Styrene was copolymerised with ethylene using the catalyst systems [rac-Et(H(4)Ind)(2)TiCl2]/MAO and [rac-Et(H(4)Ind)(2)ZrCl2]/MAO. Keeping other experimental variables under control, we tested different styrene/ethylene ratios in the reactor feed. It was found that the titanium-based catalyst showed very low activity even for ethylene homopolymerisation. In contrast, the zirconium system achieved monomer polymerisation, incorporating small amounts of styrene. When the styrene/ethylene ratio was increased, both catalyst activity and the molecular weight of the resulting copolymers decreased. However, styrene incorporation into the copolymer increased as the styrene/ethylene ratio rose. To gain insight into the copolymerisation mechanisms at play, we undertook a computational study using a high-level hybrid DFT method (B3LYP). Agreement between the experimental and theoretical results was generally good, indicating the usefulness of combined experimental/theoretical studies for clarifying mechanisms of a-olefin copolymerisation using organometallic systems. (C) 2004 Elsevier Ltd. All rights reserved.