Ethylene polymerization was carried out with two nickel(II)(alpha-diimine) complexes: 1,4-bis(2,6-diisopropylphenyl)-acenaphthenediimine-dichloro-nickel(II) (1) and 1,4-bis(2,6-diisopropylphenyl)-ethanediimine-dichloronickel(II) (2) combined with alkylaluminum activators like methylalurnoxane (MAO), trimethylaluminum (TMA), and ethylaluminum sesquichloride (EASC). The activity of these systems and the proprieties of the polymeric materials are strongly influenced by the choice of the ligand structure and, more dramatically, by the nature of the cocatalyst. EASC has been shown as giving higher productivities than TMA or MAO with both complexes, attaining 3966 kg PE/(mol Ni.h). Complex 2 gives lower productivities but produced polymers with higher polymer melting points (T-m) and crystallinity than catalyst 1. (C) 2007 Elsevier B.V. All rights reserved.