(alpha-Diiminc)nickel(II) dibromide complexes and their derivatives can be used for the polymerization and oligomerization of ethylene after activation with methyl-aluminoxane (MAO). The activities of these catalysts and the properties of the obtained polyethylenes depend on the structure of the used catalyst precursors. Therefore a variety of (a-diimine)nickel(II) dibromide complexes with chlorine and methyl substituents on the ligands and various substituents on the ligand backbone were studied as catalysts for the homogeneous polymerization of ethylene. The range of the polymerization products reaches from oligomers to polymers of low molecular weights. (C) 2002 Elsevier Science B.V. All rights reserved.