Lawsone Derivatives as Efficient Photopolymerizable Initiators for Free-Radical, Cationic Photopolymerizations, and Thiol—Ene Reactions

Abstract

Two new photopolymerizable vinyl (2-(allyloxy) 1,4-naphthoquinone, HNQA) and epoxy (2-(oxiran-2yl methoxy) 1,4-naphthoquinone, HNQE) photoinitiators derived from lawsone were designed in this paper. These new photoinitiators can be used as one-component photoinitiating systems for the free-radical photopolymerization of acrylate bio-based monomer without the addition of any co-initiators. As highlighted by the electron paramagnetic resonance (EPR) spin-trapping results, the formation of carbon-centered radicals from an intermolecular H abstraction reaction was evidenced and can act as initiating species. Interestingly, the introduction of iodonium salt (Iod) used as a co-initiator has led to (1) the cationic photopolymerization of epoxy monomer with high final conversions and (2) an increase of the rates of free-radical polymerization of the acrylate bio-based monomer; we also demonstrated the concomitant thiol–ene reaction and cationic photopolymerizations of a limonene 1,2 epoxide/thiol blend mixture with the HNQA/Iod photoinitiating system.

Bio-sourced monomers and cationic photopolymerization–The green combination towards eco-friendly and non-toxic materials

“…Over the years, several natural compounds have been identified as interesting and efficient scaffolds for radical, cationic and thiol-ene photopolymerization reactions, such as anthraquinones [235,[256][257][258][259], carotenoids [230,231], various derivatives of chlorophyll [260,261], curcuminoids [18,250,251,[262][263][264][265][266], flavonoids [267][268][269][270][271][272][273], naphthoquinones [274,275], riboflavin [276][277][278][279][280][281] and lignin derivatives [282].…”

“…Naphthoquinones generally occur in either of two isomers: 1,4-naphthoquinone and 1,2naphthoquinone. 5-hydroxy-1,4-naphthoquinone (5HNQ), which can be found in the heartwoods of Caesalpinia sappan, and 2-hydroxy-1,4-naphthoquinone (2HNQ, also typically known as lawsone), which is a yellow dye abundantly found in the leaves of Lawsonia inermis (henna plant) [304], have recently been examined for the design of natural dye-based PISs in the framework of cationic photopolymerization due to their excellent visible light absorbing properties [274,275]. Their respective chemical structures are shown in Fig.…”

“…Going deeper into the investigation of the photoinitiating ability of lawsone, Elian et al have just now reported the successful synthesis of two new biobased photopolymerizable PIs derived from 2HNQ [275]. Their aim was manifold: the natural dye-based derivatives can act as visible light photosensitizers as well as photopolymerizable monomers thanks to the grafting of both allyl and epoxy functional groups (see chemical structures in Fig.…”

A Review on Modeling Cure Kinetics and Mechanisms of Photopolymerization

“…In recent years, photoinitiated polymerizations have become an important and efficient polymerization type that has several advantages including the possibility of solvent-free formulations, low-temperature conditions, and the spatial control of the light penetration inside the photosensitive resin [ 52 , 53 ]. In principle, radical photoinitiators are compounds that decompose into radicals during exposure to shortwave or visible light.…”

“…Common type I photoinitiators are benzoin, its derivatives, dialkoxy acetophenones, aminoalkyl phenones, and bisacyl phosphinoxides ( Figure 7 ). The main drawback of type I photoinitiators is their irreversible consumption during photopolymerization [ 53 ]. In type II photoinitiators, also referred to as H-abstraction type initiators, the photoinitiator is able to abstract a hydrogen atom from adjacent molecules, forming two radicals.…”