Pressure Dependence of the Carbon Dioxide/Cyclohexene Oxide Coupling Reaction Catalyzed by Chromium Salen Complexes. Optimization of the Comonomer-Alternating Enchainment Pathway

The rate of the copolymerization reaction of cyclohexene oxide and carbon dioxide in the presence of (salen)CrIIIN3 and various cocatalysts has been determined as a function of CO2 pressure. Carbon dioxide insertion into the (salen)Cr-alkoxide intermediates, afforded following epoxide ring-opening, was shown to be rate-limiting at pressures below 35 bar. Higher pressures of carbon dioxide resulted in catalyst/substrate dilution with a concomitant decrease in the rate of copolymer formation. On the other hand, cyclic carbonate formation was inhibited as the CO2 pressure was increased. The most active (salen)CrN3 catalyst (H2salen = N,N‘-bis(3-tert-butyl-5-methoxysalicylidene)-(1R,2R)-cyclohexenediimine), along with a [PPN][N3] cocatalyst, exhibited a TOF of 1153 mol epoxide consumed/mol chromium·h at 80 °C and a CO2 pressure of 34.5 bar.