The effect of an alkyl substituted in the aromatic ring of the salen ligand on the polymerization of butadiene (Bd) with (salen)Co(II) complexes in combination with methylaluminoxane (MAO) was investigated. The activity for the polymerization of Bd was influenced significantly by the introduction of alkyl groups at the 3,3',5,5'-positions in the aromatic ring of the salen ligand, and both the polymerization rate and 1,4-cis contents increased in the following order with respect to the alkyl group: H < CH3 < t-C4H9. This is in good agreement with the bulkiness of the alkyl groups. The activity for the polymerization of the (salen)Co(II) complex possessing t-C4H9 at the 3,3'-positions was higher than that of the (salen)Co(II) bearing t-C4H9 at the 5,5'-positions. Thus, the introduction of bulky substituents at the 3,3'-positions of the salen ligand was an important factor in achieving both high activity and high 1,4-cis selectivity in the polymerization of Bd with (salen)Co(II) complexes in combination with MAO. (c) 2006 Wiley Periodicals, Inc.