A new environmentally friendly approach to lignin-based cyclic carbonates
Herein, a novel synthesis of a fully renewable lignin-based building block equipped with cyclic carbonate functionalities is presented. In an efficient two-step procedure, organosolv lignin from ethanol–water pulping of beech wood is oxyalkylated with glycerol carbonate to insert adjacent hydroxyl groups. The oxyalkylated lignin is then reacted via transesterification with dimethyl carbonate or ethylene carbonate in dimethyl sulfoxide under alkaline conditions producing cyclic carbonate functionalized lignin. This transesterification is studied as a function of time, catalyst type, and catalyst amount using NMR spectroscopy as well as size exclusion chromatography. Transesterification reactions with K2CO3 as catalyst (0.4 eq.) afford lignin-based cyclic carbonate with almost complete conversion (96%) within 6 h, as analyzed by 31P and 13C NMR spectroscopy. This green building block represents a reactive prepolymer for the synthesis of nonisocyanate polyurethanes.