Bimetallic catalysis for styrene homopolymerization and ethylene-styrene copolymerization. Exceptional comonomer selectivity and insertion regiochemistry

This communication reports the styrene homopolymerization behavior and ethylene-styrene copolymerization behavior of the covalently linked bimetallic constrained geometry catalyst (μ-CH₂CH₂-3,3′){(η⁵-indenyl)[1-Me₂Si(_tBuN)](TiMe₂)}₂ (Ti₂), which is the first single-site catalyst that effects not only styrene homopolymerization with high activity, but also efficient ethylene-styrene copolymerization over a broad styrene composition range (0-76% at 20 °C, 1.0 atm ethylene pressure). In styrene homopolymerization, a 50× increase in polymerization activity is achieved with Ti₂ vs the mononuclear analogue, Ti₁, using an identical trityl borate cocatalyst and polymerization conditions. In ethylene + styrene copolymerization, Ti₂ enchains ∼20% more styrene than Ti₁ under identical reaction conditions. ¹³C NMR spectroscopy indicates that greater than two consecutive styrene units are enchained in the copolymer backbone produced by Ti₂ + Ph₃C⁺B(C₆F₅)₄^-. End group analysis of the styrene homopolymer produced by Ti₂ + Ph₃C⁺B(C₆F₅)₄^- suggests that 1,2-regiochemistry is installed in ∼50% of the initiation steps. This unusual microstructure is believed to be related to the bimetallic catalyst structure.