Abstract: A 1,2-polybutadiene composition having an excellent reinforcing effect for rubber and synthetic resin materials including at least two polybutadiene components different in melting point from each other and each having, as a main structure, a 1,2-polybutadiene structure, produced by polymerizing a 1,3-butadiene-containing monomer in a water-containing polymerization system in the presence of a catalyst comprising a transition metal compound, an organic compound of one of the Group I to III metals, and one of carbon disulfide, phenyl isothiocyanate, and xanthic acid compounds, while adding a melting point-regulating agent comprising at least one of ketones, aldehydes, alcohols, esters, nitriles, sulfoxides, amides, and phosphoric acid esters in one or two or more separate adding operations or a continuous adding operation.

Abstract: A process for producing polybutadiene essentially consisted of syndiotactic-1,2-polybutadiene in an aqueous medium includes a step off polymerizing diene essentially consisted of 1,3-butadiene in the presence of: (A) a catalyst composition including (a) a transition metal compound, and (b) an organometallic compound which includes a metal selected from the elements consisting off groups 1, 2, 12, and 13 of Periodic Table; (B) an initiator comprising at least one component selected from a group consisting of carbon disulfide, an isothiocyanate compound, and a xanthogenic compound; and (C) an inert organic liquid having a density of at least 1.10, thereby resultant particles of syndiotactic-1,2-polybutadiene have a diameter up to 2 min.