Figure 47 Fabrication process of lignin−gelatin hybrid hydrogel.

3.5. Applications of in photopolymerization other fields

In addition to the above fields, the application of photopolymerization is gradually expanding to other fields, such as frontal polymerization, photochromic materials, self-healing materials and so on.
Frontal polymerization is a self-propagating reaction where monomers are crosslinked into polymers by the propagation from localized reaction zone to entire system.[161-162] Generally speaking, no extra energy is required except in initial stage.[162]According to the initiation sources, frontal polymerization can be divided into the thermal-induced frontal polymerization (TFP) and photo-induced frontal polymerization (PFP).[163]PFP is initiated by the irradiation usually UV light,[164-166] however, as mentioned above, UV light has many disadvantages and limitations in practical application, it is a new strategy to use more secure NIR light as irradiation source of frontal polymerization. Based on this, our group employed a two-component system including cyanine (NIR sensitizer) and iodonium salt to initiate the frontal polymerization of cationic polymerizable monomers upon 808 nm NIR laser exposure.[57] The heat released by NIR sensitizer and the polymerization reaction can dissociate AIBN or TPED to achieve frontal polymerization of vinyl ethers and epoxides. The frontal velocity and starting time can be modulate by changing the different kinds of monomers, thermal initiators and their concentration.
Solid-state photochromic materials especially photochromic polymers have potential application prospect in the fields of photoswitching devices, chemosensors, solid-state optical devices, imaging and 3D optical-data storage devices, and so on owing to their advantages of lightweight and good processability.[167-171] At present, the main preparation method of photochromic polymers is thermopolymerization, which is complicated and energy consuming. Therefore, it is extremely meaningful to develop a simple, efficient and environmentally friendly preparation method of photochromic polymers.
Du et al.[172] designed and synthesized two novel UV-LED-induced fluorophenyl oxime ester PIs (E-FBOXEs) with the ability to prepare photochromic polymers, and then photochromic polymers based on E-FBOXEs were prepared by fast photopolymerization. Proposed mechanism for the photochromism of the film initiated by E-FBOXEs is exhibited in Figure 48, Residual E-FBOXEs in photopolymerized films will generate colored iminyl radicals in the planar conformation under irradiation at 395 nm, and the recombination of the radicals to form colorless E-FBOXEs in the nonplanar conformation under heating, which results in the photochromism of the polymeric films.