The present computational study was designed to study the polymerization of ethylene catalyzed by a new Ni-based PymNox organometallic compound. Recently, we have synthesized and tested the behavior of this type of catalyst in olefin polymerization. It has been experimentally observed that the unsubstituted catalyst Ni2 (aldimino PymNox catalyst) is less active than the methyl substituted Nil (acetaldimino PymNox catalyst) analogue. The reactivity of both catalysts was examined using density functional theory (DFT) models. Our results indicate that the methyl substituted Nil introduces some additional steric hindrance that probably renders a more suitable catalyst conformation for the monomer incorporation. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48:1160-1165, 2010