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.