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 (mu-CH2CH2-3,3'){(eta5-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 degrees C, 1.0 atm ethylene pressure). In styrene homopolymerization, a 50x 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 approximately 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 approximately 50% of the initiation steps. This unusual microstructure is believed to be related to the bimetallic catalyst structure.