THEORETICAL-STUDY OF SUBSTITUENT EFFECTS IN THE DIIMINE-M(II) CATALYZED ETHYLENE POLYMERIZATION REACTION USING THE IMOMM METHOD

The integrated molecular orbital-molecular mechanics (IMOMM) method ad opting the B3LYP:MM3 combination has been used to study the full catal ysts in the diimine-M (M = Ni, Pd) catalyzed ethylene polymerization r eaction. These results have been compared with previous molecular orbi tal calculations on model systems (model). There is a lowering of the migratory insertion activation barriers when including substituent eff ects 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 d ecrease the complexation energy which leads to a lowering of the barri er. The beta-H transfer process which involves the reaction n-propyl b eta-agostic --> olefin hydride --> isopropyl beta-agostic is the likel y mechanism leading to branching of polyethylenes. In the nickel syste m, the olefin-hydride intermediate lies 13.6 (model) or 14.5 (IMOMM) k cal/mol above the n-propyl beta-agostic species, indicating that this pathway is unlikely for unsubstituted or substituted nickel diimine ca talysts. 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 t o be more prominent with bulky substituents. Although beta-H transfer is more likely for substituted palladium, the formation of the 5-coord inate intermediate is not possible due to steric effects and thus an a ssociative chain termination process is not possible for substituted p alladium while it likely can occur for unsubstituted Pd catalysts.