The copolymerization of ethylene with cyclopentene catalyzed by three alpha-diimine nickel(II) complexes in the presence of methylaluminoxane (MAO) was investigated. High-molecular-weight branched ethylene/cyclopentene copolymers with only cis-1,3-enchained cyclopentene units, which has not been reported previously, were obtained. The catalytic activity, cyclopentene incorporation, copolymer molecular weight, and molecular-weight distribution could be controlled over a wide range through the variation of the catalyst structure and polymerization conditions, including cyclopentene concentration in the feed and polymerization temperature. (C) 2008 Wiley Periodicals, Inc.