Lignin-grafting alternative copolymer of 3,4-dihydrocoumarin and epoxides as an active and flexible ingredient in sunscreen

Abstract

In this work, a series of new lignin-graft copolymers were successfully prepared through enzymatic hydrolytic lignin (EHL)-initiated ring-opening alternating copolymerization of 3,4-dihydrocoumarin (DHC) and epoxides (EPO) through organocatalysis. A series of EPOs, including styrene oxide (SO), propylene oxide (PO), cyclohexane oxide (CHO) and lignin-derived-2-(2-methoxy-4-methyl-phenoxy) (MPO), together with DHC, were selected as model compounds to prepare EHL-g-P(DHC-alt-EPO), demonstrating the tailor-made potential of this synthetic protocol. The reaction conditions were optimized and the structures and thermal properties of EHL-g-P(DHC-alt-EPO) were fully elucidated by ¹H, ¹³C and ³¹P NMR, FTIR spectroscopy, MALDI-TOF MS and GPC, DSC, and TGA. It was found that EHL-g-P(DHC-alt-SO) with a monomer conversion of 81.52%, DP of 3.21 and M_n of 62.9 kDa was prepared at a molar ratio of 5 : 5 : 1, 140 °C and 24 h. The UV shielding properties, antioxidant activity and biocompatibility of the as-prepared EHL-g-P(DHC-alt-EPO) was systematically evaluated. As a ‘proof of concept’ application, the potential of EHL-g-P(DHC-alt-SO) as an active ingredient in sunscreen was demonstrated, and a high sun protection factor (SPF) of 22.42 was achieved at 5 wt% dosage of EHL-g-P(DHC-alt-SO) in the cream, and the possible mechanism for the outstanding performance is also discussed.