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Trends in Dye Photosensitized Radical Polymerization Reactions

In this chapter, visible light-induced radical polymerization reactions in the 380-800 nm range are reviewed. The role of the absorbing species (dye) and the complete multicomponent photoinitiating systems (PISs) (dye and additives) are then emphasized. The original works on the dye-based PISs that have been proposed over the years are also outlined. However, this chapter is mainly focused on the latest developments, in the 2010-2014 period, and the actual trends of research, in particular the novel perspectives of applications under soft irradiation conditions.

1.1. Introduction

Dye photosensitized polymerization (DPP) (see [FOU 12a]) is a common expression often employed [OST 54, EAT 86, MON 93] to refer to a series of reactions where a dye1 (i) is excited by light [1.1a]2, (ii) leads [1.1b], alone or through primary or subsequent reactions involving one or more additional compounds, to radicals, cations or cation radicals, and (iii) initiates a free radical polymerization (FRP) [1.1c], a cationic polymerization (CP) [1.1d], a free radical promoted cationic photopolymerization (FRPCP) [1.1e], a concomitant cationic/radical polymerization (hybrid cure) (CCRP) [1.1f], a thiol-ene photopolymerization (TEP) [1.1g] or cross-linking reactions of prepolymers or polymers (this last point will not be covered here).

Therefore, by analogy with the term "initiator" in thermal polymerizations, the dye is also usually presented as a photoinitiator (PI), i.e. a substance that absorbs light and participates in the photoinitiation of a polymerization reaction. In the sense of photochemistry [TUR 90], however, it can also play the true role of a photosensitizer (PS) in some specific reactions3. Sometimes, there is an apparent ambiguity with the words "photoinitiator" and "photosensitizer" in papers dealing with photopolymerization. For the sake of clarity and convenience, we consider here that dye-based PISs can be classified into one-component PIS (I_Dye, [1.1a, 1.2]), two-component PIS (II_Dye, i.e. a dye and one additive ([1.1a], [1.3]-[1.5])), three-component PIS (III_Dye, i.e. a dye and two additives), etc. The additives can be electron donors (EDs) [1.3], electron acceptors (EAs) [1.4], hydrogen donors (HDs) [1.5] or electron/proton donors (EPDs) [1.6].

DPP reactions have been largely applied in various traditional and recently high-tech areas, such as radiation curing, imaging, graphic arts, optics, dentistry, medicine and nanomaterials (see [FOU 12a] and other relevant books [LAS 90, BOT 91, PAP 92, FOU 93b, KRO 94, REI 89, FOU 95a, SCR 97, KOL 97, DAV 99, NEC 99, FOU 99, CRI 99, FOU 01, DIE 02, BEL 03, FOU 06, SCH 07a, SCH 07b, LAC 08, MIS 09, ALL 10, FOU 10a, GRE 10, MAC 07]). For the past 50 years [OST 54], the large choice of available dyes and additives, and the possibility to tune the absorption in a given PIS by only changing the dye, has led to numerous research works. Recent developments have progressively allowed the use of increasingly less intense visible light sources. An important topic is now concerned with the adaptation of DPPs to irradiations with newly developed laser diodes and light emitting diodes (LEDs) for specific applications.

In many sectors, radical photopolymerization reactions are used more often than cationic photopolymerizations [FOU 12a, CRI 99, BEL 03, GRE 10]. Two historical facts can explain this difference. First, in industrial applications, the benefits versus the drawbacks are very often in favor of the radical processes. Second, many radical PISs have been successfully developed since the beginning of the 1960s. In contrast, industrial cationic PIs are only quasi-based on iodonium and sulfonium salts [CRI 79] (still largely used today) that are sensitive in the ultraviolet (UV). Later on, dialkylphenacylsulfonium, N-alkoxypyridinium, thianthrenium and ferrocenium salts, that absorb in the near UV/visible or visible range, have also been proposed [FOU 12a, CRI 99, BEL 03, KAH 10]. The photosensitization of onium salts in the visible range remains rather difficult (although successful results have been reached along the past years [FOU 12a, KAH 10]). Recent, promising developments using FRPCP should likely overcome this issue (see [FOU 12a, KAH 10] and below).

