A Density Functional Study of the Mechanism of the Diimine−Nickel-Catalyzed Ethylene Polymerization Reaction

Abstract

The mechanism of diimine−Ni-catalyzed ethylene polymerization reaction has been studied theoretically using the B3LYP density functional method. The chain initiation reaction proceeds with the coordination of ethylene to the model active catalyst [L2NiCH3]+, L2 = (HNCH)2, followed by ethylene insertion into the metal−alkyl bond with a rate-determining 11.7 kcal/mol free energy barrier to form a γ-agostic intermediate, which with a small barrier rearranges to a more stable β-agostic intermediate and then forms an olefin alkyl complex upon coordination of the next ethylene. Linear polymer propagation takes place from this olefin alkyl complex, the resting state in the catalytic cycle, via the same insertion, rearrangement, and coordination pathway. An alternative pathway from the olefin alkyl complex passes over a 14−15 kcal/mol barrier for β-hydride elimination and reinsertion for branched polymer propagation. These energetics suggest that the Ni(II)-catalyzed reaction is expected to produce more linear th...