Real-time analysis of methylalumoxane formation




Joshi, Anuj
Zijlstra, Harmen S.
Liles, Elena
Concepcion, Carina
Linnolahti, Mikko
McIndoe, J. Scott

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Chemical Science


Methylalumoxane (MAO), a perennially useful activator for olefin polymerization precatalysts, is famously intractable to structural elucidation, consisting as it does of a complex mixture of oligomers generated from hydrolysis of pyrophoric trimethylaluminum (TMA). Electrospray ionization mass spectrometry (ESI-MS) is capable of studying those oligomers that become charged during the activation process. We have exploited that ability to probe the synthesis of MAO in real time, starting less than a minute after the mixing of H2O and TMA and tracking the first half hour of reactivity. We find that the process does not involve an incremental build-up of oligomers; instead, oligomerization to species containing 12–15 aluminum atoms happens within a minute, with slower aggregation to higher molecular weight ions. The principal activated product of the benchtop synthesis is the same as that observed in industrial samples, namely [(MeAlO)16(Me3Al)6Me]−, and we have computationally located a new sheet structure for this ion 94 kJ mol−1 lower in Gibbs free energy than any previously calculated.




Joshi, A., Zijlstra, H. S., Liles, E., Concepcion, C., Linnolahti, M., & McIndoe, J. S. (2020). Real-time analysis of methylalumoxane formation. Chemical Science.