1. Introduction to photopolymerization.

As an environmentally friendly technology, photopolymerization in which liquid monomers or oligomers are transformed into solid materials under irradiation of light (ultraviolet, visible or infrared light) holds these advantages of low volatile organic compounds (VOC), fast curing, energy saving, environmental protection, and low temperature compared with traditional thermal polymerization.[1-6] In addition, photopolymerization is widely used in many fields due to its unique ā€œ5Eā€ properties (efficiency, enabling, economical, energy saving and environmental friendly).[3, 7-12] The history of photopolymerization can be traced back to the 19th century when people observed that styrene polymerized into glassy resin under light. Ostromislenski, the first researcher of photopolymerization, discovered that the number of constitutional units of product far exceeded the number of photons absorbed in photopolymerization reaction of vinyl bromide, and concluded that the reaction was chain reaction.[13] The 1960s was the heyday of basic research on photochemistry, its theory gradually improved, and began to be widely applied to actual industrial production.
A complete photopolymerization system generally consists of monomer, oligomer and photoinitiator (PI), all components are indispensable.[14] Monomers, also known as active diluents, can reduce the viscosity of polymerization system and improve solubility, more importantly, they contain one or more functional groups that can participate in the polymerization reaction such as double bonds, epoxy, oxetane, thiol, etc. Oligomers, whose molecular weights range from several hundred to tens of thousands, have high viscosity and also contain some polymerizable functional groups to participate polymerization. The proportion of monomers and oligomers is large in whole polymerization system, and the properties of polymerization materials are largely determined by them. PI, as an essential component accounting for 1-10 wt % of photopolymerization system, can produce active species under irradiation of light to initiate polymerization reaction, which has great influence on the rate of photopolymerization and the performance of polymerization material.[15-16]
The procedure of the photopolymerization reaction is exhibited in Figure 1, firstly, PI of ground state undergoes electron transition to excited state under irradiation of light, then generate active species, which are free radicals or cations generally. Active species can interact with functional groups of monomers or oligomers to initiate chain reactions, resulting in the formation of polymer.