Figure 3 Classification of photoinitiator.

2.1. UV photoinitiator

The earliest species of PIs, mainly absorb UV light to achieve initiation, have been widely employed in numerous fields of UV photopolymerization for many years. Firstly, the cleavage type (Norrish Ⅰ), refers to formation of radicals by homolytic reaction of weak covalent bond in molecular, includes these species of benzils, benzoins, oxime esters and so on.[4-5, 10, 25-26] Figure 4 presents several structures of commonly-used cleavable UV PIs such as 2-hydroxy-2-methylpropiophenone (1173), 1-hydroxycyclohexyl phenylketone (184), 2,2-dimethoxy-2-phenylacetophenone (DMPA), 2-hydroxy-4’-(2-hydroxyethoxy)-2-methylpropiophenone (2959), methyl benzoylformate (MBF), benzil, diphenyliodonium (Iod), 2-benzyl-2-(dimethylamino)-4’-morpholinobutyrophenone (369), phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide (BAPO), diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO), ethyl (2,4,6-trimethylbenzoyl) phenylphosphinate (TPO-L), oxime esters OXE-10 and OXE-02, and cleavage mechanism of PI 1173.[27]Most UV PIs of cleavage type undergo one-step cleavage to polymerize the monomers except oxime esters which requires two-step cleavage.
Figure 4 Structures of commonly-used cleavable UV PIs, and cleavage mechanism of 1173.
Different from cleavage type, the hydrogen abstraction type PIs produce radicals by interacting with hydrogen donors, then triggers photopolymerization reaction.[23, 28] In some cases, hydrogen abstraction type PIs with outstanding light absorption can also sensitize other PIs with weak light absorption, they can be called photosensitizer. Figure 5 records some structures of commonly-used hydrogen abstraction type UV PIs and photosensitizers such as benzophenone (BP), 4,4’-bis(diethylamino) benzophenone (EMK), isopropyl thioxanthone (ITX), 9,10-dibutoxyanthracene (DBA) , and initiation mechanism of BP and donor photoinitiating system.[27] In addition, several commonly-used hydrogen donors are also exhibited in Figure 5, such as N-phenylglycine (NPG), ethyl 4-dimethylaminobenzoate (EDAB), triethanolamine (TEOA), N-methyldiethanolamine (MDEOA), 2-mercaptobenzothiazole (2-MBTA), 2-mercaptobenzimidazole (2-MBMA) and 2-mercaptobenzoxazole (2-MBXA).
Figure 5 Structures of commonly-used UV PIs of hydrogen abstraction type and several hydrogen donors, initiation mechanism of BP/doner system.
Both types of PIs possess their own characteristics, for cleavage type, rapid initiation and one-component are advantages in practical application,[29] for hydrogen abstraction type, two-component PIS can effectively relieve the problem of oxygen inhibition, and is generally multifunctional.

2.2. LED photoinitiator

The traditional high-pressure mercury lamp, as the light source of UV photopolymerization, has some shortcomings such as generation of ozone, toxic mercury, short service life and large consumption of energy, which lead to its limitations in practical application. Recently, there has a tendency that the light source of photopolymerization has been moving from UV toward near-UV or visible light using light-emitting-diodes (LEDs).[5] LEDs possess many merits such as higher-operating efficiency, long service life, low cost, safety, and environmentally friendly,[3, 6-7, 11, 30-43] as a result, LED photopolymerization has boomed in recent years. Whereas the absorption wavelengths of most traditional UV PI usually are shorter than 365 nm, and do not match with the emission wavelengths (usually higher than 365 nm) of LEDs, so most of the commercially available UV PI cannot be used for LED-induced photopolymerization.[1, 30] Therefore, the development of PI suitable for LED light sources is necessary.
At present, the LED PI of cleavage type reported mainly include TPO, BAPO, ITX, coumarin-based oxime esters,[1, 16, 43]naphthalimide sulfur ether,[23, 29]glyoxylates,[3, 31, 44-45] and acylgermanes,[46-48] and so on, the relevant structural formulas are shown in Figure 6 – Figure 9. LED PI of hydrogen abstraction or photosensitizers reported mainly include anthraquinone derivatives,[35-36, 49]cyclohexanone derivatives,[37, 50]cinnamoylformate derivatives,[7] porphyrins derivatives,[51-52] and so on, the relevant structural formulas are shown in Figure 10 – Figure 13.
Figure 6 Several chemical structures of coumarin-based oxime esters.
Figure 7 Several chemical structures of naphthalimide sulfur ether.
Figure 8 Several chemical structures of glyoxylates derivatives.
Figure 9 Several chemical structures of acylgermane derivatives.
Figure 10 Several chemical structures of anthraquinone derivatives.
Figure 11 Several chemical structures of cyclohexanone derivatives.
Figure 12 Several chemical structures of cinnamoylformate derivatives.
Figure 13 Several chemical structures of porphyrins derivatives.

2.3. NIR photopolymerization

Compared with Ultraviolet (UV) light, Near-Infrared (NIR) light possesses a lower scattering coefficient which is beneficial for a deeper penetration,[53] and the release of heat is useful for the photopolymerization.[54] Cyanines are good near infrared light absorbers, can be tailor made by changing the length of methane chain and substitution pattern which can cover a large absorption wavelength region between 700 nm and 1000 nm.[55] Cyanines can react with iodonium salts to generate free radicals and conjugate acid which can efficiently initiate polymerization of acrylate monomers, epoxides and vinyl ether monomers.[54, 56-57] Oxime esters can interact with cyanines to initiate photopolymerizaiton of radical monomers.[58] Alternatively, up-conversion nanoparticles can absorb the NIR laser to emit visible light and UV light which can be absorbed by traditional photoinitiators to trigger photopolymerization.[59-60] NIR photopolymerization can be applied in digital imaging in Computer to Plate (CtP),[61] powder coating,[62] 3D printing,[63] etc.