Doctor of Philosophy (PhD)
MAO is the co-catalyst in the metallocene catalytic systems, which are widely used in single site olefin polymerization due to their high stereoselectivity. To date, the precise structure of MAO, the particular compound or compounds catalytically active in MAO, have eluded researchers. MAO, a white amorphous powder, is not a good sample for a direct spectroscopic characterization, and its NMR spectra are broad and almost featureless. Many structural models have been proposed but none are generally accepted. In the first part of this work the MAO formation mechanism was addressed. Molecular dynamics simulations at MP2 configuration interaction theory level have been carried out for elemental steps in MAO formation by trimethylaluminum hydrolysis. Methane production, in agreement with experiment, and intermediate species that are consistent with the known structural features of MAO and similar to those isolated and structurally characterized for other aluminoxanes were observed. (CH3)3Al-OH2 species, containing a stable Al-O single bond, and for whose presence there is spectroscopic evidence, emerged as the building block molecule and a cage was formed. Activation barriers for these processes were estimated. After the cage, in agreement with the multi-component nature of MAO, several distinct reaction channels were identified. It was concluded that MAO formation is a step polymerization through a bi-functional monomer, [Al(CH3)-O]is the structural unit, and a kinetic model was proposed. The emerged structures are in agreement with the crystallographic evidence for aluminoxanes, they support the experimental data regarding MAO chemical composition and explain its role in the metallocene activation, which is a failure of the previously proposed models. In the second part of this work, catalytically active MAO components were identified. In the third part, the species that were generated from the proposed mechanism were validated as suitable models for the commercial MAO structure and function by comparison of their IR and NMR parameters with experiment.
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Negureanu, Lacramioara, "Ab Initio Molecular Dynamics Simulations of Methylaluminoxane (MAO) Synthesis by Hydrolysis of Trimethylaluminum" (2006). LSU Doctoral Dissertations. 3445.
Randall W. Hall