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), (mu-CH2CH2-3,3){(eta(5)-indenyl)[1-Me2Si((BuN)-Bu-t))(TiMe2)}(2) [EBICGC(TiMe2)(2); Ti-2], (mu-CH2CH2-3,3'){(eta(5)indenyl)[1 -Me2Si((BuN)-Bu-t)](ZrMe2)}(2) [EBICGC(ZrMe2)(2); Zr-2], (mu-CH2-3,3')}{eta(5)-indenyl)[1-Me2Si((BuN)-Bu-t)](TiMe2)}(2) [MBICGC(TiMe2)(2); C1-Ti-2], and (mu-CH2-3,3'){(eta(5)-indenyl)[1-Me2Si((BuN)-Bu-t)](ZrMe2)}(2) [MBICGC(ZrMe2)(2); C1-Zr-2], in combination with the borate activator/cocatalyst Ph3C+B(C6F5)(4)(-) (B-1). Under identical styrene homopolymerization conditions, C1-Ti-2 + B-1 and Ti-2 + B-1 exhibit similar to 65 and similar to 35 times greater polymerization activities, respectively, than does monometallic [1-Me2Si(3-ethylindenyl)((BuN)-Bu-t)]TiMe2 (Ti-1) + B-1. C1-Zr-2 + B-1 and Zr-2 + B-1 exhibit similar to 8 and similar to 4 times greater polymerization activities, respectively, than does the monometallic control (1-Me2Si(3-ethylindenyl)((BuN)-Bu-t)]ZrMe2 (Zr-1) + B-1. NMR analyses show that the bimetallic catalysts suppress the regiochemical insertion selectivity exhibited by the monometallic analogues. In ethylene copolymerization, Ti2 + B, 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 Ti-1 + B-1. This observed bimetallic chemoselectivity effect follows the same general trend as the pi-electron density on the styrenic ipso carbon (D > E > IF > C > B). Kinetic studies reveal that both Ti2 + B-1 and Ti-1 + B-1-mediated ethylene-styrene copolymerizations follow second-order Markovian statistics and tend to be alternating. Moreover, calculated reactivity ratios indicate that Ti-2 + B-1 favors styrene insertion more than does Ti-1 + B-1. All the organozirconium complexes (C1-Zr-2, Zr-2, and Zr-1) are found to be incompetent for ethylene-styrene copolymerization, yielding only mixtures of polyethylene and polystyrene. Model compound (mu-CH2CH2-3,3'){(eta(5)-indenyl)[1-Me2Si((BuN)-Bu-t)][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(C6F5)(3) suggest an eta(l) -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 icorporation selectivity. Both homopolymerization and copolymerization results argue that substantial cooperative effects between catalytic sites are operative.