Abstract: A diene polymer composition having improved fracture properties and a small change in the shape of extruded product is disclosed, which comprises at least 5% by weight of a copolymer composed of at least one conjugated diene (component A) having the following general formula: ##STR1## wherein R.sub.1 and R.sub.2 are a hydrogen atom or a methyl group, and 2,3-dimethylbutadiene (component B), said copolymer having a content of cis-1,4-bond of not less than 80%.

Abstract: A process for preparing polybutadiene having a 1,2-configuration content of 5-40% and a cis-1,4-configuration content of 60% or more, which comprises polymerizing 1,3-butadiene in the presence of a catalyst consisting of:(A) a cobalt compound,(B) at least one member selected from the group consisting of:(i) a trialkylaluminum,(ii) a reaction product of a trialkylaluminum and an alcohol or water,(iii) a mixture of a trialkylaluminum and a dialkylaluminum halide,(iv) a dialkylaluminum hydride, and(v) an organolithium compound, and(C) at least one compound selected from the group consisting of phenyl isothiocyanate and carbon disulfide, in a hydrocarbon or halogenated hydrocarbon solution of a high cis-1,4-polybutadiene obtained by polymerizing 1,3-butadiene with a catalyst consisting of:(D) at least one compound selected from the group consisting of the organocarboxylates of nickel and organic complex compounds of nickel,(E) at least one compound selected from the group consisting of boron trifluoride, boron tr