Structure of methylaluminoxane (MAO): Extractable [Al(CH 3 ) 2 ] + 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 [Al 33 O 26 (CH 3 ) 47 ][Al(CH 3 ) 3 ] 2 featuring two trimethylaluminum units, Al(CH 3 ) 3 , coordinated to two unsaturated aluminum sites. Our structural data are consistent with the hypothesis that the active sites bear coordinated Al(CH 3 ) 3 and provide [Al(CH 3 ) 2 ] + for precatalyst activation. Quantum chemical calculations revealed the most preferred sites for [Al(CH 3 ) 2 ] + 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.