## 1 Introduction
### The Development of New Photosensitive Systems  
It clearly appeared that the development of PIS should proceed through new concepts, ensuring an increase of their photochemical/chemical reactivity. In this direction, a noticeable improvement was noted with the introduction of the silyl chemistry into PISs [99; 100]. The silane (e.g., tris-(trimethylsilyl)silane (TTMSS)) becomes a magic additive, which renders more feasible the photopolymerization reactions in aerated conditions. In a silane containing PIS for FRP, initiating silyl radicals are generated: (i) they consume oxygen (15); (ii) scavenge the peroxyls (16 and 17) and (iii) regenerate new silyls. As a consequence, the oxygen inhibition is reduced, and the total amount of interesting R\({}_{3}\)Si\({}^{\bullet}\) increases, so that oxygen becomes a mediator in the initiating radical production. \[\mathrm{R}_{3}\mathrm{Si}^{\bullet}+\mathrm{O}_{2}\rightarrow\mathrm{R}_{3} \mathrm{Si}\mathrm{-O}_{2}{}^{\bullet} \tag{15}\]  
\[\mathrm{R}_{3}\mathrm{Si}\mathrm{-O}_{2}{}^{\bullet}+\mathrm{R}_{3}\mathrm{ Si}\mathrm{-H}\rightarrow\mathrm{R}_{3}\mathrm{Si}\mathrm{-O}_{2}\mathrm{H}+\mathrm{R}_{3} \mathrm{Si}^{\bullet} \tag{16}\]  
\[\mathrm{RM}_{\mathrm{n}}\mathrm{-O}_{2}{}^{\bullet}+\mathrm{R}_{3}\mathrm{Si} \mathrm{-H}\rightarrow\mathrm{RM}_{\mathrm{n}}\mathrm{-O}_{2}\mathrm{H}+ \mathrm{R}_{3}\mathrm{Si}^{\bullet} \tag{17}\]The same holds true in FRPCP, which is usually affected by the presence of oxygen. In a silane containing PIS, the oxygen inhibition is dramatically decreased as resulting from (15-17). Moreover, the addition of the iodonium salt allows an oxidation of the silyl radical (18): a R\({}^{+}\) cation is formed and can serve as a very efficient initiating species (19). In such PIS, an interesting feature relates to the possibility of forming the same cationic species, R\({}^{+}\), whatever the starting absorbing radical source (RS) (contrary to reaction 10, where the nature of the cation is dependent on the starting PI). RS can be a usual PI or PS (but also any other compound) being able to form sillys by cleavage of, e.g., a C-Si or a Si-Si bond (20), and an electron/proton transfer with, e.g., ketones or dyes (21). \[\mathrm{R_{3}Si^{\star}+Ph_{2}I^{+}\to R_{3}Si^{+}+Ph_{2}I^{\bullet}} \tag{18}\]  
\[\mathrm{R_{3}Si^{+}+monomer\to Polymer} \tag{19}\]  
\[\mathrm{RS\ (\textit{hv})\to R_{3}Si^{\star}+counter\ radical} \tag{20}\]  
\[\mathrm{RS+R_{3}SiH\ (\textit{hv})\to R_{3}Si^{\star}+RSH^{\star}} \tag{21}\]  
Therefore, the novel introduction of this silyl chemistry opens a new way to cure coatings under UV and visible lights. Interestingly, this also allows photocuring under soft conditions (visible light using exposure to Xe lamps, household halogen lamps, diode lasers (405, 457, 532, 635 nm), LED bulbs and sun; low intensity sources), under air, using relatively low viscosity matrices [101]. Figure 3 shows the role of the silane in the typical FRP and FRPCP of aerated curable formulations under near UV/visible lights. The germyl [102] and boryl [103] chemistries can play a similar role. In the same way, _N_-vinyl carbazole (NVK) was recently advantageously introduced into a formulation instead of the silane [104]; NVK is a cheap and efficient alternative to _tris_(trimethylsilyl)silane. Using other PISs and monomers (e.g., divinyl ethers), higher final conversions can be reached. Figure 3: Photopolymerization kinetics of (**A**) an acrylate monomer (trimethylol-propane triacrylate (TMPTA) from Cytec) using a phosphine oxide as the photoinitiator in the absence (1) or in the presence (2) of _tris_-(trimethylsilyl)silane (TTMSS) (3% _w_/_w_); (**B**) an epoxy monomer ((3,4-epoxycyclohexane)methyl 3,4-epoxycyclohexarylcarboxylate (EPOX), Uvacure 1500 from Cytec) using an hydroxyl alkyl acetophenone/iodonium salt (1%/1% _w_/_w_) as the photoinitiating system in the absence (**1**) or in the presence (**2**) of TTMSS (3% _w_/_w_); under air; Xe lamp exposure.