Photopolymerization Reactions: On the Way to a Green and Sustainable Chemistry

The present paper reviews some aspects concerned with the development of green technologies in the photopolymerization area: use of visible light sources (Xe and Hg-Xe lamps, diode lasers), soft irradiation conditions (household lamps: halogen lamp, fluorescence bulbs, LED bulbs), sunlight exposure, development of very efficient photoinitiating systems and use of renewable monomers. The drawbacks/breakthroughs encountered when going on the way of a greener approach are discussed. Examples of recent achievements are presented.

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  , 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  is formed or new ion radicals are created, respectively.PI → PI* (h) → radicals R  (1) PS → PS* (h) → PS + + PI − →→→ radicals (3) In CP (5)(6)(7)(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).
H + (or PS + ) + cationic monomer → polymer (8) In free radical promoted cationic polymerization (FRPCP) (9)(10)(11), a radical, R • , 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 • and a cation, R + , suitable for the ring opening reaction (ROP) of epoxides or the cationic polymerization of vinyl ethers (the Ph 2 I • species readily decomposes into PhI and Ph • ) [19].
R + + monomer → polymer (11) 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  > 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 ~2-10 mW/cm 2 ), visible light irradiation (400 nm <  < 800 nm), use of renewable monomers, etc.

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.The absorption properties of PI, PS and PIS (ground state spectra and molar extinction coefficients, ε) play a decisive role, as the polymerization rate, Rp, is directly connected with the amount of light absorbed (I abs ): I abs = I 0 (1-10 −cl ) where I 0 , c and l stand for the incident light intensity, the molar extinction coefficient, the photoinitiator concentration and the sample thickness, respectively.The delivered flux of photons can be very high with Hg lamps (Hg arc lamp, doped Hg lamps, electrodeless Hg lamps; typically > 1-2 W/cm²), highly packed arrays of light emitting diodes (LED) at 365 or 395 nm (a few W/cm²), Hg-Xe or Xe lamps and quite low with household devices (halogen lamps, fluorescent bulbs and white or blue LED bulbs; <10 mW/cm²), diode lasers (10-100 mW/cm²) or sun (2 mW/cm²).Typical examples of emission spectra are given in Figure 1 for various sources.
The same holds true in CP as, in addition, oxygen inhibition does not occur.Fast curing speeds are reached under light exposure below 400 nm.
The situation is more complicated in FRPCP, as the usual photoinitiating systems are naturally less efficient and sensitive to the presence of oxygen, but new PISs have led to promising developments (see below).
Going to longer wavelength exposures (450-700 nm) can also be achieved in FRP, CP and FRPCP using appropriate conventional PISs, provided that relatively high intensity light sources and viscous media are used.A real progress, however, has been realized in recent works and many PISs that meet this challenge (even with low intensity lights and low viscosity media) have been proposed in the last five years (see, e.g., [19, and references therein).

The Oxygen Inhibition
In FRP and FRPCP, a well-known drawback [19] concerns the oxygen inhibition (12)(13)(14), which is due to the excited triplet state quenching ( 3 PI or 3 PS) by O 2 and the scavenging of the initiating R  and propagating RM n  radicals by O 2 (a nearly diffusion controlled reaction; highly stable peroxyl radicals are formed).The polymerization starts in the film as soon as oxygen is consumed.The practical effects of this phenomenon strongly depend on the experimental conditions.In highly viscous or thick samples (e.g., epoxy acrylate matrices), the re-oxygenation process is slow, which leads to an efficient polymerization after an inhibition period.The top layer in contact with air is easily polymerized, provided that a high PI concentration and a high light intensity are used: this is easily feasible in thin samples; it might be more complicated in thick samples.On the opposite, in very low viscosity media (e.g., di-or tri-functional monomers, such as trimethylol-propane triacrylate (TMPTA) or 3,4-epoxycyclohexane)methyl 3,4-epoxycyclohexylcarboxylate (EPOX), the re-oxygenation remains efficient, thereby reducing the monomer conversions.In addition, when the light intensity is attenuated, the oxygen inhibition has a dramatic effect on the polymerization profile, due to (i) the lowering of the initial O 2 consumption process and, as a consequence, (ii) the decrease of the initiating radical concentration (as a higher amount of these radicals are trapped by O 2 ).As is known, decreasing the oxygen inhibition effect can be achieved through various strategies (see a review in [19]).The recent introduction of a novel approach has led to successful results (see below).

