## 1 Introduction  
Photopolymerization reactions are commonly presented as belonging to a green technology characterized by low electrical power input and energy requirements, low temperature operation and no volatile organic compounds release (solvent-free systems) [1; 2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18]. In industrial sectors, such as radiation curing, imaging, microelectronics, medicine or optics (with various and very different applications, e.g., in coatings, varnishes, paints, adhesives, graphic arts, printing plates, stereolithography, photoresists, laser direct imaging, computer-to-plate technology, holographic optical elements or tooth repair), light-induced polymerization reactions have been very well-known for many years [19]. These reactions involve a polymerizable radical or cationic matrix and a more or less complex photoinitiating system (PIS) [19, 20, 21, 22, 23, 24, 25, 26]. Mercury lamps are largely used as light sources together with doped Hg lamps or microwave powered lamps. The delivered lights in the 280-450 nm range often satisfactorily match the absorption of PIS. In the context of green chemistry, the avoidance of volatile organic compounds is also an important issue. Free radical photopolymerization (FRP) is undoubtedly the most popular compared to cationic photopolymerization (CP). A PIS contains at least a photoinitiator (PI) and/or a photosensitizer (PS): PI (or PS) has to absorb the light [19]. Upon excitation, in FRP, PI becomes excited (PI*) and generates (1-4) a radical, R\({}^{\bullet}\), either directly through cleavage or in the presence of an electron/hydrogen donor. When PS is used and excited, the excitation has to move from PS* to PI by energy (2) or electron transfer (3): the same R\({}^{\bullet}\) is formed or new ion radicals are created, respectively. \[\mathrm{PI}\rightarrow\mathrm{PI}\mathrm{*}\;(\mathbf{h}\mathbf{v}) \rightarrow\mathrm{radicals}\;\mathrm{R}^{\bullet} \tag{1}\]  
\[\mathrm{PS}\rightarrow\mathrm{PS}\mathrm{*}\;(\mathbf{h}\mathbf{v}) \rightarrow\mathrm{PI}\mathrm{*}\rightarrow\mathrm{R}^{\bullet} \tag{2}\]  
\[\mathrm{PS}\rightarrow\mathrm{PS}\mathrm{*}\;(\mathbf{h}\mathbf{v}) \rightarrow\mathrm{PS}^{\bullet+}+\mathrm{PI}^{\bullet-}\rightarrow\rightarrow\rightarrow\mathrm{radicals} \tag{3}\]  
\[\mathrm{R}^{\bullet}+\mathrm{radical\ monomer}\rightarrow\mathrm{polymer} \tag{4}\]  
In CP (5-8), onium salts (e.g., the iodonium salt referred to here as Ph\({}_{2}\)I\({}^{+}\); several commercial derivatives that do not release benzene are known) are used as PI [19]. Their direct homolytic/heterolytic decomposition followed by hydrogen transfer reactions leads to a proton. Their photosensitized decomposition occurs according to energy (6) or electron transfer (7). \[\mathrm{PI}\;(\mathrm{Ph}_{2}\mathrm{I}^{+})\;(\mathbf{h}\mathbf{v}) \rightarrow\mathrm{PI}\mathrm{*}\rightarrow\rightarrow\rightarrow\mathrm{H} ^{+} \tag{5}\]  
\[\mathrm{PS}\rightarrow\mathrm{PS}\mathrm{*}\;(\mathbf{h}\mathbf{v}) \rightarrow\mathrm{PI}\mathrm{*}\rightarrow\rightarrow\rightarrow\mathrm{H} ^{+} \tag{6}\]  
\[\mathrm{PS}\rightarrow\mathrm{PS}\mathrm{*}\;(\mathbf{h}\mathbf{v}) \rightarrow\mathrm{PS}^{\bullet+}+\mathrm{PI}^{\bullet-}\;(\mathrm{e.g.,}\; \mathrm{Ph}_{2}\mathrm{I}^{\bullet}) \tag{7}\]  
\[\mathrm{H}^{+}\;(\mathrm{or}\;\mathrm{PS}^{\bullet+})+\mathrm{cationic\ monomer} \rightarrow\mathrm{polymer} \tag{8}\]  
In free radical promoted cationic polymerization (FRPCP) (9-11), a radical, R\({}^{\bullet}\), is produced from a radical source (RS) (a PI or a PS can play such a role) and then oxidized by Ph\({}_{2}\)I\({}^{+}\) to form Ph\({}_{2}\)I\({}^{\bullet}\) and a cation, R\({}^{+}\), suitable for the ring opening reaction (ROP) of epoxides or the cationic polymerization of vinyl ethers (the Ph\({}_{2}\)I\({}^{\bullet}\) species readily decomposes into PhI and Ph\({}^{\bullet}\)) [19].