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.