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-CH2CH2-3,3‘){(η5-indenyl)[1-Me2Si(tBuN)](TiMe2)}2[EBICGC(TiMe2)2; Ti2]
and (μ-CH2-3,3‘){(η5-indenyl)[1-Me2Si(tBuN)](TiMe2)}2[MBICGC(TiMe2)2; C1−Ti2], in combination with the
bifunctional bisborane activator 1,4-(C6F5)2BC6F4B(C6F5)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, Ti2 + 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 (Ti1) + 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.