The Soft Irradiation Conditions
The above mentioned considerations explain why the FRP is difficult when using visible light and low intensity sources under air.For example, the development of sunlight photosensitive formulations (see, e.g., [77][78][79][80][81][82][83][84][85] and references therein; see also the patent literature aiming at industrial applications) for the drying of paints for crack-bridging applications, anti-soiling properties, the manufacture of interpenetrating polymer networks (IPN) usable as protective coatings and glues, the fabrication of glass fiber reinforced composites, hard and rigidified four layer glass cloth laminate, clearcoats and polymer-clay composites has been realized in the past, but these systems, except some of them (e.g., those described in [79,80,84]) suffer from oxygen inhibition and a relatively low photosensitivity.
As stated above, except some colored systems (e.g., the ferrocenium salts), the usually employed cationic PIs (onium salts) for CP absorb in the UV.Even in academic laboratories, efficient photosensitization reactions of cationic PISs upon visible light is rather limited, as the possible efficient electron donor/onium salt couples are in a very limited number, despite careful research [19].
In FRPCP, the main problem concerns the choice of PIs, leading to an efficient R + initiating cation (3): few examples were known; most systems operated in the near UV; the efficiency/reactivity was not so high [86][87][88][89][90][91][92][93][94][95].Interesting systems have been shown to work under sunlight, but in laminated conditions [96][97][98], they have, however, opened up promising perspectives.Through the very recent development of efficient visible light sensitive systems, FRPCP has known a substantial progress (see below).

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  increases, so that oxygen becomes a mediator in the initiating radical production.
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)(16)(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 silyls 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).

3a/ Soft or Eco-Friendly Photopolymerization of Synthetic Monomers
In this part, we will show some examples (extracted from our own work ), which illustrate today's green character of the photopolymerization reactions of synthetic monomers (other experiments using renewable monomers will be presented below with more details).TMPTA (trimethylol propane acrylate) and EPOX ((3,4-epoxycyclohexane)methyl 3,4-epoxycyclohexylcarboxylate) will be used as representative low viscosity monomers.Divinyl ethers (e.g., triethylene glycol divinyl ether (DVE-3)) can also be photopolymerized.All the formed coatings are tack-free.

Design of New PIS Allowing a UV-Free Exposure and Ensuring the Use of Visible Light
Figure 4 shows some polymerization profiles of EPOX using typical visible light absorbing PISs.A Xe lamp ensures fast CP and FRPCP processes.The FRP of acrylates is also feasible under such irradiation conditions.Therefore, visible photons can be successfully used and Hg lamps avoided.The recent development of di-and tri-functional architectures of PIs, light harvesting PIs and push-pull and multicolor PIs opens a route towards highly absorbing PIs in the 400-800 nm range [57,58,64,73].

Use of Newly Developed LEDs and Laser Diodes Avoiding Hg-Based Lamps
Excellent conversion vs. time curves can be recorded upon excitation with a laboratory LED device at 365 nm (Figure 5) [64].Commercial highly packed LED systems lead to the cure speeds attained with Hg lamps.According to the usual absorption spectra of PIs in the UV, many PIs work in these conditions [19].A smaller number of systems can operate at 395 nm.Recently developed PISs operating in the near-UV/visible range (e.g., [57,58]) noticeably extend the scope of the existing structures and should be efficient upon a 395 nm LED irradiation.1) bis(acyl)phosphine oxide (BAPO)/iodonium salt (1%/1% w/w); (2) BAPO/iodonium salt/TTMSS (1%/1%/3% w/w); (3) BAPO/iodonium salt/tetraphenyldisilane (1%/1%/3% w/w).Instead of BAPO, more colored structures, such as titanocenes and other dyes, can be used.Laser diodes also lead to efficient FRP, CP and FRPCP .New PIS exhibit an absorption that satisfactorily matches the emission of the sources from the blue to the red: this is exemplified in Figure 6, where three kinds of PI can be used with four examples of LED bulbs; TMPTA, as well as EPOX can be polymerized.Laser diode arrays obviously allow faster curing speeds.(2) Napht/Ethyl-dimethylaminobenzoate (EDB) (0.5%/4.5% w/w); (3) Napht/EDB/phenacyl bromide (0.5%/4.5%/3% w/w).(B) Compared polymerization profiles of EPOX under air upon a red LED bulb irradiation in the presence of: (1) Pent/Ph 2 I + (0.5%/ 2% w/w) and (2) Pent/TTMSS/Ph 2 I + (0.5%/3%/ 2% w/w).Insert: emission spectra of the used LED bulbs (2-12 mW/cm²); different photoinitiators recently proposed [75,76].Household devices, such as halogen lamps, fluorescent bulbs and LED bulbs, deliver low intensity visible light and are used in organic synthesis.They have been recently introduced for the photopolymerization of low viscosity monomers under air  (see, again, the polymerization profiles under a red LED bulb exposure in Figure 6).Today, many PIS allow FRP, CP and FRPCP in these irradiations conditions: e.g., Figure 7 shows an efficient polymerization of EPOX under halogen lamp exposure.It is obvious that all the work on the design of PISs carried out in this area should be very helpful for potential and promising applications with more energetic light sources in both laboratory scale devices and industrial lines.

