title: Mass spectrometric characterization of methylaluminoxane-activated metallocene complexes.
abstract: Electrospray-ionization mass spectrometric studies of poly(methylaluminoxane) (MAO) in the presence of [Cp2 ZrMe2 ], [Cp2 ZrMe(Cl)], and [Cp2 ZrCl2 ] in fluorobenzene (PhF) solution are reported. The results demonstrate that alkylation and ionization are separate events that occur at competitive rates in a polar solvent. Furthermore, there are significant differences in ion-pair speciation that result from the use of metallocene dichloride complexes in comparison to alkylated precursors at otherwise identical Al/Zr ratios. Finally, the counter anions that form are dependent on the choice of precursor and Al/Zr ratio; halogenated aluminoxane anions [(MeAlO)x (Me3 Al)y-z (Me2 AlCl)z Me](-) (z=1, 2, 3…︁) are observed using metal chloride complexes and under some conditions may predominate over their non-halogenated precursors [(MeAlO)x (Me3 Al)y Me](-) . Specifically, this halogenation process appears selective for the anions that form in comparison to the neutral components of MAO. Only at very high Al/Zr ratios is the same "native" anion distribution observed when using [Cp2 ZrCl2 ] when compared with [Cp2 ZrMe2 ]. Together, the results suggest that the need for a large excess of MAO when using metallocene dichloride complexes is a reflection of competitive alkylation vs. ionization, the persistence of unreactive, homodinuclear ion pairs in the case of [Cp2 ZrCl2 ], as well as a change in ion pairing resulting from modification of the anions formed at lower Al/Zr ratios. Models for neutral precursors and anions are examined computationally.
authors.affiliation: Department of Chemistry, University of Victoria, P.O. Box 3065 Victoria, BC V8W3V6 (Canada).
authors.forename: Tyler K
authors.initials: TK
authors.lastname: Trefz
authors.forename: Matthew A
authors.initials: MA
authors.lastname: Henderson
authors.forename: Mikko
authors.initials: M
authors.lastname: Linnolahti
authors.forename: Scott
authors.initials: S
authors.lastname: Collins
authors.forename: J Scott
authors.initials: JS
authors.lastname: McIndoe
keywords: electrospray ionization; mass spectrometry; metallocenes; methylaluminoxane; polymerization