Bimetallic effects in homopolymerization of styrene and copolymerization of ethylene and styrenic comonomers: Scope, kinetics, and mechanism

Neng Guo, Charlotte L. Stern, Tobin J. Marks

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This contribution describes the homopolymerization of styrene and the copolymerization of ethylene and styrenic comonomers mediated by the single-site bimetallic "constrained geometry catalysts" (CGCs), (μ-CH 2CH2-3,3′){(η5-indenyl)[1-Me 2Si(tBuN)](TiMe2)}2 [EBICGC(TiMe2)2; Ti2], (μ-CH 2CH2-3,3′){(η5-indenyl)[1-Me 2Si(tBuN)](ZrMe2)}2 [EBICGC(ZrMe2)2; Zr2], (μ-CH 2-3,3′){(η5-indenyl)[1-Me2Si( tBuN)](TiMe2)}2 [MBICGC(TiMe2) 2; C1-Ti2], and (μ-CH2-3,3′) {(η5-indenyl)[1-Me2Si(tBuN)](ZrMe 2)}2 [MBICGC(ZrMe2)2; C1-Zr 2], in combination with the borate activator/cocatalyst Ph 3C+B(C6F5)4- (B1). Under identical styrene homopolymerization conditions, C1-Ti2 + Bi and Ti2 + B1 exhibit ∼65 and ∼35 times greater polymerization activities, respectively, than does monometallic [1-Me2Si(3-ethylindenyl)(tBuN)]TiMe 2 (Ti1) + B1. C1-Zr2 + B1 and Zr2 + B1 exhibit ∼8 and ∼4 times greater polymerization activities, respectively, than does the monometallic control [1-Me2Si(3-ethylindenyl)(tBuN)]ZrMe2 (Zr 1) + B1. NMR analyses show that the bimetallic catalysts suppress the regiochemical insertion selectivity exhibited by the monometallic analogues. In ethylene copolymerization, Ti2 + B1 enchains 15.4% more styrene (B), 28.9% more 4-methylstyrene (C), 45.4% more 4-fluorostyrene (D), 41.2% more 4-chlorostyrene (E), and 31.0% more 4-bromostyrene (F) than does Ti1 + B1. This observed bimetallic chemoselectivity effect follows the same general trend as the π-electron density on the styrenic ipso carbon (D > E > F > C > B). Kinetic studies reveal that both Ti2 +- B1 and Ti 1 + B1-mediated ethylene-styrene copolymerizations follow second-order Markovian statistics and tend to be alternating. Moreover, calculated reactivity ratios indicate that Ti2 + B1 favors styrene insertion more than does Ti1 + B1. All the organozirconium complexes (C1-Zr2, Zr2, and Zr 1) are found to be incompetent for ethylene-styrene copolymerization, yielding only mixtures of polyethylene and polystyrene. Model compound (μ-CH2CH2-3,3′){(η5-indenyl)[1- Me2Si(tBuN)][Ti(CH2Ph)2]} 2 {EBICGC[Ti(CH2Ph)2]2; Ti 2-(CH2Ph)4} was designed, synthesized, and structurally characterized. In situ activation studies with cocatalyst B(C 6F5)3 suggest an η1-coordination mode for the benzyl groups, thus supporting the proposed polymerization mechanism. For ethylene-styrene copolymerization, polar solvents are found to increase copolymerization activities and coproduce atactic polystyrene impurities in addition to ethylene-co-styrene, without diminishing the comonomer incorporation selectivity. Both homopolymerization and copolymerization results argue that substantial cooperative effects between catalytic sites are operative.

Original languageEnglish
Pages (from-to)2246-2261
Number of pages16
JournalJournal of the American Chemical Society
Issue number7
Publication statusPublished - Feb 20 2008


ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

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