Oxidation of Methylalumoxane Oligomers: A Theoretical Study Guided by Mass Spectrometry

Methylalumoxane (MAO) plays a critical role in catalytic polymerization of olefins, both as activator of the precatalyst and scavenger of impurities. The latter involves oxidation of MAO, which is not well understood at a molecular level. On the basis of our previous computational explorations of the structure of MAO and electrospray ionization mass spectrometry (ESI-MS) studies of its oxidation, we report here a systematic theoretical study to shed light on the oxidation process. We identify the structural features of the MAO that are essential for oxidation by screening the thermodynamics of the oxidation reactions as a function of shape and size of the previously justified model MAOs. In correlation with previously reported ESI-MS studies of the corresponding anions (Chem. Eur. J. 2018, 24, 5506−5512, DOI: 10.1002/chem.201705458), the calculations indicate that Al sites of high Lewis acidity could be of major relevance in the oxidation process. Such sites are abundant in the size domain observed relevant for the corresponding anions.