste discharged from the polymerisation reactor to a blender containing a gas phase, a stirring device with at least one stirring rotor and a discharge device, d.sub.2) additionally treating a physical nature, chemical nature, or both, of the said elastomer paste in the blender, and d.sub.3) continuously discharging the elastomer paste derived from the additional treatment step.

9. The process according to claim 8, wherein the stirring device of the blender sweeps at least 90% of the volume of the blender, ensuring self-cleaning.

10. The process according to claim 8, wherein the additional treatment comprises concentration of the reaction medium.

11. The process according to claim 8, wherein the additional treatment comprises a reaction subsequent to the polymerisation.

12. The process according to claim 8, wherein the discharge of the elastomer paste in step d) and, where appropriate, in step d.sub.3) is performed via the combined action of at least one emptying screw and of a gear pump which constitutes the discharge device.

13. The process according to claim 1, wherein the heat of the polymerisation reaction and, where appropriate, of the treatment subsequent to the polymerisation is governed by the at least partial vaporization of the constituents of the non-polymeric phase of the reaction medium.

14. The process according to claim 13, wherein the gas phase resulting from the at least partial vaporization of the constituents of the non-polymeric phase of the reaction medium is extracted from the polymerisation reactor and, where appropriate, from the blender.

15. The process according to claim 14, wherein the gas phase resulting from the vaporization of the constituents of the non-polymeric phase of the reaction medium is extracted and condensed to be totally or partly recycled into the polymerisation reactor and/or, where appropriate, into the blender.

16. The process according to claim 1, wherein the polymerisation takes place by coordination catalysis in the presence of a catalytic system based on at least: (i) a rare-earth metal organic salt, (ii) an alkylating agent and, where appropriate, (iii) a halogen donor, and/or (iv) a preformation diene monomer.

17. The process according to claim 16, wherein, during step a, at least one alkylaluminium compound of formula AIR.sub.3 or HAIR.sub.2 wherein R represents an alkyl radical containing from 1 to 20 carbon atoms and H represents a hydrogen atom, which is identical to or different from that of the catalytic system, is introduced continuously into the polymerisation reactor, via a stream separate from that of the catalytic system.

18. The process according to claim 17, further comprising, prior to step a), placing the alkylaluminium compound, the solvent and the monomer to be polymerised in contact to form a mixture, which is introduced into the polymerisation reactor during step a).

19. The process according to claim 16, wherein the diene monomer to be polymerised is chosen from butadiene and isoprene, or a mixture thereof.

20. The process according to claim 1, wherein the polymerisation is of anionic type and takes place in the presence of an anionic polymerisation initiator chosen from organic compounds of an alkali metal, introduced continuously into the polymerisation reactor during step a).

21. The process according to claim 20, wherein, in step a), the mixture of monomers i) also comprises at least one vinylaromatic monomer.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) As non-limiting illustrations, two installations for the continuous synthesis of a diene elastomer in accordance with two embodiments of the synthetic process of the invention are more particularly described with reference to FIGS. 1 and 2, each constituting a schematic representation of an installation.

(2) FIG. 1 is a scheme of an installation according to one embodiment of the invention for the continuous synthesis of a diene elastomer by coordination catalysis incorporating a polymerisation reactor and a granulator.

(3) FIG. 2 is a scheme of an installation according to an embodiment of the invention for the continuous synthesis of a diene elastomer by coordination catalysis incorporating, besides a polymerisation reactor and a granulator, a self-cleaning blender.

(4) FIG. 3 is a graph showing a residence time distribution according to an embodiment of the invention.

(5) FIG. 4 is a graph showing a residence time distribution according to another embodiment of the invention.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

(6) The installation illustrated in FIG. 1 essentially comprises a polymerisation reactor 1 which contains a gas phase and is equipped with at least one stirring rotor and a discharge device containing at least one screw. The reactor 1 is equipped with means for reaching, in concentrated medium, a specific flow such that the residence time distribution function is such that the standard deviation of this function is greater than the mean residence time divided by 2√3, preferably greater than the mean residence time divided by 2.

(7) The reactor 1 is thus equipped with a stirring system suited to the high viscosities that the reaction medium may reach. Specifically, the synthetic process of the invention is also adapted to bulk polymerisation. This particular variant of the invention allows a person skilled in the art to estimate the type of stirring to provide. Stirring systems that may be envisaged are described above.

