Modification of kraft lignin to expose diazobenzene groups: Toward pH- and light-responsive biobased polymers

A pH- and light-responsive polymer has been synthesized from softwood kraft lignin by a two-step strategy that aimed to incorporate diazobenzene groups. Initially, styrene oxide was reacted with the phenolic hydroxyl groups in lignin, to offer the attachment of benzene rings, thus creating unhindered reactive sites for further modifications. The use of advanced spectroscopic techniques ((1)H and (31)P NMR, UV and FTIR) demonstrated that the reaction was quantitative and selective toward the phenolic hydroxyl groups. In a second step, the newly incorporated benzene rings were reacted with a diazonium cation to form the target diazobenzene motif, whose formation was again thoroughly verified. As anticipated, the diazobenzene-containing kraft lignin derivatives showed a pH-dependent color change in solution and light-responsive properties resulting from the cis-trans photoisomerization of the diazobenzene group

Reversible crosslinking of lignin via the furan-maleimide Diels-Alder reaction

Two distinct functionalization schemes for Kraft lignin (KL) were developed to selectively incorporate furan and/or maleimide motifs as chain ends. The incorporation of furan functionalities was carried out by the selective and quantitative reaction of the lignin's phenolic OH groups with furfuryl glycidyl ether (FGE). Maleimide groups were introduced by esterifying the lignin's aliphatic and phenolic OH groups with 6 maleimidohexanoic acid (6-MHA), offering a high loading despite a somewhat incomplete conversion. Furan- and maleimide-functionalized lignins were subsequently combined to generate crosslinking via the Diels-Alder (DA) [4 + 2] cycloaddition reaction. The formation of the DA adduct was confirmed by 1H NMR. Under appropriate conditions, the formation of a gel was apparent, which turned back into the liquid state after performing the corresponding retro-DA reaction upon heating to 120 °C. This study reveals the significant versatility and potential of the developed strategy for the utilization of lignin-based recyclable networks