Different chlorine-free alkylaluminum compounds were active cocatalysts for ethylene polymerization in the presence of 1,4-bis(2,6-diisopropylphenyl)-acenaphthenediimine-dichloronickel (II) (1). The combination of 1 with trimethylaluminum or triisobutylaluminum produced catalytically active species that polymerized ethylene with productivities up to 469 kg(polymer)/(mol(Ni).h). The activity of the catalytic system and the properties of the polymeric materials were influenced strongly by the reaction temperature. The polymers had a high molecular weight (up to 642 x 10(3) g.mol(-1)), and the molecular weight increased with the reaction time. The polyethylenes were branched, and the branching could be modulated by the proper choice of reaction parameters.