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.