Bio-based design and synthesis of lignin-graft-lignin copolymers and their nanomaterials

Lignin, the most abundant natural aromatic biopolymer, is a promising alternative to petroleum-based polymers. Extensive efforts have been devoted to its chemical modifications for high-value applications, with lignin-grafted copolymer nanomaterials emerging as a key advancement. However, the relationships between copolymerization and nanomaterial properties of lignin are rarely studied. Here, we copolymerized three lignin derivative monomers (FMA, VMA, and SMA) onto lignin via reversible addition-fragmentation chain transfer (RAFT) copolymerization and investigated how these lignin copolymers improve the functionality and processability of lignin and affect the properties of resulting lignin nanomaterials. The successful copolymerization was verified by NMR and GPC. The grafting kinetics were revealed, with FMA exhibiting the highest reactivity, followed by VMA and SMA. Lignin-copolymer nanobeads were prepared via a solvent-shifting process, showing spherical shapes with diameters of 152 nm for Lignin-PFMA, 125 nm for Lignin-PVMA, and 176 nm for Lignin-PSMA, respectively. The lignin nanofibers are also obtained by electrospinning without the assistance of other synthetic polymers with fiber diameters ranging from 310 to 1180 nm. The cell viability attained from the incubation of lignin nanobeads and nanofibers for either CCL9.1 or WS1 cells was above 80 %, which indicates their good biocompatibility. Together their unique antioxidant and UV-absorbing properties, the lignin nanomaterials demonstrate great potential for biomedical applications, including UV-protective pharmaceutical formulations and antioxidative biomaterials for wound healing/tissue engineering.