Structure-Reactivity Relationships in Methylaluminoxane: Ionization Potentials Across Compositions and Morphologies

Methylaluminoxane (MAO) is the most widely used cocatalyst in metallocene-catalyzed olefin polymerization, yet its structural basis for reactivity remains poorly understood. Here, we present a computational survey of ionization potentials across 118 MAO species with the general formula (AlOMe)_n(AlMe₃)_m (n  =  4-10), covering over 1100 aluminum sites classified by local coordination environment. The analysis is extended to experimentally relevant sizes through evaluation of sheet and cage models up to n = 18, including an ovalene-type sheet model and crystallographically characterized 26,9 species. All energies are evaluated at the RI-MP2 level. Ionization potentials become systematically more negative with increasing oligomer size, and sheet structures are more reactive than cages across the sizes studied. At experimentally relevant sizes, the reactive site landscape converges to two characteristic types: one for methide abstraction and one for dimethylaluminum cation cleavage, with the latter pathway becoming exceptionally favorable for the ovalene-type sheet topology that matches the crystallographically confirmed MAO structure. The identification of specific sheet-type species as the most reactive MAO components is consistent with the experimental observation that the catalytically active component of MAO is a minor species within the bulk material, providing a quantitative basis for understanding structure-reactivity relationships in this industrially important cocatalyst.