A DENSITY-FUNCTIONAL STUDY OF THE MECHANISM OF THE DIIMINE-NICKEL-CATALYZED ETHYLENE POLYMERIZATION REACTION

The mechanism of diimine-Ni-catalyzed ethylene polymerization reaction has been studied theoretically using the B3LYP density functional met hod. The chain initiation reaction proceeds with the coordination of e thylene to the model active catalyst [L(2)NiCH(3)](+), L(2) = (HNCH)(2 ), followed by ethylene insertion into the metal-alkyl bond with a rat e-determining 11.7 kcal/mol free energy barrier to form a gamma-agosti c intermediate, which with a small barrier rearranges to a more stable beta-agostic intermediate and then forms an olefin alkyl complex upon coordination of the next ethylene. Linear polymer propagation takes p lace from this olefin alkyl complex, the resting state in the catalyti c cycle, via the same insertion, rearrangement, and coordination pathw ay. An alternative pathway from the olefin alkyl complex passes over a 14-15 kcal/mol barrier for beta-hydride elimination and reinsertion f or branched polymer propagation. These energetics suggest that the Ni( II)-catalyzed reaction is expected to produce more linear than methyl- branched polymers, and that higher temperature increases and higher et hylene pressure decreases the branching. Hydrogenolysis is an energeti cally favorable termination pathway, proceeding via coordination of a hydrogen molecule to the metal center, followed by H-H activation thro ugh a four-centered ''metathesis-like'' transition state and reductive elimination of alkane. A comparison with zirconocene-catalyzed ethyle ne polymerization shows that the Ni(II)catalyzed polymerization should be slightly slower and should give more branching.