This report describes a computational DFT study, based on the hybrid density functional B3LYP method, designed to investigate the hydrogenolysis process that occurs in ethylene polymerisation catalysed by a nickel-bulk diimine catalyst (N,N'-bis(2,6-diisopropylphenyl)2,3-butanediimine nickel(II) methyl cation [[(2,6-(Pr-i)(2)Ph)N=C(Me)-C(Me) = N(2,6-(Pr-i)(2)Ph))NiCH3](+). Mechanisms of ethylene and hydrogen insertions into the growing polymer chain and ethylene insertion into the hydride complex formed after hydrogenolysis were investigated. The results obtained for these catalysts were then compared to those calculated for metallocene catalysts. Due to a higher difference in energy barriers between ethylene and hydrogen insertion, it may be concluded that controlling the molecular weight of the resultant polymer using nickel diimine catalysts is even more difficult than for the metallocene catalysts. Through statistical thermodynamics and kinetics calculations it was shown that the activity and molecular weights of polymers produced by nickel-diimine catalysts are dramatically decreased in the presence of hydrogen with respect to those of polymers generated by metallocene catalysts. These findings help to clarify the contradictory experimental results published in the scientific and technical literature regarding the effect of hydrogen on the catalytic activity and molecular weight of polymers produced using nickel diimine catalysts. (C) 2003 Elsevier Science Ltd. All rights reserved.