Electrospray ionization mass spectrometric studies of poly(methylaluminoxane) (MAO) in the presence of Cp 2 ZrMe 2 , octamethyltrisiloxane (OMTS), or [Bu 4 N]Cl in fluorobenzene solution are reported. The results concur with the hypothesis that MAO is

Mass spectrometric analysis of aluminoxanes is difficult due to their high reactivity, thermal instability, involatility, and propensity to form complex mixtures of oligomers. However, the controlled hydrolysis of fluorobenzene solutions of triisobutylaluminum provides surprisingly simple electrospray ionization mass spectra (ESI-MS), consisting of just two main charged species. The most abundant of these can be characterized as [Al3O3 i Bu4(Al i Bu2H)2(Al i Bu3)]−, using a combination of m/z value, isotope pattern, MS/MS studies, and isotopic labeling.