Theoretical studies of the mechanism of ethylene polymerization reaction catalyzed by diimine-M(II) (M = Ni, Pd and Pt) and Ti- and Zr-chelating alkoxides

We have analyzed the computational results for several elementary reactions of the ethylene polymerization process catalyzed by an alternative (to the existing metallocene catalysts) "non-cyclopentadienyl" catalysts such as d iimine-M(II) (where M = Ni and Pd) and chelating bridged Ti- and Zr-complex es. The obtained data have been compared with those for the existing zircon ocene-based catalysts. In general, it was shown that: (i) the resting stage of the process is a metal-olefin-alkyl complex, the olefin coordination en ergy of which is a few kcal/mol larger for diimine-M(II) systems than zirco nocene or dialkoxide systems; (ii) the rate-determining barrier is a migrat ory insertion barrier calculated from the metal-olefin-alkyl complex, which is found to be a few kcal/mol larger for the diimine-M(II) system compared to the Cp2ZrCH3+ catalyst. The presence of certain flexible bridging ligan ds X in the Ti-alkoxide complex, [Y(Ph)X(Ph)Y]TiCH3+, which are capable of donating electron density to the cationic metal center at various stages du ring the reaction makes this barrier a few kcal/mol smaller for the dialkox ide than the Cp2ZrCH3+ catalyst. It was shown that an increase in the metal -bridge interaction decreases the migratory insertion barrier and, conseque ntly, increases the catalytic activity of these complexes. Although the dii mine-M(II) catalysts are less active than zirconocene-based ones, the micro structure of the polymers produced by the former catalyst, which is found t o be a function of temperature, ethylene, steric bulkiness of the auxiliary ligands, and transition metal center, makes them attractive for practice. We also have studied the mechanisms of several chain termination/transfer r eactions, as well as the role of steric effects in the studied elementary r eactions. We have clearly demonstrated tremendous possibilities of the comp utational chemistry in solving complex problems of the homogenous catalyst, and its high capability of predicting new and more active catalysts for di fferent commercially important processes including olefin polymerization re actions.