Experimental and theoretical investigations of the structure of methylaluminoxane (MAO) cocatalysts for olefin polymerization

The structure of methylaluminoxane (MAO), used as a cocatalyst for olefin p olymerization, has been investigated by Raman and in situ IR spectroscopy, polymerization experiments, and density functional calculations. From exper imental results, a number of quantum chemical calculations, and bonding pro perties of related compounds, we have suggested a few Me18Al12O9 cage struc tures, including a highly regular one with C-3h symmetry, which may serve a s models for methylaluminoxane solutions. The cages themselves are rigid bu t may contain up to three bridging methyl groups on the cage surfaces that are labile and reactive. Bridging methyls were substituted with Cl atoms to form a compound otherwise similar to MAO. Chlorinated MAO is unable to act ivate a metallocene catalyst, even in the presence of trimethylaluminum (TM A), but allows subsequent activation by regular MAO. With bis(pentamethylcy clopentadienyl)zirconium dichloride, MAO and TMA seem to influence chain te rmination independently. Several findings previously poorly explained are r ationalized with the new model, including the observed lack of reaction pro ducts with excess TMA. (C) 2000 John Wiley & Sons, Inc.