Process for preparing random butadiene-isoprene copolymers having a high content of cis-1,4 units

A process for the preparation of a random butadiene-isoprene copolymer having a high content of cis-1,4 units comprising copolymerizing butadiene and isoprene in the presence of at least one organic solvent and of a catalytic system prepared in situ comprising:

• (a1) at least one neodymium carboxylate which is soluble in the organic solvent, containing a variable amount of water, the H2O/Nd molar ratio being between 0.001/1 and 0.50/1;
• (a2) at least one aluminum alkyl compound;
• and
• (a3) at least one aluminum alkyl compound containing at least one halogen atom,
• wherein the molar ratio between the halogen present in the at least one aluminum alkyl compound containing at least one halogen atom (a3) and the neodymium carboxylate (a1) is between 2.5/1 and 5.5/1,
• wherein the aluminum alkyl compound containing at least one halogen atom (a3) is used in such an amount that the molar ratio between the halogen present in the aluminum alky compound (a3) and the aluminum alkyl compound (a2) is between 0.4 and 5, and
• wherein the random butadiene-isoprene copolymer has a molecular weight distribution, indicated as a polydispersion index corresponding to the ratio between the weight average molecular weight (Mw) and the number average molecular weight (Mn) (i.e. the ratio Mw/Mn) of between 2.0 and 3.2.