A new series of monochelate nickel(II) catalysts, obtained by oxidative addition of salicylaldimine ligands to bis(1,5-cyclooctadiene)nickel(0), was reported for the polymerization of ethylene (E). Almost stoichiometric amounts of MAO as co-catalyst were necessary in order to activate these systems, the productivity being dependent on the Al/Ni molar ratio. The activity and the average molecular weight of the resulting PE were mainly influenced by the nature of the substituents on both the phenolate moiety and on the N-aryl ring. In particular, when two nitro groups are present on the 3,5-positions of phenolate moiety high activity (up to 120,000 g PE/(mol Ni X h)) was observed. The obtained PE samples showed a high linearity and average molecular weight in the 100,000-500,000 Da range. The obtained results seem to suggest that, at least for these catalysts based on salicylaldiminate ligands, the presence of electron withdrawing nitro groups on the phenoxy moiety plays a more important role with respect to the bulkiness of the substituents on both the ortho positions of the N-aryl group in order to prevent chain transfer and thus enhance the molecular weight of the polymer. (C) 2003 Elsevier Science Ltd. All rights reserved.