Additive and Aging Effects on Methylalumoxane Oligomers

In
the presence of the weak donor octamethyltrisiloxane (OMTS),
methylalumoxane (MAO) undergoes ionization through [Me2Al]+ abstraction. The [Me2Al·OMTS]+[(MeAlO)x(Me3Al)yMe]− ion pairs formed can
conveniently be studied by electrospray ionization mass spectrometry.
The anion distribution changes with OMTS:Al ratio, with low molecular
weight (MW) anions forming at high ratios due to oligomer degradation.
Monitoring of room-temperature aging shows that MAO oligomers of low
MW are slowly and systematically converted to species with higher
MW through addition of (MeAlO) units, and intermediate oligomers are
observed and assigned. This process occurs at room temperature but
does not occur significantly over a period of months at low temperatures.
Computational investigations of these species show large cagelike
clusters with four-coordinate Al and three-coordinate O. All neutral
structures have sites that could readily give up [Me2Al]+ to form the observed ions, though sites that can lose [Me2Al]+ without leaving a two-coordinate oxygen site
are favored. These studies provide concrete insights into the absolute
MW of MAO oligomers and their exchange over time.

在弱给体八甲基三硅氧烷（octamethyltrisiloxane，OMTS）存在下，甲基铝氧烷（methylalumoxane，MAO）可通过夺取[Me₂Al]⁺阳离子发生电离。所形成的[Me₂Al·OMTS]⁺[(MeAlO)ₓ(Me₃Al)ᵧMe]⁻离子对，可通过电喷雾电离质谱实现便捷表征。阴离子的分布随OMTS与Al的摩尔比发生变化：当OMTS与Al比例较高时，由于低聚物发生降解，会生成低分子量（MW）阴离子。对室温陈化过程的监测显示，低分子量的MAO低聚物会通过添加(MeAlO)单元，缓慢且系统地转化为更高分子量的物种，期间可观测到中间态低聚物并对其进行归属。该转化过程可在室温下发生，但在低温环境下数月内均不会显著进行。针对这些物种的计算研究表明，其存在大量由四配位铝与三配位氧构成的笼状簇结构。所有中性结构均存在可轻易脱去[Me₂Al]⁺以形成观测到的离子的位点，不过那些脱去[Me₂Al]⁺后不会残留两配位氧位点的位点更受青睐。本研究为MAO低聚物的绝对分子量及其随时间的转化过程提供了明确的认知。