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)(CrN3)-N-III 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 (H-2-salen = N,N'-bis(3-tert-butyl-5-methoxysalicylidene)-(1R,2.R)-cyclohexenediimine), along with a [PPN] [N-3] cocatalyst, exhibited a TOF of 1153 mol epoxide consumed/mol chromium(.)h at 80 degreesC and a CO2 pressure of 34.5 bar.