Thousands of papers have been already published in the field of DPPs. Before writing a novel review chapter, one question we can ask is: how can we present the most up-to-date situation? The first possibility, as is very often done, consists of providing a broad overview of the available systems which results in a qualitative list of PI, PS and compound/additive combinations. For obvious reasons, it is impossible to discuss their relative efficiency as the experimental conditions are completely different from one paper to another (film vs. solution, high-viscosity media vs. low-viscosity media, in laminate vs. under air, high light intensity vs. low light intensity, short exposure time vs. long exposure time, etc.). It could even happen that a proposed system (presented as a new system) is in the end not interesting from a practical point of view. The main interest of such reviews undoubtedly remains the gathering of the available systems. The second possibility, as sometimes encountered, consists of presenting the systems that are effectively used in a given field of applications. This represents a real interest as only the efficient systems are reported in that case. However, it requires us to have a review paper for each domain of applications. The third possibility, that is rather rare, consists of outlining the key points and the breakthroughs in the design of ever more reactive and efficient compounds, discussing the relative interest of the different systems toward the different uses, comparing the performances when possible and stressing the actual trends of development at a given time.

A recent book provides a detailed analysis of the encountered systems up to 2010/2011 (large overview, mechanism, reactivity and efficiency) [FOU 12a]; other previous books on the subject include [LAS 90, BOT 91, PAP 92, FOU 93b, KRO 94, REI 89, FOU 95a, SCR 97, KOL 97, DAV 99, NEC 99, FOU 99, CRI 99, FOU 01, DIE 02, BEL 03, FOU 06, SCH 07a, SCH 07b, LAC 08, MIS 09, ALL 10, FOU 10a, GRE 10] and [MAC 07]. Many review papers typically focusing on dyes as PIs or PSs [EAT 86, FOU 90, SCH 90b, URA 03, FOU 03, FOU 93a, FOU 95b, FOU 95c, FOU 00, TIM 93, CUN 93, GRE 93, CRI 93, FOU 11, FOU 10b, YAG 10, FOU 07, FOU 12b, FOU 12c, LAL 11b, XIA 15, LAL 15, SHA 14] or partly dealing with photoinitiation under visible lights [PAC 01, LAL 09b, LAL 09, LAL 10, LAL 12, IVA 10, MUF 10, LAL 14b, BON 14, SAN 14] have also been already published. As a result, we chose here to (i) provide a very brief overview of the PISs developed over the last 50 years, (ii) focus on the most recent literature and (iii) illustrate the today's trends of development. Examples of these trends will concern the search of novel dyes for (i) polychromatic light excitations, (ii) blue, green and red laser light-induced polymerizations, (iii) photoinitiation under soft irradiation conditions, (iv) sunlight exposure, (v) enhanced absorption properties (red-shifted spectra and high molar extinction coefficients), (vi) the use as photoinitiator catalysts (PICs), (vii) dual radical/cationic PISs, (viii) performances attained under specific LED and laser diode exposures, (ix) concomitant radical/cationic photopolymerizations and the elaboration of interpenetrated polymer networks (IPNs), (x) TEPs, (xi) photopolymerizable panchromatic films or (xii) the manufacture of in situ nanoparticle (NP) containing films.

Our review here is limited to systems operating in dye photosensitized radical polymerizations (DPRPs) (see Figure 1.1). Their behavior in FRPCP reactions will be only evoked as all CPs are specifically covered in another covered in Chapter 2 of this book. In the same way, the role of dyes as part of PISs in the medical area, controlled radical photopolymerization reactions, printing technologies, stereolithography, optics or dyes under two-photon excitation are also discussed in detail in other chapters of this book.

Figure 1.1. Chemical mechanisms for dye photosensitized radical polymerizations

1.2. A brief overview of dye-based PISs

As stated in the previous section, various presentations of DPP reactions are available in review papers [EAT 86, FOU 90, SCH 90b, URA 03, FOU 03, FOU 93a, FOU 95b, FOU 95c, FOU 00, TIM 93, CUN 93, GRE 93, CRI 93, FOU 11, FOU 10b, YAG 10, FOU 07, FOU 12b, FOU 12c, LAL 11b, XIA 15, PAC 01, LAL 09b, LAL 09, LAL 10, LAL 12, IVA 10, MUF 10, LAL 14b], in general books [LAS 90, BOT 91, PAP 92, FOU 93b, KRO 94, REI 89, FOU 95a, SCR 97, KOL 97, DAV 99, NEC 99, FOU 99, CRI 99, FOU 01, DIE 02, BEL 03, FOU 06, SCH 07a, SCH 07b, LAC 08, MIS 09, ALL 10, FOU 10a GRE 10, MAC 07] and in a recent specialized monograph [FOU 12a]. In this section, a short...