Real-time analysis of methylalumoxane formation

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...

Spectroscopic Studies of Synthetic Methylaluminoxane: Structure of Methylaluminoxane Activators

“…Finally, these preparative data conclusively illustrate that the ESI‐MS of the hydrolysis experiments reported here and earlier [32] reflect the specific conditions of this reaction (possibly isothermal, dilute solution or suspension). Theoretical work ( vide infra ) suggests the neutral precursor to [ 16,6 ] − represents a local rather than a global minimum.…”

“…In this paper, we provide full details on these hydrolysis experiments, and interpret some of the transient behaviour seen. We also provide additional experimental data which shows that the anion distribution of h‐MAO or non‐hydrolytic MAO, as well as other properties can be quite different from that reported previously [32] . In other words, the properties of MAO, including its ion‐pair speciation, are largely determined by the conditions and precursors used for synthesis.…”

“…Evidently, the formation of aluminoxane activators involves several different, but slower processes occurring at somewhat different rates. One of these could involve incremental growth through step‐wise reaction of lower MW species with an oligomer, or as proposed previously, mutual reaction of higher MW species with each other [32] . In fact, it is possible there is more than one way to form a particular reactive species, and/or that there are different, reactive species with the same composition formed by different routes.…”

“…In both solvents the average m/z ratio of the mixture changes in the manner shown in Figure 6 [32] . For the different reactions which have been studied, this change does not follow simple exponential growth but instead is better described by bi‐exponential growth (see Supporting Information for details).In other words there are at least two stages, one rapid and the other slower, that characterize the change in composition of the mixture with time [42] …”

“…In the absence of oxidation, the changes in anion speciation in the two different solvents are similar, though there are obvious differences in rate and conditions under which the reaction reaches completion (defined as no perceptible change in anion distribution with time). As shown in Figure 3 in Ph‐F a 2‐fold or more excess of Me 3 Al over water is needed to drive the reaction to completion at room temperature, while in o ‐F 2 C 6 H 4 the entire process is much faster ( vide infra ) and only a slight excess of Me 3 Al results in completion, regardless of whether OMTS is present or not during hydrolysis [32] …”