Process for the preparation of polybutadiene with a low branching degree

A process for the preparation of polybutadiene with a low branching degree having the following characteristics:

• branching index value (gM) lower than 1;
• damping coefficient value (tans), defined as the trigonometric tangent of the ratio between the viscous modulus (G″) and the elastic modulus (G′) [tan δ=G″/G′] measured at 0.01 Hz, 100° C. and 1% of strain, ranging from 0.80 to 1.40;
• Mooney viscosity lower than 49;
• Mw/Mn lower than 2.9;
• percentage of 1,4-cis units higher than 93%;
• the above process being effected by the polymerization of 1,3-butadiene in the presence of neodymium catalysts, the above process comprising:
• (a) a first polymerization step of butadiene in the presence of organic solvents and in the presence of a catalytic system comprising (a1) a derivative of neodymium selected from neodymium carboxylates with a molar ratio H2O/Nd lower than 1, and a molar ratio —COOH/Nd lower than 2;
• (a2) an aluminum alkyl having the general formula (I) AlR1R2R3 wherein R1 and R2, the same or different, are an alkyl radical containing from 1 to 10 carbon atoms or a hydrogen atom, R3 is an alkyl radical containing from 1 to 10 carbon atoms;
• (a3) an organo aluminum derivative containing at least one halogen atom;
• the above first step being carried out up to a butadiene conversion ≧98% to give a linear polybutadiene (gM=1) with a content of 1,4-cis units greater than 93%, and an Mw/Mn ranging from 2.2 to 2.9;
• (b) treatment of the polymeric solution obtained at the end of step (a) with a peroxide until polybutadiene with a low branching degree is obtained;
• (c) recovery of the polybutadiene with a low branching degree obtained at the end of step (b).