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Published on: October 18, 2019
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Mass spectrometric characterization of methylaluminoxane-activated metallocene complexes.
Tyler K Trefz1, Matthew A Henderson, Mikko Linnolahti
1Department of Chemistry, University of Victoria, P.O. Box 3065 Victoria, BC V8W3V6 (Canada).
Chemistry (Weinheim an Der Bergstrasse, Germany)
|January 6, 2015
Summary
Electrospray ionization mass spectrometry reveals that alkylation and ionization are separate, competing processes in poly(methylaluminoxane) (MAO) solutions. Different zirconium precursors influence ion-pair speciation and anion formation, impacting catalyst behavior.
Area of Science:
- Organometallic Chemistry
- Polymer Chemistry
- Mass Spectrometry
Background:
- Poly(methylaluminoxane) (MAO) is a crucial co-catalyst in olefin polymerization.
- Understanding MAO's interaction with metallocene catalysts is key to controlling polymerization.
- Previous studies have focused on MAO's role but detailed speciation remains complex.
Purpose of the Study:
- To investigate the electrospray ionization mass spectrometry (ESI-MS) behavior of MAO in the presence of different zirconium precursors.
- To elucidate the mechanisms of alkylation and ionization in polar solvents.
- To characterize the resulting ion-pair speciation and anion formation.
Main Methods:
- Electrospray ionization mass spectrometry (ESI-MS) was employed.
- Studies were conducted in fluorobenzene (PhF) solution.
- Various zirconium complexes ([Cp2 ZrMe2 ], [Cp2 ZrMe(Cl)], and [Cp2 ZrCl2 ]) were used at different Al/Zr ratios.
Main Results:
- Alkylation and ionization were identified as distinct, competing reactions.
- Significant differences in ion-pair speciation were observed between metallocene dichloride and alkylated precursors.
- Halogenated aluminoxane anions were formed with metal chloride complexes, sometimes dominating over non-halogenated anions.
Conclusions:
- The excess MAO required for metallocene dichloride catalysts is due to competitive alkylation/ionization and persistent unreactive ion pairs.
- Anion modification and altered ion pairing at lower Al/Zr ratios contribute to catalyst performance differences.
- Computational models were used to examine neutral precursors and anions.
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