Cyclic organic carbonates as oxyalkylating reagents for lignin

This work presents an efficient, non-toxic and solvent-free procedure for the oxyalkylation of lignins using different cyclic organic carbonates. Comparative studies of the oxyalkylation of beech wood organosolv lignin with ethylene carbonate (EC), propylene carbonate (PC) and glycerol carbonate (GC) were performed. In addition, the oxyalkylation method was successfully adapted to technical lignins (kraft, soda). The structural changes of modified lignins were examined by 31P NMR spectroscopy as well as size exclusion chromatography (SEC). All of the phenolic and almost all of the aliphatic hydroxyl groups could be converted to hydroxyalkyl units using 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as catalyst and reaction temperatures of 170°C at atmospheric pressure. This oxyalkylation procedure allows the preparation of lignin polyols with extended carbon skeleton and exclusively aliphatic hydroxyl groups applicable as bio-based precursor for polyurethane and polyester synthesis.