Photoinitiators (PIs) and photoinitiating systems (PISs) that contain a PI and additive(s) are encountered in polymer synthesis where they initiate polymerization or/and crosslinking reactions under exposure to a light source. PIs and PISs play a key role in the starting point of a polymerization reaction under exposure to UV or visible light. They have to be well designed and adapted to the experimental conditions used in a given application.
A PI is excited by light and leads to various initiating species (Scheme 1) as a function of its chemical structure through primary (when alone) or subsequent reactions involving one or more additional compounds (when incorporated into a PIS). Thus, various types of reaction can be observed: free-radical polymerization, cationic polymerization, free-radical-promoted cationic polymerization, concomitant cationic/radical polymerization (hybrid cure), thiol-ene polymerization, acid- or base-catalyzed polymerization, or crosslinking reactions of monomers, prepolymers or polymers.
First, huge sectors of applications include the traditional UV curing area. UV curing is a green technology that is environmentally friendly, with nearly no release of volatile organic compounds (VOC), room temperature operations and possible use of renewable materials, which is successful, for example, in the industries of varnishes, paints, coatings, adhesives, composites or graphic arts. Second, they are currently encountered in high-tech areas where micro- and nano-patterns, architectures or objects can be created, for example, in (laser) imaging, microlithography, stereolithography, 3D printing, microelectronics, optics, and holography. Third, they are rapidly developing in the medical area (e.g. dental materials, tissue engineering, surgical sealants or drug release, bioprinting) thanks to the elaboration of suitable chemicals, media, products, materials or processes.
The progress is largely driven by the development of (i) new monomers and oligomers with enhanced end-user properties (mechanical properties, adhesion, flexibility, etc.), (ii) novel light sources, such as laser diodes or light-emitting diodes (LEDs), that avoid the hazardous Hg lamps and (iii) new PIs and PISs combining, for example, near-UV or visible (and even NIR) light absorptions, high molar extinction coefficients and a high (photo)chemical reactivity. Irradiation under soft conditions becomes feasible (even under sunlight for fully optimized systems).
As far as photochemistry is concerned, it has clearly evolved strongly. The first generation of industrially used PIs and PISs disclosed in the 1970s, and the structurally related compounds proposed later on still operate efficiently today in numerous industrial processes where the development cost of totally novel compounds is a serious brake. On the other hand, in emerging applications and sectors where the cost is not a decisive parameter, novel ideas and concepts as well as novel end-user demands require the design of PIs and PISs with the properties of five-legged sheep. In all sectors, however, the man ingenuity that is the heart and engine of the authentic development of leading-edge research continuously leads to a lot of work for the synthesis of high-performance systems that can operate at any wavelength, under low light intensity and under air, ensure better safety and provide novel handling or end-use properties.
More than 40 years of research allows today the tailor-made photochemistry and chemistry of photosensitive formulations. Originally based on the synthesis of compounds through a trial and error strategy or/and the screening of available products, this research has been accompanied by a more and more important insight into the involved chemical mechanisms in solvent media through steady-state photolysis experiments, high-resolution electrospray ionization-mass spectrometry, analysis of the polymer end-groups, (time-resolved) ESR and NMR techniques, photothermal methods, and time-resolved absorption and fluorescence spectroscopies on the nano-, pico- and even femto-second scales. The study of the behaviour of excited states and transient species in bulk is also possible. The availability and introduction of molecular orbital (MO) calculation techniques leads today to remarkable theoretical investigations of, for example, the light absorption properties and the reactivity, which in turn has a recent and strong influence on the synthesis of original PIs. The increased incursion into other domains, such as photoredox catalysis, natural products as PIs, compounds encountered in organic electronics, and systems exhibiting non-linear properties, photocontrolled polymerization reactions have recently paved the way to the proposal of novel PIs and PISs.
The finding of novel structures and their use in novel applications both form part of the development of the PI/PIS area. Fantastic progress is currently underway, as supported by the flow of papers in so many different scientific and technological fields where a photoinduced polymerization/crosslinking reaction is the basic step. This PI/PIS area is seeing continuously increasing interest from both an academic and a practical point of view. A convincing argument can be found in a comparison of what was known and gathered in books in 1992,1 1995,2 1999,3–5 2002,6 2010,7 and 2012.8 In the past 5 years, the breathless pace of papers has not stopped. On the contrary, the pace is accelerating. Plenty of papers published in journals or as book chapters outline many innovative developments. This is the reason for the present book, which is focused on a series of relatively short chapters dealing with examples of the latest developments in the field of PIs and PISs in the years beyond 2011 as a recently published authored book8 extensively covered the history of PIs and PIS until 2010/2011. These chapters must be seen as examples of research directions of growing importance, which outline the dominant trends of the current research, the novel or potential applications, and the challenges that have to be overcome to gain successful results in the future.
Therefore, some general chapters will briefly review the progress realized in recent years in UV radical photoinitiators, long-wavelength-sensitive radical photoinitiators, cationic photoinitiators, macromolecular photoinitiators, and unusual photoinitiators. As discussed here, the better understanding of the involved mechanisms and the use of MO calculations allow the design of tailor-made compounds. In this direction, special emphasis is given to recent developments concerned with photoinitiators for blue to red LED exposures, photoredox catalysts as photoinitiators, NIR photoinitiating systems for thick materials, and D–π–A type photoinitiators for radical and cationic photopolymerizations under near-UV and visible LEDs. Light-controlled polymerizations (photoiniferters, NMP, ATRP, RAFT) have received considerable attention thanks to novel ideas for PIs and PISs. The possibilities of visible-light-induced click chemistries are reviewed. Several chapters where the challenges and progress are discussed illustrate the role of well-adapted PIs and PISs for specific uses or applications: photoinitiators for the manufacture of nanoparticle-containing matrices, photoinitiators in ionic liquids, photoinitiators in dentistry, water-soluble photoinitiators for use in water-borne or hydrophilic media, photoinitiators for photopolymers involved in newly proposed solar cells or for photopolymerizations in self-assembled systems and emulsions.
We feel that this book will be an interesting contribution to the state-of-the-art in the area of photosensitive systems encountered today in polymerization reactions and a useful presentation of trends of development in ever-growing or newly emerging applications.
Jacques Lalevée and Jean Pierre Fouassier