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Counterion effects on propylene polymerization using two-state ansa-metallocene complexes.

Muqtar Mohammed1, Marcio Nele, Abdulaziz Al-Humydi

  • 1Department of Chemistry, University of Waterloo, Waterloo, Ontario, Canada, N2L 3G1.

Journal of the American Chemical Society
|June 26, 2003
PubMed
Summary
This summary is machine-generated.

This study investigated propylene polymerization using metallocene catalysts and various co-initiators. The polymer microstructure was largely unaffected by counterion choice, with catalyst behavior primarily differing in operating regimes rather than intrinsic stereoselectivity.

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Area of Science:

  • Polymer Chemistry
  • Organometallic Chemistry
  • Catalysis

Background:

  • Metallocene catalysts are crucial for olefin polymerization, offering tunable properties.
  • Understanding the influence of co-initiators and counterions on catalyst performance is key to controlling polymer microstructure.
  • Propylene polymerization provides a model system for studying complex catalytic mechanisms.

Purpose of the Study:

  • To investigate the effect of different co-initiators (methylaluminoxane, B(C(6)F(5))(3), [Ph(3)C][B(C(6)F(5))(4)]) on propylene polymerization using unsymmetrical ansa-metallocene complexes.
  • To elucidate the relationship between catalyst structure, co-initiator, and resulting polypropylene (PP) microstructure.
  • To explore the impact of monomer concentration on catalyst behavior and polymer tacticity.

Main Methods:

  • Synthesis and application of unsymmetrical ansa-metallocene complexes (Me(2)Y(Ind)CpMMe(2)).
  • Propylene polymerization experiments conducted at varying monomer concentrations.
  • Analysis of polymer microstructure, including tacticity and pentad distributions.
  • Kinetic modeling to determine fundamental catalytic parameters (e.g., propagation, inversion rates).

Main Results:

  • Catalysts derived from Me(2)Si(Ind)CpZrMe(2) operated under fast chain inversion conditions, with PP microstructure largely independent of the counterion.
  • Polypropylene tacticity was sensitive to monomer concentration with specific catalyst/co-initiator combinations (Me(2)C(Ind)CpHfMe(2)/PMAO or [Ph(3)C][B(C(6)F(5))(4)]).
  • Catalysts activated with B(C(6)F(5))(3) produced stereoregular PP, with microstructure invariant to monomer concentration, suggesting consistent operating regimes.

Conclusions:

  • The primary difference in catalyst performance stems from the operating regime (relative rates of insertion vs. inversion) rather than intrinsic stereoselectivity.
  • The observed ordering of counterions with respect to operating regime deviates from expectations based on coordinating ability.
  • This study provides insights into the nuanced control of polymer microstructure through catalyst design and reaction conditions in metallocene-catalyzed propylene polymerization.