Cyclic Carbonates as Safe and Versatile Etherifying Reagents for the Functionalization of Lignins and Tannins

The potential of cyclic carbonates for the functionalization of lignins and condensed tannins was studied in detail. Four different cyclic carbonates, namely, ethylene, propylene, vinyl ethylene, and glycerol carbonates, were evaluated. Full conversion of the phenolic hydroxyl groups was observed within very short reaction times (less than 2 h and as low as 15 min with ethylene carbonate). Comparison among the different cyclic carbonates shows that the substituent influences the reactivity as follows: CH₃ < CHCH₂ < CH₂OH < H. The developed method is a safe alternative to the use of organohalides or epoxides for the introduction of functional groups of interest onto lignins and tannins. The prepared derivatives expose primary or secondary hydroxyl groups, vinyl groups, 1,2- and 1,3-diols, or five-membered cyclic carbonates. All of the derivatives have an enhanced thermal stability and a lowered glass transition temperature (T_g) compared to the neat lignins and condensed tannins and thus present a high potential for the preparation of various types of biobased and aromatic polymers. The proposed method also allows the grafting of oligomeric chains when the reaction time is extended and thus represents an alternative to the use of the toxic and hazardous ethylene or propylene oxides for the oxyalkylation of polyphenols.