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

Abstract

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.

Conversion of carbon dioxide to cyclic carbonates using diimine Ru(II) complexes as catalysts

“…4). A similar effect was reported by Darensbourg: [21] catalytic transformation of CO 2 to polycarbonate increased with increasing the pressure, but the value of TOF was decreased.…”

Aspects of carbon dioxide utilization

“…It actually allows one to avoid fighting the equilibrium in the direct synthesis of dimethyl carbonate from carbon dioxide and methyl alcohol [110]. Many catalysts have been investigated for the reaction of carbon dioxide with various kinds of oxiranes as shown in Table 6 [118][119][120][121][122][123][124][125][126][127][128].…”

Catalytic Processes for Chemical Conversion of Carbon Dioxide into Cyclic Carbonates and Polycarbonates

“…In order to further understand copolymerization process, Darensbourg's group studied the role of the initiator and cocatalyst [17], the effects of the substituents on the salen ligand, co-catalyst loading [16a] and the pressure of carbon dioxide [18]. First, the rate of copolymerization depends on both the electron-donating ability of the salen ligand and the co-catalyst such as, N-heterocyclic amines, phosphines, and ionic salts [17].…”

“…Higher pressure of carbon dioxide results in catalyst / substrate dilution, and thus causes the rate of copolymerization decreased. On the other hand, the formation of cyclic carbonate is inhibited as the CO 2 pressure increases [18]. Fourth, initiation step takes place via second-order activation (Scheme 2).…”