Synthesis and photoinitiation properties of lignin model compounds

Lignin could generate active free radicals under light irradiation and be directly used as photoinitiator (PI). However, the photoinitiation unit is unclear due to its complex molecular structure. Some previous studies indicate that the photoyellowing of lignin is due to the photosensitivity of Cα‑carbonyl β-O-4 unit in its molecular structure. Herein, four lignin model compounds D2, S3, G4 and F4 containing Cα‑carbonyl β-O-4 structure unit were synthesized and characterized. Among them, F4 was obtained by introducing co-initiator 4-(dimethylamino) benzoic acid ethyl ester (EDAB) to the lignin structural unit. The results demonstrated that under 365 nm LED irradiation, the four model compounds all could directly initiate the polymerization of acrylate monomer PEGDA but their photoinitiation efficiency was quite different. F4 exhibited the best photoinitiation performance compared with the other three PIs. As a one-component photoinitiator (PI), the photoinitiation efficiency of F4 was equal to that of a commercial two-component system benzophenone (BP)/EDAB. In addition, when combined with a hydrogen donor triethanolamine (TEOA), the photoinitiation efficiency of the two-component system F4/TEOA was higher than that of BP/TEOA. Furthermore, the photolysis mechanism of F4 was clarified through identifying the generated active species under light irradiation by electron spin resonance and laser flash photolysis.