This report describes complete density functional theory studies performed to elucidate ethylene polymerisation mechanisms of the Brookhart-type catalysts N,N'-(2,6-dimethylphenyl)ediylenediimine nickel (II) and N-(2,6-dimethylphenyl)pyridine-2-carboxaldiimine nickel (II). Both catalysts showed conformations that blocked the active site and thus hindered ethylene coordination. These conformations were related to the rotational capacity of the ancillary aryl group attached to the ligands, which generates agostic interactions between nickel and hydrogen atoms. It was calculated that these conformations were 8.3 and 10.7 kcal/mol (respectively for each catalyst) more stable than non-blocking cationic conformations. From a design perspective, catalytic systems would need to include bulky constituents to avoid these types of blocking conformation. The two possible mechanisms already proposed for the chain initiation step evoke approaches for ethylene inside and out of the equatorial plane of the catalyst. Herein, the in-plane mechanism was found to be the most favourable. However, for chain propagation, the out-plane approach for ethylene proved to be the most feasible, due to nickel agostic interactions with the growing alkyl chain. The energy barriers calculated for the propagation step were 9.3 and 10.6 kcal/mol for the two catalysts, respectively. These findings indicate the ethylenediimine catalyst is slightly more active than the pyridine catalyst. Three possible chain transfer mechanisms were also considered for the two catalysts: beta-hydride transfer, beta-hydride transfer to monomer and an association mechanism; the last two being the mostly likely mechanisms in thermodynamics and kinetic terms. Chain transfer processes for the pyridine-2-carboxaldiimine based catalyst were found to be more favourable than those of the ethylenediimine based catalyst. This finding is in agreement with experimental results which indicate that the former catalyst gives rise to lower molecular weight polymers. (C) 2003 Elsevier Science Ltd. All rights reserved.