Mass Spectrometric Characterization of Methylaluminoxane

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 partially ionized in sufficiently polar media, where the contact ion pairs are of the general formula [Me 2 Al][(MeAlO) x (AlMe 3 ) y Me]. A limited number of compositions of this type are detected; the most abundant has x = 23 and y = 7, where the cation [Me 2 Al(OMTS)] + (with m / z 293) and the anion (with m / z 1853) were detected in the presence of OMTS, which stabilizes the Me 2 Al cation. The results demonstrate that MAO ionizes Cp 2 ZrMe 2 by acting as a source of this electrophilic cation, giving rise to an ion pair featuring a large and weakly coordinating aluminoxanate counterion.