(8) According to preferred configurations, the polymerisation reactor is more particularly of Z-shaped arm blender technology (or sigma blender). The term “Z-shaped arm blender technology” more particularly means a mixer or blender formed from a tank equipped with two Z-shaped arms, each driven about a rotary axle that is preferably horizontal. Each of the arms is supported on both of the two opposite sides of the tank. When the arms are imbricated, they are driven in rotation with fixed speed ratios, so that they do not hit each other. When the arms are tangential, they are driven in rotation independently of each other, or otherwise. The two Z-shaped arms are preferentially driven counter-rotatively so as to force-feed the emptying device at the bottom of the tank.

(9) The polymerisation reactor 1 is at least connected to several continuous feed sources including at least one source 2 for feeding with catalytic system, a source 3 for feeding with at least one monomer, where appropriate mixed with the inert hydrocarbon-based solvent, and an outlet adapted to remove from the said reactor 1, continuously, the elastomer paste as a stream leaving via a discharge device.

(10) According to the variant of the installation illustrated by FIG. 1, the polymerisation reactor 1 is connected to a source 3 for feeding with a mixture of at least one monomer and of solvent. A mixer 4, located upstream of the polymerisation reactor 1, performs this mixing. This mixer 4 is fed with a source of at least one monomer 5 and with a source of solvent 6, and is connected to the polymerisation reactor 1.

(11) According to variants of the catalytic polymerisation process in accordance with the invention, a predetermined additional amount of at least one alkylaluminium compound may be introduced into the polymerisation reactor, via a stream that is separate and independent from the introduction of the catalytic system used for the polymerisation reaction. According to an installation variant illustrated by dashed lines in FIG. 1, the polymerisation reactor 1 is also connected to a source for feeding with additional alkylating agent 7. The alkylating agent may, alternatively, be added to the mixture of monomer and solvent before the inlet into the reactor 1, the source for feeding with additional alkylating agent 7 thus being connected to the source for feeding with monomer and solvent 3, as is indicated with dashed lines in FIG. 1. Alternatively, according to another alternative that is also illustrated in FIG. 1 by dashed lines, the alkylating agent may be injected directly into the mixer 4 in order to be mixed therein with the monomer and the solvent, the source for feeding with additional alkylating agent 7 then being connected to the mixer 4.

(12) According to an installation variant illustrated in FIG. 1, the source for feeding with catalytic system 2 may be connected to an installation, not shown, for the continuous preparation of the catalytic system as described in EP 1 968 734 A1

(13) The polymerisation reactor 1 is equipped with a discharge device containing at least one screw which makes it possible to continuously discharge the elastomer paste as an exiting stream 9. This discharge device consists of at least one emptying screw, integrated into the reactor 1, and of a gear pump 10 downstream of the reactor 1. The discharge device is particularly adapted to continuously discharge a product of high viscosity, and thus makes it possible to regulate the flow rate leaving the reactor 1 so that it is identical to the flow rate entering this same reactor 1. The discharge device that may be envisaged is more fully described above.

(14) The presence of a gas phase in the reactor 1, the stirring system adapted to high viscosities and the continuous discharge device constitute, with the various feeds of monomers, solvent and catalytic systems, essential means for being able to perform the process for the continuous synthesis of a diene elastomer especially characterized by a degree of conversion of at least 60%, at the limit of the first third of the reaction volume of the polymerisation reactor in combination with a specific flow such that the standard deviation of the residence time distribution function in the polymerisation reactor is greater than the mean residence time divided by 2√3.

(15) According to certain embodiments of the polymerisation process, the heat of the polymerisation reaction is regulated by the at least partial vaporization of the non-polymeric constituents of the reaction medium. In FIG. 1 illustrating a variant of this process, the polymerisation reactor 1 is equipped with a condensation loop which allows the condensation of the gases derived from the reactor 1 via a condenser 8, and the return of all or part of the condensate into the reactor 1.

(16) According to an optional embodiment, not shown, the condensation loop may advantageously be connected to a device for removing the uncondensable products, in a manner known per se.

(17) According to the embodiment of the invention illustrated in FIG. 1, a stopper and an antioxidant are then injected into the stream of elastomer paste 9 discharged from the reactor 1, by means, respectively, of feed sources 11 and 12 upstream of the gear pump 10.

(18) The installation illustrated in FIG. 1 also comprises a granulator 13, preferably under water. The granulator 13 is equipped with an inlet connected to the gear pump 10 via which the discharge device feeds the granulator 13 with elastomer paste to be chopped. The granulator 13 is also equipped with an outlet adapted to remove from the said granulator 13, continuously, the particles of elastomer paste as a stream 14 especially towards a device for removing the solvent and the monomer(s).

