The homopolymerization of methyl methacrylate was investigated with bis(salicylaldiminate)nickel(II) complexes, such as bis[3,5-dinitro-N(2,6-diisopropylphenyl)salicylaldiminate]nickel(II) (IIIa) and bis[3,5-dinitro-N(phenyl)salicylaldiminate]nickel(II) (IIIb), and with methylaluminoxane (MAO) as an activator. In particular, the effect of the Al/Ni molar ratio on the catalytic activity and on the properties of the resulting poly(methyl methacrylate) (PMMA) was checked. The maximum activity was ascertained when an Al/Ni molar ratio equal to about 100 was used. However, the productivity of the catalytic systems was rather low. When the IIIa/MAO catalytic system was prepared under an ethylene atmosphere, an extremely high activity was observed, a productivity value of up to around 150,000 g of PMMA/(mol of Ni X h) being obtained, the highest ever found with nickel-based catalysts. No appreciable presence of ethylene counits in the polymeric products was also ascertained. When the IIIb/MAO system was used, similar results were found, and high molecular weight PMMAs were obtained, despite the absence of bulky isopropyl substituents in positions ortho and ortho' to the N-aryl moiety of the salicylaldiminate ligand. (C) 2003 Wiley Periodicals, Inc.