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.
Highlights
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 (ESIMS) is capable of studying those oligomers that become charged during the activation process
The TMA in MAO can be divided into two kinds: “bound TMA” which is incorporated in the MAO and “free TMA” which can be removed under vacuum to form TMA-depleted MAO (DMAO).[18]
Summary
Methylalumoxane (MAO) is an oligomeric activator for singlesite ole n polymerization precatalysts, prepared by the reaction of trimethylaluminum (TMA) with water.[1,2,3,4,5,6,7] MAO is a complete activator[8,9,10] through playing multiple roles: it acts as a scavenger of oxygen and water; it can alkylate the precatalyst; and it can ionize the precatalyst via abstraction of a methyl group.[11,12] Trimethylaluminum is a capable scavenger on its own,[13] and will methylate metal–halogen bonds,[14,15] but it is not able to ionize the precatalyst.[16]. When water and TMA are combined, a fast exothermic reaction generates MAO with methane as a byproduct.[50] We faced severe methodological challenges in studying this system mass spectrometrically, because of the evolution of methane, the exothermicity of the reaction, the low polarity of the toluene solvent[51] generally used in synthesis, the propensity of the reacting solution to cause capillary blockages during analysis, the complexity of the mixture, and the inapplicability of normalization in the context of a system whose total ion count is changing.
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