It was found that poly(butadiene), poly(isoprene), and poly(2,3-dimethylbutadiene) with high cis-1,4 content were obtained with Nd(OCOR)(3)-(i-Bu)(3)Al-Et2AlCl catalysts (R = CF3, CCl3, CHCl2, CH2Cl, CH3) in hexane at 50 degrees C [cis-1,4 content : poly(BD), > 98%; poly(TP), greater than or equal to 96%; poly(DMBD), greater than or equal to 94%]. Copolymerization of IF and styrene (St) was carried out at various monomer feed ratios to evaluate the monomer reactivity ratio and cis-1,4 content of the diene unit and then to elucidate the cis-l,4 polymerization mechanism of IF. The cis-1,4 content of the IP unit in the copolymers decreased with increasing St content in the copolymers. The cis-l,4 polymerization was disturbed by incorporating St unit in the copolymers, since the penultimate St unit hardly coordinates to the neodymium metal, resulting in a decrease of the cis-1,4 content in the copolymers. That is, the cis-1,4 polymerization of IP is suggested to be controlled by a back-biting coordination of the penultimate diene unit. On the other hand, in the case of poly(BD-co-IP) and poly(BD-co-DMBD), the cis-1,4 content of the ED, IF, and DMBD units in the copolymers was almost constant (cis : 94-98%), irrespective of the monomer feed ratios and polymerization temperature. Consequently, the penultimate IP and DMBD units favorably control the terminal ED, IF, or DMBD unit to the cis-1,4 configuration through the back-biting coordination. For the monomer reactivity ratios, a clear difference was observed in each system : r(BD) = 1.22, r(IP) = 1.14; r(BD) = 40.9, r(DMBD) = 0.15 LOW polymerizability of DMBD was mainly ascribed to the steric effect of the methyl substituents.