Real-Time Analysis of Methylalumoxane Formation
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’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.
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