Pressure dependence of the carbon dioxide/cyclohexene oxide coupling reaction catalyzed by chromium salen complexes. Optimization of the comonomer-alternating enchainment pathway
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The rate of the copolymerization reaction of cyclohexene oxide and carbon dioxide in the presence of (salen)Cr III and various cocatalysts has been determined as a function of CO 2 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 CO 2 pressure was increased. The most active (salen)CrN 3 catalyst (H 2-salen = N,N'-bis(3-teri-butyl-5-methoxysalicylidene)-(1R,2R)-cyclohexenediimine), along with a [PPN][N 3] cocatalyst, exhibited a TOF of 1153 mol epoxide consumed/mol chromiumh at 80C and a CO 2 pressure of 34.5 bar.