The ligand structure of (alpha-diimine)nickel(II) catalyst precursors has a significant influence on the behavior of these catalysts in the polymerization of ethylene. After activation with MAO, homogeneous nickel catalysts of this type are suitable to polymerize olefins to low molecular oligomers and/or more or less branched polymers. The variation of the ligand framework of the catalysts with halogen substituents in different positions shows clear dependencies between these substituents and the properties of the polymerization products. Unsaturated oligomers with an even number of carbon atoms (4-50) and all possible isomers as well as polymers up to a molecular weight of 36,000 g/mol can be obtained. (C) 2002 Elsevier Science B.V. All rights reserved.