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Structure of methylaluminoxane (MAO): Extractable [Al(CH3)2]+ for precatalyst activation
Lubin Luo1, Jarod M Younker1, Alexander V Zabula1
1ExxonMobil Technology and Engineering Company, Baytown, TX 77520, USA.
Abstract:
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 [Al33O26(CH3)47][Al(CH3)3]2 featuring two trimethylaluminum units, Al(CH3)3, coordinated to two unsaturated aluminum sites. Our structural data are consistent with the hypothesis that the active sites bear coordinated Al(CH3)3 and provide [Al(CH3)2]+ for precatalyst activation. Quantum chemical calculations revealed the most preferred sites for [Al(CH3)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.
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