Kinetic Study on the Coupling of CO₂ and Epoxides Catalyzed by Co(III) Complex with an Inter- or Intramolecular Nucleophilic Cocatalyst

Trivalent cobalt complexes of salicylaldimine in the presence of an inter- or intramolecular nucleophilic cocatalyst have proven to be excellent catalysts for the copolymerization of CO₂ and epoxides to selectively afford the corresponding polycarbonates in perfectly alternating nature. Especially, bifunctional cobalt­(III)–salen complexes bearing an appended quaternary ammonium salt are more efficient in catalyzing this copolymerization even at high temperatures and extremely low catalyst loading. The present study focuses on comparative kinetics of two different catalyst systems (binary catalyst system of salen­(III)­X 1/ⁿBu₄NX and bifunctional catalyst 2 bearing an appended quaternary ammonium salt, X = 2,4-dinitrophenoxide) for coupling CO₂ and epoxides (propylene oxide or cyclohexene oxide) by means of in situ infrared spectroscopy. An induction period was readily found in the binary catalyst system, and its length significantly depends on catalyst loading. Contrarily, no induction period was observed in the bifunctional catalyst 2, in which the overall reaction pathway is consistent with the first-order dependence on catalyst concentration. A reaction order of 1.61 of catalyst concentration was obtained from the binary 1/ⁿBu₄NX catalyst system, indicating the complexity of the copolymerization. The energies of activation determined for cyclic carbonate and copolymer formation in the coupling reaction of CO₂ and propylene oxide catalyzed by the binary 1/ⁿBu₄NX system are 50.1 and 33.8 kJ/mol, respectively, compared to the corresponding values in the bifunctional catalyst 2 of 77.0 and 29.5 kJ/mol. The big difference in the energies of activation for cyclic carbonate versus copolymer formation accounts for the excellent selectivity for copolymer formation in the bifunctional catalyst systems even at elevated temperatures. In the coupling system of CO₂ and cyclohexene oxide, the energy of activation for copolymer (E_a) formation is 47.9 kJ/mol for the binary 1/ⁿBu₄NX catalyst system, higher than 31.7 kJ/mol determined in the bifunctional catalyst 2.