(19) The installation illustrated in FIG. 2 comprises the same elements upstream of the polymerisation reactor 1 as those represented in FIG. 1, up to the polymerisation reactor 1. It also comprises a blender 15 downstream of the polymerisation reactor 1 and connected thereto via the gear pump 10. The blender 15 has a stirring device which sweeps at least 90% of the volume of the blender and preferentially 95% of its volume, thus ensuring self-cleaning.

(20) The blender 15 is at least equipped with an inlet connected to a source for feeding with elastomer paste discharged from the polymerisation reactor 1 via the gear pump 10, an outlet adapted to remove from the said blender 15, continuously, the elastomer paste as an exiting stream 17 towards the pump 18.

(21) The blender 15 is equipped with a discharge device containing at least screw which allows the continuous discharge of the elastomer paste as an exiting stream 17. This discharge device consists of at least one emptying screw, integrated into the blender 15, and a gear pump 18 located downstream of the reactor 15.

(22) The exiting stream of elastomer paste 17 is conveyed towards the granulator 13, preferably under water, in order to be chopped thereat via the gear pump 18 which is connected to the granulator.

(23) According to certain embodiments of the polymerisation process, the blender 15 may have various roles to perform various steps subsequent to the polymerization. Thus, the blender 15 may be used to concentrate the reaction medium by removing, where appropriate, part of the solvent, and/or unreacted monomers. Reactions subsequent to the polymerisation may also be performed in this blender 15. These various steps may take place simultaneously in the blender 15.

(24) In FIG. 2 illustrating an installation variant for one of these embodiments of the process, the blender 15 is equipped with a condensation loop which allows: the condensation of the gases derived from the reactor 15 via a condenser 16, and the return of all or part of the condensate into the blender 15.

(25) This condensation loop also ensures control of the heat of the reactions operating in the blender 15 by the at least partial vaporization of the non-elastomeric constituents of the reaction medium.

(26) According to an optional embodiment, not shown, the condensation loop may advantageously be connected to a device for removing the uncondensable products in a manner that is known per se.

(27) According to the embodiment of the invention illustrated in FIG. 2, a stopper and an antioxidant are then injected into the stream of the elastomer paste 17 discharged from the blender 15, by means, respectively, of feed sources 11 and 12 upstream of the gear pump 18.

(28) The installation illustrated in FIG. 2 also comprises a granulator 13, preferably under water. The granulator 13 is equipped with an inlet connected to the gear pump 18 via which the discharge device feeds the granulator 13 with elastomer paste to be chopped. The granulator 13 is also equipped with an outlet adapted to remove from the said granulator 13, continuously, the particles of elastomer paste as a stream 14, especially towards a device for removing the solvent and the monomer(s).

(29) FIGS. 1 and 2 are schemes of embodiments of the invention for the continuous synthesis of a diene elastomer via coordination catalysis. A person skilled in the art will understand that an installation for performing a process for the continuous synthesis of a diene elastomer via anionic polymerisation, which constitutes other embodiments of the invention, does not comprise a source for feeding with alkylating agent 7. According to this embodiment, the polymerisation reactor is connected, respectively, to several feed sources including at least one source for feeding with polymerisation initiator which then replaces the feed source referenced 2 in FIGS. 1 and 2.

Example 1

(30) Process for Synthesizing a Polyisoprene Via Coordination Catalysis in Highly Concentrated Medium According to One Variant of the Invention

(31) A few definitions: diethylaluminium chloride=DEAC diisobutylaluminium hydride=DiBaH

(32) The polymerisation process was performed on a continuous line in the installation represented in FIG. 1 comprising a gas-phase reactor 150 liters in total with twin Z-shaped arms, equipped with a discharge device with an emptying screw and a gear pump.

(33) Isoprene and pentane are mixed in a dynamic mixer 4 provided for this purpose upstream of the polymerisation reactor 1. The mixture obtained is injected in the form of a stream entering 3 directly into the reactor 1.

(34) The isoprene is injected at a rate of 7.52 kg/h as a stream 5 into the mixer 4.

(35) The pentane is injected at a rate of 12.40 kg/h as a stream 6 into the mixer 4, i.e. about 37% by mass of monomer in the reaction medium.

