Toward Controlling the Metallocene/Methylaluminoxane-Catalyzed Olefin Polymerization Process by a Computational Approach

We describe and compare the proposed mechanisms of ethene polymerization by the metallocene/methylaluminoxane (MAO) catalyst in terms of quantum chemical calculations. In combination with the Cp₂ZrMe₂ precatalyst, we employ two models for MAO, produced by hydrolysis of trimethylaluminum (TMA). Both MAOs contain associated TMA as a key ingredient for cocatalytic activity. The TMA association/dissociation equilibrium in the MAOs controls the mechanism of catalyst activation and suggests preference for catalyst activation via [AlMe₂]⁺ abstraction from the MAO by the precatalyst rather than via Lewis-acidic abstraction of the leaving group from the precatalyst by the MAO. Solvent interactions increase the relative concentration of Lewis-acidic sites. Chlorination of MAO facilitates the catalytic processes. Studies as a function of precatalyst structure reproduce the general experimental observations of the easier catalyst activation by zirconocenes than by hafnocenes and the positive effects of adding a dimethylsilyl bridge and replacing the cyclopentadienyl with an indenyl ligand. This study provides a starting point for rational control of the behavior of the metallocene/MAO catalyst system.