Real-time analysis of methylalumoxane formation

Date

2020

Authors

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

Journal Title

Journal ISSN

Volume Title

Publisher

Chemical Science

Abstract

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.

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Citation

Joshi, A., Zijlstra, H. S., Liles, E., Concepcion, C., Linnolahti, M., & McIndoe, J. S. (2020). Real-time analysis of methylalumoxane formation. Chemical Science. https://doi.org/10.1039/d0sc05075j