Copolymerization of styrene (St) and butadiene (Bd) with CpTiCl3/methylaluminoxane (MAO) catalyst in the presence or absence of chloranil (CA) was investigated. The CpTiCl3/MAO catalyst showed a high activity for the copolymerization of St with Bd. The 1,4-cis contents in the Bd units for the copolymerization of St and Bd with the CpTiCl3/MAO catalyst was observed, and the 1,4-cis content was optimum at a MAO/Ti mole ratio of around 225. The effect of the polymerization temperature on the copolymerization was noted, as was the effect of the 1,4-cis microstructure in the Bd units for the copolymerization of St and Bd. The addition of CA to the CpTiCl3/MAO catalyst was found to influence the molecular weight of the copolymer. The high weight-average molecular weight copolymer (M = ca. 50 X 10(4)) consisting of mainly a 1,4-cis microstructure of Bd units (1,4-cis = 80.0%) was obtained from the copolymerization with the CpTiCl3/MAO catalyst in the presence of CA (CA/Ti mole ratio = 1) at 0degreesC. (C) 2003 Wiley Periodicals, Inc.