Use of Sunlight Irradiation
Sun is the lowest intensity source used in this paper (2 mW/cm²).Efficient photopolymerization reactions still appear as relatively extremely hard.The FRP was mainly restricted to complex paint formulations (see the patent literature) or acrylates dispersed in a solid matrix [80,92].On the opposite side, CP and FRPCP were reported as possible [96][97][98].
Recently, the FRP of a viscous matrix under air (e.g., an epoxy-acrylate having a viscosity of ~14,000 cP) has been carried out using efficient PISs based on silyl radical chemistry (50% conversion within 20 s and a final conversion of 75% at t = 8 mn using a bis(acyl)phosphine oxide (BAPO) and a silane) [77].CP and FRPCP now appear as relatively easily feasible (see e.g., Figure 9); once again, the use of a three-component photoinitiating system based on a photoinitiator, an iodonium salt and a silane (or N-vinylcarbazole) allows an efficient curing of a usual difunctional epoxide matrix under air (see, e.g., in ).

3b-Photopolymerization of Renewable Monomers
Some typical examples of photopolymerization profiles of renewable epoxy resins upon visible light exposure (Xe lamp) are displayed in Figure 10 (see also Table 1).Among the different compounds depicted in Scheme 1, LDO is the most reactive monomer.This is in agreement with the cyclohexyl epoxy structure, where the ring opening process is highly favorable [19].The polymerization is slower with ESO, ELO (epoxidized linseed oil) and EFA, but quite good final monomer conversions can be reached (40%-60%; Figure 10B) using a combination of the photoinitiator with an iodonium salt and a silane; moreover, tack-free coatings are formed.In any case, a decrease of the band at ~790 cm −1 (due to the epoxy ring) is monitored, whereas an increase of the IR absorption band of the polyether network is observed in the 1050-1150 cm −1 range.The photoinitiating system is important for getting a high reactivity, as exemplified by Figures 10A and 11, where different photoinitiating systems lead to very different polymerization profiles.Photoinitiating systems based on bis-acylphosphine-oxides (BAPO) are very efficient (Table 2).
Extremely soft irradiation conditions can also be used.Figure 12 shows the epoxide consumption and the formation of the polyether network under a household fluorescent bulb or sunlight exposure under air.In outdoor conditions, tack-free coatings are obtained with LDO, ELO and ESO (Table 2).
As before, the polymerization profiles of these monomers are also clearly improved by the presence of a silane, i.e., for ELO, a tack-free coating is obtained within only 9 min in the presence of a silane (TTMSS) vs. 50 min in the absence of the silane.As the polymerization efficiency in the presence of EPOX, LDO or ESO in the same experimental conditions are relatively close, it is obvious that renewable monomers can be successfully used in photocurable formulations operating in a large range of excitation wavelengths delivered by polychromatic (Xe lamps, household lamps) and (quasi) monochromatic (LED and laser diodes) light sources and sun.Some high performance PISs developed in the last year (see, e.g., the 2012 and 2013 references in [57][58][59][60][61][62][63][64][65][66][67][68][69][70][71][72][73][74][75][76]) should ensure a faster formation of tack-free coatings upon sunlight exposure under air: work is under progress.

Conclusions
This paper has reviewed some aspects concerned with the development of green technologies in the photopolymerization area.Interesting visible light irradiation sources (Xe lamps, diode lasers, LEDs, household lamps and, obviously, sun) today allow large possibilities of excitation from the near-UV to the near-infra-red.The development of very efficient PISs sensitive in the blue-to-red wavelength range for radical and cationic polymerization reactions undoubtedly opens new opportunities of polymerization reactions.Working in the absence of UV lights under air is on the right path today.Harmful Hg lamps can be avoided.Applications where (i) low light intensities are available (e.g., with sunlight) or required or (ii) quite low viscosity monomers (particular acrylates or cationic monomers) or thin films have to be employed become possible.Using sunlight, which has been a dream for a long time, might be within reach.The photopolymerization of renewable monomers is quite feasible.However, such monomers have to be designed as a function of the applications and the desired final material properties.
Many new additional works have to likely be proposed, for example, in the radiation curing area.It seems difficult today to find renewable acrylates exhibiting a performance close to that of the usual synthetic di-and tri-functional monomer/oligomers.The situation is different with the renewable epoxides, i.e., the compared performance of LDO and artificial epoxides are close in terms of polymerization rates and conversions, and the fabrication of glass fiber-reinforced composites with epoxidized vegetable oils has already been reported.Important questions may appear, e.g., about the physical/mechanical/surface, etc., properties of the cured material when starting from a conventional synthetic monomer or a modified natural raw compound.All the work described here was conducted in organic media: the use of water-borne formulations is noticeably less developed and the investigation of the photopolymerization of water-reducible, as well as water-based dispersions upon visible light exposure under air might also deserve to be carried out.
In the different topics discussed throughout this paper, much has been done, but much still remains to be done.The efforts deployed during the last thirty years to develop green aspects of the photopolymerization area begin to change, however, what was a challenge into a reality.

3 .
Development of PISs for Soft Irradiation Conditions
a in absence of silane; b in presence of silane; *: no tack-free coating after 1 h.