Coordination copolymerization of severely encumbered isoalkenes with ethylene: Enhanced enchainment mediated by binuclear catalysts and cocatalysts

This contribution describes the implementation of the binuclear organotitanium "constrained geometry catalysts" (CGCs), (μ-CH ₂CH₂-3,3′){(η⁵-indenyl)[1-Me ₂Si(^tBuN)](TiMe₂)}₂[EBICGC(TiMe ₂)₂; Ti₂] and (μ-CH₂-3,3′) {(η⁵-indenyl)[1-Me₂Si(^tBuN)](TiMe ₂)}₂[MBICGC(TiMe₂)₂; C1-Ti ₂], in combination with the bifunctional bisborane activator 1,4-(C₆F₅)₂BC₆F₄B(C ₆F₅)₂ (BN₂) in ethylene + olefin copolymerization processes. Specifically examined are the classically poorly responsive 1,1-disubstituted comonomers, methylenecyclopentane (C), methylenecyclohexane (D), 1,1,2-trisubstituted 2-methyl-2-butene (E), and isobutene (F). For the first three comonomers, this represents the first report of their incorporation into a polyethylene backbone via a coordination polymerization process. C and D are incorporated via a ring-unopened pathway, and E is incorporated via a novel pathway involving 2-methyl-1 -butene enchainment in the copolymer backbone. In ethylene copolymerization, Ti ₂ + BN₂ enchains ∼2.5 times more C, ∼2.5 times more D, and ∼2.3 times more E than the mononuclear catalyst analogue [1-Me₂Si(3-ethylindenyl)-(^tBuN)]TiMe₂ (Ti ₁) + B(C₆F₅)₃ (BN) under identical polymerization conditions. Polar solvents are found to weaken the catalyst-cocatalyst ion pairing, thus influencing the comonomer enchainment selectivity.