The synthesis of cyclic carbonates from epoxides and carbon dioxide using metal-free catalyst systems is critically reviewed.

In the water-assisted coupling reaction of CO₂ and propylene oxide (PO) catalyzed by halide salts, the PO conversion was closely correlated with the electrical conductivity, suggesting that simple conductivity measurements might be used as a potential method for the selection of the catalyst.

A new, natural lysine-based (salen)Cr^IIICl ((lys-salen)Cr^IIICl) complex was prepared and its catalytic activity for the copolymerization of CO₂ and cyclohexene oxide (CHO) was described in the presence of PPNCl (PPN⁺ = bis(triphenylphosphoranylidene)ammonium) as cocatalyst. The influence of the reaction time, operating temperature and the molar ratio of the catalyst components on the copolymerization was investigated in detail. The results showed that the (lys-salen)Cr^IIICl, synthesized from non-ortho-diamine, could effectively catalyze the alternating copolymerization (carbonate linkages = 94.6–99.0%). The selectivity was >95%, and was less sensitive to the temperature and the molar ratio of catalyst components, compared to that of the copolymerization catalyzed by traditional salen–metal complexes. The ESI-MS analyses of oligomer and (lys-salen)Cr^IIICl indicated that a possible chain-transfer reaction had taken place, which might be induced by the water coordinating to the central metal ion.

Metal β-diketonate complexes could efficiently catalyze the chemical fixation of CO₂ at 1 atm and near room temperature.