Abstract: A chloroprene rubber composition having a high damping performance, comprising a solid chloroprene polymer and a low molecular weight chloroprene polymer having a number average molecular weight of from 500 to 50,000, obtained by polymerization in the presence of an alkylmercaptan and/or a mercaptoalcohol.

Abstract: A process for producing a conjugated diolefinic polymer by polymerizing at least one conjugated diolefin or copolymerizing at least one conjugated diolefin with at least one vinyl aromatic hydrocarbon in a hydrocarbon solvent in the presence of an alfin catalyst and at least one molecular weight regulator selected from the group consisting of (A) unsaturated halohydrocarbons represented by the general formula, RCX .dbd. CYZ, wherein R represents hydrogen, an alkyl group having 1 to 7 carbon atoms, a vinyl group, a phenyl group, a substituted phenyl group, or a halogen atom and X, Y and Z represent independently a hydrogen or halogen atom, at least one of said R, X, Y and Z being a halogen atom, (B) halogenated aromatic hydrocarbons, (C) ethers, polyethers, and acetals and (D) tertiary amines and, if necessary, a dihydro aromatic hydrocarbon, characterized in that said polymerization or copolymerization is effected in the presence of 0.03 to 0.