The copolymerization of styrene and 1,3-butadiene (Bd) or isoprene (Ip) was carried out with half-sandwich titanium(IV) Cp'TiCl3 catalysts (where Cp' is cyclopentadienyl 1, indenyl 2, or pentamethyleyclopentadienyl 3) with methylaluminoxane as a cocatalyst. For the copolymerization with Bd, catalyst 3 gave the copolymers containing the highest amount of Bd among the catalysts used. The resulting copolymers were composed of a styrene-Bd multiblock sequence. High melting points were observed in the copolymers prepared with catalyst 1. The structures of hydrogenated poly(styreneco-Bd) were studied by C-13 NMR spectroscopy, and the long styrene sequence length was detected in the copolymers prepared with catalyst 1. For styrene/Ip copolymerization, random copolymers were obtained. Among the used catalysts, catalyst 1 gave the copolymers containing the highest amount of Ip. The copolymers prepared with catalyst 1 showed a steep melting point depression with increasing Ip content because of the high ratio of 1,4-inserted Ip units and/or the low molecular weights of the copolymers. (C) 2003 Wiley Periodicals, Inc.