Structure of methylaluminoxane (MAO): Extractable [Al(CH)] for precatalyst activation.

Methylaluminoxane (MAO) is used as a precatalyst activator for million ton-scale production of commercial polyolefins, but its precise structure and associated activation mechanisms have been a fundamental research puzzle for more than 40 years. We report here the crystallographic characterization of an active MAO component, which reveals a discrete two-dimensional sheet cluster [AlO(CH)][Al(CH)] featuring two trimethylaluminum units, Al(CH), coordinated to two unsaturated aluminum sites. Our structural data are consistent with the hypothesis that the active sites bear coordinated Al(CH) and provide [Al(CH)] for precatalyst activation. Quantum chemical calculations revealed the most preferred sites for [Al(CH)] abstraction (a change in Gibbs free energy of 0.0 kilocalories per mole). Olefin polymerization tests on metallocenes activated with the crystallized MAO show higher activities than with the commercial MAO.