Oxidation of Methylalumoxane Oligomers

The anions formed from methylalumoxane (MAO) and suitable donors (e.g. octamethyltrisiloxane) are amenable to mass spectrometric (MS) analysis. Their composition as deduced from this data allows direct insight into the chemical transformations of their neutral precursors. One such process is oxidation, which is well‐known to be facile for MAO without any clear idea of what actually occurs at a molecular level. Addition of O2 to MAO results in immediate gelation, but MS analysis reveals no corresponding change to the composition of the principal oligomeric anions. A slow (hours) reaction does occur that involves net incorporation of Me2AlOMe into the oligomeric anions, and the identities of the OMe‐containing anions were confirmed by 1H NMR spectroscopy, MS/MS analysis, and addition of an authentic sample of Me2AlOMe to MAO. The result tallies with the fact that addition of O2 to MAO produces Me2AlOMe from free Me3Al which eventually leads to formation of oxidized MAO oligomers and changes in ion abundance. Aging of the oxygenated MAO results in further growth of the oligomers similar to that of the non‐oxidized species. Mass spectrometric analysis therefore reveals useful insights into the environmental history of a given MAO batch. Oxidation of methylalumoxane (MAO) oligomers is studied using mass spectrometry. Exposure of MAO to O2 shows formation of oligomers with OMe units formed through Me2AlOMe incorporation. This reactivity and the subsequent changes in MAO's structure provide new insights into the complex nature of this important activator.