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Real-time analysis of methylalumoxane formation.

Anuj JoshiHarmen S ZijlstraElena LilesCarina ConcepcionMikko LinnolahtiJ Scott McIndoe
Published in: Chemical science (2020)
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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