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The Dynamic Equilibrium Between (AlOMe)n Cages and (AlOMe)n·(AlMe3)m Nanotubes in Methylaluminoxane (MAO): A First-Principles Investigation

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journal contribution
posted on 23.12.2014 by Zackary Falls, Nina Tymińska, Eva Zurek
Species likely to be present in methylaluminoxane (MAO) are studied via dispersion-corrected DFT, which we show is able to accurately predict thermochemical parameters for the dimerization of trimethylaluminum (TMA). Both cage-like, (AlOMe)n,c, and TMA-bound nanotubes, (AlOMe)n,t·(AlMe3)m, are found to be important components of MAO. The most stable structures have aluminum/oxygen atoms in environments whose average hybridization approaches sp3/sp2. The (AlOMe)n,t·(AlMe3)m isomers with the lowest free energies possess Al−μ-Me–Al bonds. At 298 K a novel Td-(AlOMe)16,c oligomer is one of the most stable structures among the six stoichiometries with the lowest free energies: (AlOMe)20,c·(AlMe3)2, Td-(AlOMe)16,c, (AlOMe)18,c, (AlOMe)20,c·(AlMe3), (AlOMe)10,t·(AlMe3)4, and (AlOMe)20,c. As the temperature rises, the abundance of (AlOMe)n,t·(AlMe3)m decreases, and that of (AlOMe)n,c increases. Because the former are expected to be precursors for the active species in polymerization, this may in part be the reason why the cocatalytic activity of MAO decreases at higher temperatures.