The integrated molecular orbital-molecular mechanics (IMOMM) method adopting the B3LYP:MM3 combination has been used to study the full catalysts in the diimine-M (M = Ni, Pd) catalyzed ethylene polymerization reaction. These results have been compared with previous molecular orbital calculations on model systems (model). There is a lowering of the migratory insertion activation barriers when including substituent effects from 9.9 (model) to 3.8 (IMOMM) kcal/mol for nickel and from 16.2 (model) to 14.1 (IMOMM) kcal/mol for palladium. Steric interactions decrease the complexation energy which leads to a lowering of the barrier. The beta-H transfer process which involves the reaction n-propyl beta-agostic --> olefin hydride --> isopropyl beta-agostic is the likely mechanism leading to branching of polyethylenes. In the nickel system, the olefin-hydride intermediate lies 13.6 (model) or 14.5 (IMOMM) kcal/mol above the n-propyl beta-agostic species, indicating that this pathway is unlikely for unsubstituted or substituted nickel diimine catalysts. For palladium, where the olefin-hydride intermediate resided 5.4 kcal/mol above the beta-agostic species in model B3LYP predictions, IMOMM reduces this difference to almost zero, suggesting branching to be more prominent with bulky substituents. Although beta-H transfer is more likely for substituted palladium, the formation of the 5-coordinate intermediate is not possible due to steric effects and thus an associative chain termination process is not possible for substituted palladium while it likely can occur for unsubstituted Pd catalysts.