Under natural light, lignin can produce a small amount of free radicals, resulting in yellowing of lignin-related products. However, the activity of these free radicals is very low, so that few studies on exploring the application potential of lignin as photoinitiator (PI) in photopolymerization have been reported. In this study, based on dealkaline lignin (DAL), two alkylated DAL (DAL-11ene and DAL-12ane) were synthesized through facile one-step esterification reactions between DAL and undecanoyl chloride or dodecanoyl chloride. The photoinitiation properties of DAL, DAL-11ene, and DAL-12ane were investigated by real-time FTIR spectroscopy and NIR-photorheology using 1,6-hexanediol diacrylate as a monomer. The formation of reactive species was elucidated by laser flash photolysis and electron spin resonance. The results indicated that DAL itself showed a certain photoinitiating ability, especially when it was combined with an amine co-initiator, while both DAL-11ene and DAL-12ane presented much higher photoinitiating efficiencies compared with DAL. Moreover, DAL-11ene containing a polymerizable group could be bonded into the network of final polymer products, which can increase their biosafety. 3D structures of polyacrylates were fabricated successfully using DAL-11ene and DAL-12ane as PIs through digital light processing 3D printing upon 405 nm exposure, indicating their large potentials in photopolymerization.

3D printing of bio-based and renewable polymers such as lignin has gained research attention during the last few decades. We report on the synthesis and characterization of a liquid lignin-based photopolymer and its application in additive manufacturing (AM). Wheat straw soda lignin is liquified in an oxyalkylation reaction with propylene oxide under alkaline conditions and modified with methacryloyl chloride to obtain a lignin-based methacrylate resin. Ninety percent of the functional hydroxyl groups are grafted during the synthesis. The photopolymerization efficiency was evaluated by real-time-NIR-photorheology experiments with two different photoinitiators, leading to double bond conversions (DBC) of ≥80%. 3D-printing experiments of the methacrylated lignin were performed with the hot lithography technology. For the first time, a light-curable lignin derivative with a lignin content of over 30% was successfully 3D printed via vat photopolymerization without any reactive diluents, which is a significant improvement over current state-of-the-art solutions. This outstanding result is a motivating proof of concept and a promising starting point for the in-depth evaluation of bio-based precursors as an alternative to nonrenewable derivatives for 3D printing.