Synthesis of 6-membered cyclic carbonates from 1,3-diols and low CO2 pressure: a novel mild strategy to replace phosgene reagents

Unlike aromatic polycarbonates, aliphatic polycarbonates (APCs) have been little explored commercially. However, their low toxicity and biodegradability make them excellent candidates for biomedical applications. Recently, they have also been investigated as thermoplastics, binders for photovoltaics, polymer electrolytes and adhesives. Part of this renewed attention stems from the possibility to obtain APCs via the copolymerisation of CO2 and epoxides, an attractive reaction which uses an abundant and virtually free source of carbon, but is limited by the scope of usable epoxides. APCs can also be synthesised by polycondensation of diols with phosgene derivatives or dialkyl carbonates, but the control of the polymer molecular weight is highly sensitive to the reaction conditions. Hence, Ring-Opening Polymerisation (ROP) of cyclic carbonates has become the method of choice for APC synthesis, as the development of ROP catalysts has enabled controlled polymerisation of highly functionalised monomers under mild conditions (usually 6-, 7-, or strained 5-membered rings). A common preparation of cyclic carbonate monomers involves the transesterication of diols with phosgene, a toxic reagent synthesised from CO and Cl2 in an energy intensive process. Nevertheless, phosgene is still widely employed due to its efficiency and the lack of a more versatile and sustainable alternative. Safer phosgene derivatives include di-tert-butyl dicarbonate, 1,10-carbonyldiimidazole,16 and aromatic