Homo-and copolymerizations of butadiene (BD) and styrene (St) with rareearth metal catalysts, including the most active neodymium (Nd)-based catalysts, have been examined, and the cis-1,4 polymerization mechanism was investigated by the diad analysis of copolymers. Polymerization activity of BD was markedly affected not only by the ligands of the catalysts but also by the central rare-earth metals, whereas that of St was mainly affected by the ligands. In the series of Nd-based catalysts [Nd(OCOR) 3 : R Γ
CF 3 , CCl 3 , CHCl 2 , CH 2 Cl, CH 3 ], Nd(OCOCCl 3 ) 3 gave a maximum polymerization activity of BD, which decreased with increasing or decreasing the pKa value of the ligands. This tendency was different from that for Gd(OCOR) 3 catalysts, where the CF 3 derivative led to the highest polymerization activity of BD. For the polymerization of St and its copolymerization with BD, the maximum activities were attained at R Γ
CCl 3 for both Nd-and Gd-based catalysts. The copolymerization of BD and St with Nd(OCOCCl 3 ) 3 catalyst was also carried out at various monomer feed ratios, to evaluate the monomer reactivity ratios as r BD Γ
5.66 and r St Γ
0.86. The cis-1,4 content in BD unit decreased with increasing St content in copolymers. From the diad analysis of copolymers, it was indicated that Nd(OCOCCl 3 ) 3 catalyst controls the cis-1,4 structure of the BD unit by a back-biting coordination of the penultimate BD unit. Furthermore, the long range coordination of polymer chain by the neodymium catalyst was suggested to assist the cis-1,4 polymerization.