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Real-Time Analysis of Methylalumoxane Formation

submitted on 20.08.2020, 02:06 and posted on 20.08.2020, 10:17 by Anuj Joshi, Harmen S. Zijlstra, Elena Liles, Carina Concepcion, Mikko Linnolahti, J Scott McIndoe

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’ve 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 energy than any previously calculated.


NSERC Strategic Project Grant #478998-15

Finnish Grid Infrastructure and Finnish Grid and Cloud Infrastructure resources (urn:nbn:fi:research-infras-2016072533)


Email Address of Submitting Author


University of Victoria



ORCID For Submitting Author


Declaration of Conflict of Interest

Work partially funded by Nova Chemicals' Centre for Applied Research