(36) A coordination catalytic system is also injected directly into the reactor 1 as a stream 2. The catalytic system is based on neodymium tris[bis(2-ethylhexyl)phosphate], butadiene as preformation monomer, DiBaH as alkylating agent and DEAC as halogen donor. It was prepared according to the preparation method described in paragraph I of the abovementioned document WO-A-03/097 708 in the name of the Applicants.

(37) The catalytic system has the following mole ratios relative to the neodymium: Nd/butadiene/DiBaH/DEAC=1/30/1.8/2.6

(38) The amount of neodymium is 105 μmol per 100 g of isoprene. The mole concentration of neodymium in the catalyst is 0.02 mol/L.

(39) DiBaH is also injected directly into the reactor 1 as a separate and independent stream 7. The amount of DiBaH injected is 110 μmol/100 g of isoprene. The mole concentration of the DiBaH solution is 0.1 mol/L.

(40) The pressure of the gas phase is regulated at 0.16 barg, the vapours are condensed in an external condenser 8 and returned into the polymerisation reactor 1 with twin Z-shaped arms.

(41) The jacket temperature is maintained at 35° C.

(42) The mean residence time t.sub.0 of the reaction is 80 minutes.

(43) The discharge screw makes it possible to transfer the product from the reactor 1 to a gear pump 10.

(44) The stopper and the antioxidant used are oleic acid, at 1 phr (phr: parts by mass per 100 parts by mass of elastomers) and N-1,3-dimethylbutyl-N′-phenyl-para-phenylenediamine, at 0.5 pcm (pcm: parts by mass per 100 parts by mass of isoprene monomer). This stopper and this antioxidant are injected one after the other at the outlet of reactor 1, upstream of the gear pump 10, as streams 11 and 12.

(45) The gear pump 10 transfers the elastomer paste to the underwater granulator 13. The flow rate transferred by the gear pump 10 is equal to the sum of the flow rates injected into the reactor 1 to which is added the flow rates of stopper and of antioxidant.

(46) No expansion phenomenon is observed.

(47) The conversion measured on a withdrawn sample, at the limit of the first third of the reactor volume, is 89%. The conversion measurement was established from a GC measurement of the isoprene in the withdrawn sample. The residual isoprene in the withdrawn sample was assayed at 4% by mass.

(48) The flows are characterized by the residence time distribution given in FIG. 3.

(49) The experimental points for establishing this residence time distribution were obtained by gas chromatographic measurement of the changes in concentration of a tracer following a very rapid injection according to a method of pulse introduction of a chemically inert product according to the principle described in the book by Jacques Villermaux, Génie de la reaction chimique: conception et fonctionnement des réacteurs. Editors. 1993, TEC & DOC—LAVOISIER, pages 170 to 172. The experimental points are derived from samples taken from stream 9 of FIG. 1 at the outlet of reactor 1, at the end of the discharge screw.

(50) Modelling of these experimental points by a gamma law made it possible to determine the variance of the residence time distribution function E.

(51) The gamma law for modelling the residence time distribution function E is as follows:

(52) $E=\frac{t^{k-1}*e^{\frac{-t}{t_0}}}{\Gamma \left(k\right)t_0^k}$ with:
Γ(k): gamma function of k;
k: constant;
t.sub.0: mean residence time, i.e. 80 minutes;
t: residence time.

(53) The variance of this residence time distribution function is equal to k*t.sub.0.sup.2.

(54) Adjustment of the function to the experimental points makes it possible to determine the parameter k, which is equal to 1.22.

(55) The standard deviation of the residence time distribution function is deduced therefrom, which is equal to 88.4, which is greater than the mean residence time divided by 2√3, i.e.

(56) $\frac{t_0}{2\sqrt{3}}=\frac{80}{2\sqrt{3}}=23.1.$

(57) This polyisoprene synthetic process according to an embodiment of the invention, in highly concentrated medium and continuously, shows that if the combination of the established conditions of the process according to an embodiment of the invention, namely a minimum conversion of 60% and a standard deviation of the residence time distribution function in the polymerisation reactor greater than the mean residence time divided by 2√3, is respected, no expansion phenomenon is observed. This process in accordance with the invention provides a solution to the technical problem of providing a flexible process, especially over a wide temperature range, that is adaptable to an economically advantageous industrial production due to the reduced amount of solvent used, while at the same time ensuring an increased production efficiency.

