The homopolymerization of ethylene by using different catalytic systems based on dinitro-substituted bis(salicylaldiminate)nickel(II) precursors such as bis[3,5dinitro-N(2,6-diisopropylphenyl)]nickel(II) and bis[3,5-dinitro-N(phenyl)]nickel(II) in combination with organoaluminum compounds was investigated. In particular, the catalytic performances were studied as a function of the main reaction parameters, such as temperature, pressure, Al/Ni molar ratio, and duration. Methylaluminoxane resulted in the best co-catalyst. Activities up to 200 kg polyethylene/(mol Ni X h) to give a linear high-molecular-weight polymer were achieved. The influence of the bulkiness of the substituents on the N-aryl group of the aldimine ligand was also checked; it resulted in a determinant for catalytic activity rather than for polymer characteristics. (C) 2004 Wiley Periodicals, Inc.