Synthetic polyisoprenes and their preparation method

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The present invention relates to synthetic polyisoprenes having a high content of cis-1, 4 linkages and a process for their preparation. The synthetic polyisoprenes according to the invention have a content of cis-l, 4 linkages, measured using the technique of carbon-13 nuclear magnetic resonance or of medium-wave infrared radiation analysis, which is strictly greater than 99.0%. The process according to the invention for the preparation of these synthetic polyisoprenes consists essentially of reacting a catalytic system in the presence of isoprene, and it consists of : • using, as catalytic system, a system based on at least : - a conjugated diene monomer, - an organic phosphoric acid salt of one or more rare earths, - an alkylaluminium alkylating agent of the formula AIR 3 or HAIR 2 , and - a halogen donor consisting of an alkylaluminium halide, the molar ratio (alkylating agent / rare earth salt) being from 1 to 5, and • carrying out the polymerisation reaction of the isoprene at a temperature less than or equal to 5°C.