Mass Spectrometric Characterization of Methylaluminoxane‐Activated Metallocene Complexes |
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| Authors: | Dr Tyler K Trefz Dr Matthew A Henderson Prof Mikko Linnolahti Dr Scott Collins Prof J Scott McIndoe |
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| Affiliation: | 1. Department of Chemistry, University of Victoria, P.O. Box 3065 Victoria, BC V8W3V6 (Canada);2. Department of Chemistry, University of Eastern Finland, P. O. Box 111, 80101 Joensuu (Finland) |
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| Abstract: | Electrospray‐ionization mass spectrometric studies of poly(methylaluminoxane) (MAO) in the presence of Cp2ZrMe2], Cp2ZrMe(Cl)], and Cp2ZrCl2] 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(Me3Al)y?z(Me2AlCl)zMe]? (z=1, 2, 3…?) are observed using metal chloride complexes and under some conditions may predominate over their non‐halogenated precursors (MeAlO)x(Me3Al)yMe]?. 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 Cp2ZrCl2] when compared with Cp2ZrMe2]. 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 Cp2ZrCl2], 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. |
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| Keywords: | electrospray ionization mass spectrometry metallocenes methylaluminoxane polymerization |
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