Homo- and copolymerizations of ethylene have been carried out by zirconocarborane [Cp*(C2B9H11)Zr(Me)](n) (Cp*: C5Me5, Me: methyl) catalyst in the presence of common alkylaluminum or methyl aluminoxane (MAO) as a cocatalyst. The polymerization rate was not proportional to the concentration of catalyst and was strongly dependent upon the type and amount of cocatalyst used for activation. The catalytic activity was sensitive to the polymerization temperature, due to the complicated equilibrium of the cyclopentadienyl dicarbollide complexes formed at different temperatures. Copolymerizations of ethylene and 1-hexene or I-octene showed rate enhancement behavior, while the catalyst system was characterized by negligible comonomer reactivity. The multiple melting peaks of polyethylenes that can be fractionated demonstrated that the active sites are not uniform. End group analysis of the polymer showed that beta-hydride elimination is a main chain transfer process, but the chain transfer to cocatalyst becomes dominant as the cocatalyst concentration increases. (C) 2003 Elsevier B.V. All rights reserved.