Example 2

(58) Process for Synthesizing a Polyisoprene by Coordination Catalysis in Highly Concentrated Medium According to Another Variant of the Invention

(59) The polymerisation proceeds in the same manner as in Example 1. The reagents and the amounts injected into reactor 1 are the same. The stopper and the antioxidant are no longer injected at the outlet of reactor 1.

(60) The gear pump 10 transfers the elastomer paste to a self-cleaning blender with a gas phase 15. The flow rate transferred by the gear pump 10 is equal to the sum of the flow rates injected into reactor 1.

(61) The self-cleaning blender is a twin-arm blender-reactor with a total volume of 39 liters, with a discharge device consisting of an emptying twin screw and a gear pump 18. The flow in this twin-arm mixer-reactor may be likened to a piston flow.

(62) The pressure of the gas phase is set at 0.16 barg relative, and the vapours are condensed in an external condenser 16 and conveyed to the self-cleaning blender 15.

(63) The jacket temperature is maintained at 35° C.

(64) The mean residence time t.sub.0 of the reaction in the self-cleaning blender 15 is 37 minutes.

(65) The stopper and the antioxidant used are oleic acid, at 1 phr (phr: parts by mass per 100 parts by mass of elastomers) and N-1,3-dimethylbutyl-N′-phenyl-para-phenylenediamine, at 0.5 pcm (pcm: parts by mass per 100 parts by mass of isoprene monomer). This stopper and this antioxidant are injected one after the other at the outlet of reactor 15, upstream of the gear pump 18, as flows 11 and 12.

(66) The gear pump 18 transfers the elastomer paste to the underwater granulator 13. The flow rate transferred by the gear pump 18 is equal to the sum of the flow rates injected into reactor 1, to which is added the flow rates of the streams of stopper 11 and of antioxidant 12.

(67) No expansion phenomenon is observed.

(68) The conversion measured on a sample taken from stream 17 upstream of the gear pump 18 is 99.98%. The conversion measurement was established from a GC measurement of the isoprene in the sample taken. The residual isoprene in the sample taken was assayed at 74 ppm.

(69) This polyisoprene synthetic process according to an embodiment of the invention, in highly concentrated medium and continuously, shows that if the combination of established conditions of the process according to an embodiment of the invention, namely a minimum conversion of 60% and a standard deviation of the residence time distribution function in the polymerisation reactor greater than the mean residence time divided by 2√3, is respected, no expansion phenomenon is observed. This process in accordance with the invention provides a solution to the technical problem of providing a flexible process, especially over a wide temperature range, that is adaptable to an economically advantageous industrial production due to the reduced amount of solvent used, while at the same time ensuring increased production efficiency, without expansion of the reaction medium. This process also makes it possible to increase the degree of conversion up to 99.98%.

Example 3

(70) Process for Synthesizing a Polyisoprene by Coordination Catalysis in Highly Concentrated Medium not in Accordance with the Invention

(71) The polymerisation process was performed on a continuous line in the installation comprising a stirred and self-cleaning reactor of continuous blender technology, with a gas phase and twin arms, with a total volume of 29 liters, equipped with a discharge device consisting of a twin screw and a gear pump. The speed of the arms is 20 rpm.

(72) Isoprene and pentane are mixed in a dynamic mixer provided for this purpose upstream of the polymerisation reactor. The mixture obtained is injected in the form of a stream entering directly into the polymerisation reactor.

(73) The isoprene is injected at a flow rate of 3.63 kg/h into the dynamic mixer.

(74) The pentane is injected at a flow rate of 4.47 kg/h into the dynamic mixer, i.e. about 42% by mass of monomer in the reaction medium.

(75) A coordination catalytic system is also injected directly into the polymerisation reactor. The catalytic system is based on neodymium tris[bis(2-ethylhexyl)phosphate], butadiene as preformation monomer, DiBaH as alkylating agent and DEAC as halogen donor. It was prepared according to the preparation method described in paragraph I of the abovementioned document WO-A-03/097 708 in the name of the Applicants.

(76) The catalytic system has the following mole ratios relative to the neodymium: Nd/butadiene/DiBaH/DEAC=1/30/1.4/2.6

(77) The amount of neodymium is 350 μmol per 100 g of isoprene. The mole concentration of neodymium in the catalyst is 0.02 mol/L.

(78) DiBaH is also injected directly into reactor 1 as a separate and independent stream 7. The amount of DiBaH injected is 50 μmol/100 g of isoprene. The mole concentration of the DiBaH solution is 0.01 mol/L.

