Highly Branched High cis Polybutadiene

PURPOSE : A method for preparing a highly branched cis-1, 4-polybutadiene having a high cis-1, 4 content is provided, to reduce the cold flow without the increase of Mooney viscosity, thereby improving processability and storage stability. CONSTITUTION : The method comprises the steps of polymerizing 1, 3-butadiene or butadiene derivative in the presence of a polymerization catalyst comprising a rare earth metal compound, a halogen-containing compound and an organic aluminum compound in a nonpolar solvent, to prepare cis-1, 4-polybutadiene having 95 % or more of cis-1, 4 content; and adding sulfur chloride and mercaptan to the cis-1, 4 polybutadiene for the radical reaction. Preferably the butadiene derivatives are 1, 3-butadiene, isoprene, 1, 3-pentadiene, 2, 3-dimethyl-1, 3-butadiene, myrcene or their derivatives; the rare earth metal compound is a salt of organic acids or inorganic acids; the halogen-containing compound is a halogen-containing Lewis acid or an organic halogen compound capable of donating halogen easily; and the nonpolar solvent is butane, pentane, hexane, isopentane, octane, isooctane, cyclopentane, methyl cyclopentane, cyclohexane, methyl cyclohexane, ethyl cyclohexane, benzene, toluene, ethyl benzene or xylene.