Formation and Structure of Hydrolytic Methylaluminoxane Activators

Methylaluminoxane (MAO) activators have sheet structures which form ion-pairs on reaction of neutral donors such as octamethyltrisiloxane (OMTS). The ion-pairs can be detected by electrospray ionization mass spectrometry (ESI-MS) in polar media. The growth of these reactive precursors during hydrolysis of Me3Al can be monitored using ESI-MS. Density functional theory, combined with numerical simulation of growth, indicates that this process involves rapid formation of low MW oligomers, followed by assembly of these species into low MW sheets. These can grow through further addition of low MW oligomers or by fusion into larger sheets. The mechanism of these growth processes leads to the prediction that even-numbered sheets should be favored, and this surprising result is confirmed by ESI-MS monitoring experiments of both activator growth and MAO aging.

Automated summary

Methylaluminoxane (MAO) activators have sheet structures that form ion-pairs during hydrolysis, as demonstrated by electrospray ionization mass spectrometry (ESI-MS). Density functional theory and numerical simulation indicate rapid formation of low MW oligomers, followed by assembly into low MW sheets during this process. The study predicts that even-numbered sheets should be favored, which is confirmed by ESI-MS monitoring experiments of activator growth and MAO aging.