(79) The pressure of the gas phase is set at 0.5 barg, and the vapours are condensed in an external condenser and conveyed to the polymerisation reactor.

(80) The jacket temperature is maintained at 48° C.

(81) The mean residence time t.sub.0 of the reaction is 54 minutes.

(82) The discharge twin screw makes it possible to transfer the product from the reactor to a gear pump.

(83) The stopper and the antioxidant used are oleic acid, at 1 phr (phr: parts by mass per 100 parts by mass of elastomers) and N-1,3-dimethylbutyl-N′-phenyl-para-phenylenediamine, at 0.5 pcm (pcm: parts by mass per 100 parts by mass of isoprene monomer). This stopper and this antioxidant are injected one after the other at the reactor outlet, upstream of the gear pump.

(84) The gear pump transfers the elastomer paste to an underwater granulator. The flow rate transferred by the gear pump is equal to the sum of the flow rates injected into polymerisation reactor, to which is added the flow rates of stopper and of antioxidant.

(85) During this test, expansion of the reaction medium was observed. This expansion did not make it possible to control the heat of the reaction.

(86) The conversion measured on a sample taken, at the limit of the first third of the reactor volume, is 56%, i.e. less than the minimum limit required for the process of the invention. The conversion measurement is established from a GC measurement of the isoprene in the sample taken. The residual isoprene, in the sample taken, is assayed at 18% by mass.

(87) The flows are characterized by the residence time distribution given in FIG. 4.

(88) The experimental points for establishing this residence time distribution were obtained by gas chromatographic measurement of the changes in concentration of a tracer following a very rapid injection according to a method of pulse introduction of a chemically inert product according to the principle described in the book by Jacques Villermaux, Génie de la réaction chimique: conception et fonctionnement des reacteurs. Editors. 1993, TEC & DOC—LAVOISIER, pages 170 to 172. The experimental points are derived from samples taken at the polymerisation reactor outlet, at the end of the discharge screw.

(89) Modelling of these experimental points by a gamma law made it possible to determine the variance of the residence time distribution function E.

(90) The gamma law used for modelling the residence time distribution function E is the following:

(91) $E={\left(\frac{J}{t_0}\right)}^J\frac{t^{J-1}*e^{\frac{-\mathrm{Jt}}{t_0}}}{\left(J-1\right)!}$ with:
J: number of reactors in series;
t.sub.0: mean residence time, i.e. 54 minutes;
t: residence time.

(92) The variance of this residence time distribution function is equal to

(93) $\frac{t_0^2}{J}.$

(94) Adjustment of the function to the experimental points makes it possible to determine the parameter J, which is equal to 14.

(95) The standard deviation of the residence time distribution function is deduced therefrom, which is equal to 14.43, which is less than the mean residence time divided by 2√3, i.e.

(96) $\frac{t_0}{2\sqrt{3}}=\frac{54}{2\sqrt{3}}=15.59,$
which is the minimum limit required for the process of the invention.

(97) This process not in accordance with an embodiment of the invention, for the synthesis of polyisoprene in highly concentrated medium and continuously, shows that if the combination of the established conditions of the process according to an embodiment of the invention, namely a minimum conversion of 60% and a standard deviation of the residence time distribution function in the polymerisation reactor greater than the mean residence time divided by 2√3, is not respected, an expansion phenomenon is observed, which no longer makes it possible to control the heat of the reaction. This process not in accordance with the invention does not provide a solution to the technical problem of providing a flexible process, especially within a wide temperature range, which is adaptable to an economically advantageous industrial production while at the same time ensuring increased production efficiency.

APPENDIX 1

(98) The inherent viscosity is determined by measuring the flow time (t) of the polymer solution and the flow time of toluene (t.sub.0), in a capillary tube. The flow time of toluene and that of the polymer solution (C) at 0.1 g/dl are measured in an Ubbelhode tube (capillary diameter 0.46 mm, volume 18 to 22 ml), placed in a bath thermostatically maintained at 25±0.1° C.

(99) The inherent viscosity is obtained by the following relationship:

(100) ${\eta }_{\mathrm{inh}}=\frac{1}{C}\ln \left[\frac{\left(t\right)}{\left(t_0\right)}\right]$ with: C: polymer concentration of the toluene solution in g/dl; t: flow time of the toluene solution of polymer in seconds; t.sub.0: flow time of the toluene in seconds; η.sub.inh: inherent viscosity expressed in dl/g.