Prising open the black box of methylaluminoxane with mass spectrometry.

Methylaluminoxane (MAO) is the most widely used cocatalyst in olefin polymerization, yet its molecular structure and mode of action have remained elusive due to its oligomeric, dynamic, and poorly defined nature. This review summarizes how electrospray ionization mass spectrometry (ESI-MS) has enabled direct interrogation of MAO speciation under conditions relevant to catalysis. Key experimental advances, including glovebox-ESI interfacing, the use of weakly coordinating donors, and nonpolar solvent systems, have allowed the identification of dominant ionic components such as [Me2Al]+ and a family of aluminoxane-derived anions of general formula [(MeAlO)m(Me3Al)nMe]-, with [(MeAlO)16(Me3Al)6Me]- emerging as a "magic-number" species. These findings, combined with complementary insights from crystallography, solid-state NMR, and total scattering, support sheet-like structural motifs for MAO oligomers. Mass spectrometric studies further reveal dynamic processes including alkyl exchange, oxidation, and aging, providing a molecular-level picture of MAO formation and reactivity. Collectively, these advances begin to resolve the long-standing "black box" nature of MAO and provide guiding principles for the design of next-generation cocatalysts with improved efficiency and defined composition.