Bimetallic Effects in Homopolymerization of Styrene and Copolymerization of Ethylene and Styrenic Comonomers: Scope, Kinetics, and Mechanism

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-CH2CH2-3,3‘){(η5-indenyl)[1-Me2Si(tBuN)](TiMe2)}2 [EBICGC(TiMe2)2; Ti2], (μ-CH2CH2-3,3‘){(η5-indenyl)[1-Me2Si(tBuN)](ZrMe2)}2 [EBICGC(ZrMe2)2; Zr2], (μ-CH2-3,3‘){(η5-indenyl)[1-Me2Si(tBuN)](TiMe2)}2
[MBICGC(TiMe2)2; C1-Ti2], and (μ-CH2-3,3‘){(η5-indenyl)[1-Me2Si(tBuN)](ZrMe2)}2 [MBICGC(ZrMe2)2; C1-Zr2], in combination with the borate activator/cocatalyst Ph3C+B(C6F5)4- (B1). Under identical styrene
homopolymerization conditions, C1-Ti2 + B1 and Ti2 + B1 exhibit ∼65 and ∼35 times greater polymerization
activities, respectively, than does monometallic [1-Me2Si(3-ethylindenyl)(tBuN)]TiMe2 (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 (Zr1) + 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 Ti1 + 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 Zr1) 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; Ti2(CH2Ph)4} was designed, synthesized, and structurally characterized. In situ activation studies with cocatalyst
B(C6F5)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.