## 1 Introduction
\[\mathrm{PI}\rightarrow\mathrm{PI}\mathrm{*}\;(\mathbf{h}\mathbf{v}) \rightarrow\mathrm{R}^{\bullet} \tag{9}\]  
\[\mathrm{R}^{\bullet}+\mathrm{Ph}_{2}\mathrm{I}^{+}\rightarrow\mathrm{R}^{+} +\mathrm{Ph}_{2}\mathrm{I}^{\bullet}\;(\rightarrow\mathrm{R}^{+}+\mathrm{Ph} \mathrm{I}+\mathrm{Ph}^{\bullet}) \tag{10}\]  
\[\mathrm{PS}\rightarrow\mathrm{PS}\mathrm{*}\;(\mathbf{h}\mathbf{v}) \rightarrow\mathrm{PI}\mathrm{*}\rightarrow\mathrm{R}^{\bullet} \tag{2}\]0  
The PI, PS and RS have to be selected to absorb the irradiation wavelengths [19]. In FRP, the selection of near UV/visible photosensitive systems for industrial applications is quite easy (and almost feasible on laboratory scale experiments at any UV-visible wavelength). In CP, as the PIs mainly absorb in the UV, the search and the design of suitable PS compounds as energy or electron donors for visible light-induced polymerizations are necessary, but this appears as a rather complex task for the photocuring of coatings in industrial lines. Due to its versatility, FRPCP is certainly one of the most interesting and promising ways for a cationic polymerization under exposure at \(\lambda>350\) nm (up to 700 nm), but the occurrence of efficient reactions (10 and 11) is not so trivial, and the oxygen quenching of the radicals is detrimental. The development of photopolymerization reactions towards a green technology can be found in five directions:  
(1) search for new PI or PS being able (i) to absorb the visible lights that are very often lost when employing conventional mercury lamps and PISs and/or (ii) to move the system towards a UV-free exposure (doped Hg lamps, Xe-Hg lamps, Xe lamps). (2) use of newly developed high intensity LED or laser diodes operating at well-defined near UV/visible wavelengths avoiding the use of Hg-based lamps and the presence of more energetic UV wavelengths (254, 313 nm). Today, in industrial applications, LED technology allows highly packed arrays operating at 365 or 395 nm, together with a low heat generation, low energy consumption, low cost and low maintenance; the development of laser diode arrangements ensures high intensity monochromatic irradiations from the blue to the red part of the spectrum. (3) development of PISs for soft irradiation conditions and use of low visible light intensity sources, e.g., household devices: halogen lamp, fluorescent bulbs and LED bulbs. (4) use of sunlight, which is a cheap and inexhaustible energy source (but strongly affected by the weather and location) that might be of interest for (i) particular outdoor applications (e.g., for paint drying) and (ii) the possibility of curing large dimensioned pieces or surfaces without requiring any irradiation device. (5) search of natural products or renewable monomers (the plant oil derivatives present attractive features, such as versatility, biodegradability and low cost). In a general way, the questions that have to be solved for getting a high polymerization efficiency concern the PISs and the starting monomers, as well as their adaptation to the available light sources. In the present paper, we will (i) discuss the drawbacks/breakthroughs encountered when going on the particular way of a greener approach for photopolymerization reactions, (ii) define the key points for the design of a high performance PIS in such conditions and (iii) show, as examples, some of our new or recent achievements using soft illumination conditions (e.g., household lamps and sunlight exposure; typically \(\sim\)2-10 mW/cm\({}^{2}\)), visible light irradiation (400 nm \(<\lambda<\) 800 nm), use of renewable monomers, _etc_. **2. Drawbacks/Breakthroughs on the Way to Greener Photopolymerization Reactions**  
_2.1. The Photopolymerization Reactions_  
In photopolymerization reactions [19], the matching of the PIS absorption spectrum with the emission spectrum of the light source, as well as the number of available incident photons, I\({}_{0}\), is crucial.