Theoretical studies of the Cp2ZrR+-catalyzed propylene polymerization reactions and a comparison with ethylene polymerization

The polymer chain initiation and two chain termination processes, beta-H el imination and hydrogenolysis, were studied with density functional calculat ions for polymerization of propylene using Cp2ZrCH3+ as the model catalyst. The first step of the chain initiation, coordination of propylene to the m etal center of the bare catalyst to form the pi-complex, is calculated to b e 35 kcal/mol exothermic, which is about 15 kcal/mol larger than for coordi nation of ethylene. The rate-determining insertion barrier from the pi-comp lex is calculated to be 12 kcal/mol, significantly higher than 6.5 kcal/moI for ethylene. Thus, propylene polymerization using this catalyst is expect ed to be slower than ethylene polymerization, which agrees well with experi mental observations. Three beta-agostic intermediates, with two methyl grou ps positioned cis, trans and gem to each other, are found. The activation o f the beta-agostic C-H bond from these intermediates, with an activation en ergy of 4-6 kcal/moI, is more facile and causes more branching for propylen e polymerization than for ethylene polymerization, in agreement with the ex periments. The olefin dissociation after beta-H elimination is difficult an d is not likely to be a major mechanism for chain termination for propylene polymerization. Hydrogenolysis of the beta-agostic intermediates, proceedi ng through a metathesis transition state, is a facile chain termination pro cess. (C) 1999 Elsevier Science B.V. All rights reserved.