Methylaluminoxane as a Cocatalyst for Olefin Polymerization. Structure, Reactivity and Cocatalytic Effect

The structure and reactivity of methylaluminoxane (MAO), used as a cocatalyst for olefin polymerization, has been investigated by in situ IR spectroscopy, polymerization experiments and density functional calculations. We have suggested a few Me18Al12O9 cage structures, including a highly regular one with C3h symmetry, which may serve as models for methylaluminoxane solutions. Three reactive methyl bridges, presumably the key elements in metallocene activation, are situated at the cage surfaces. Further, exchange reactions show that the methyl groups are readily exchanged with chlorine, while non-bridging methyl groups are inert. The chlorinated MAO thus formed (MAO-Cl) is unable to activate bis(pentamethylcyclopendadienyl)zirconium dichloride (Cp*2ZrCl2), even with a surplus of added trimethylaluminium (TMA). MAO and TMA are present as separate FTIR-spectroscopic entities, with TMA acting independently as chain transfer agent for this catalysts.