Introduction

As already known, a photoinitiator (PI) or a photoinitiating system (PIS), where a PI is introduced, allows the initiation of a polymerization reaction under exposure to a light source.

Light-induced polymerization reactions are largely encountered in many industrial applications or in promising laboratory developments. The basic idea is to readily transform a liquid monomer (or a soft film) into a solid material (or a solid film) upon light exposure at ambient temperature. The huge sectors of applications are found in both traditional and high tech areas such as radiation curing, laser imaging, 3D printing, microelectronics, optics, biosciences, dentistry, nanotechnology, etc. Radiation curing is considered as a green technology that continues its rapid development.

Photopolymerization reactions can be carried out in various experimental conditions, such as in films, gas phase, aerosols, multilayers, (micro)heterogeneous media or solid state, on surface, in ionic liquids, in situ in the manufacturing of microfluidic devices, in vivo, even under magnetic field, etc. Very different aspects are concerned in gradient, template, frontal, controlled, sol-gel, two-photon, redox, laser-induced, or spatially controlled photopolymerizations, etc.

A photopolymerization reaction involves a PI or a PIS, a polymerizable medium, and a light source. The photoinitiator plays a crucial role as it absorbs the light and starts the reaction. Its reactivity governs the efficiency of the polymerization. The literature shows that a considerable number of works are devoted to the design of photosensitive systems being able to operate in many various (and sometimes exotic) experimental conditions.

Fantastic developments have occurred all along the past five decades. Significant achievements have been done since the early works on photopolymerization in the 1960s and the traditional developments of the radiation curing area. Today, high tech applications are continuously emerging. A tailor-made photochemistry and chemistry for the design of high-performance systems (that can operate at any wavelength, under low light intensity and under air, ensure a better safety, and provide novel handling or end use properties) have appeared in this area and has achieved remarkable success. The search for a safe and green technology is growing. Interesting items first relate to the polymer science and technology field but also to the photochemistry, physical chemistry, and organic chemistry areas.

In the past 40?years, many aspects of light-induced polymerization reactions have been discussed in books and review papers. Each of these books, however, usually cover more deeply selected aspects depending first on the origin (university and industry) and activity sector of the author/editor (photochemistry, polymer chemistry, applications, etc) and second on the goals of the book (general presentation of the technology, guide for end users, and academic scope, etc). Our previous general book published more than 20?years ago (Hanser, 1995) and devoted to the three photoinitiation/photopolymerization/photocuring complementary aspects already provided a first account on the photosensitive systems. Unfortunately, for obvious reasons, all these three fascinating aspects that continuously appeared in the literature could no longer be developed in detail in a single monograph because of the rapid growth of the research. This was the reason why our second book ( Photoinitiators for Polymer Synthesis) published in 2012 (Wiley) was only focused on the photosensitive systems that are used to initiate the photopolymerization reaction, their adaptation to the light sources, their excited state processes, their interaction with the different available monomers, their working out mechanisms, and the approach for a complete understanding of the (photo)chemical reactivity. This second book showed the huge progress made between 1995 and 2012.

Why a new book? Indeed, by the end of 2010, one could have had the feeling that almost everything had been foreseen and verified in the design of photoinitiators and PISs. However, science is ever going on! Thanks to novel methods of investigation (both experimental and theoretical) and novel ideas accompanied by searches in other areas (fine product catalogs, natural compounds, optoelectronics, organic light-emitting diodes (OLEDs), solar cells, composites: lightweight materials, etc), a huge progress has been done in the past 10?years! A lot of novel structures previously unimaginable and usable in novel and promising applications have been published and have opened new unsuspected horizons.

Huge challenges remain in the area of PIs and PISs, but a constant development is noted in (i) nontoxic (or less toxic) systems, e.g. biological and medical applications, food packaging, and for the safety of the end users (the toxicity of many classical PIs has been re-evaluated in the context of the REACH registration); (ii) high-performance systems for the access to larger objects in 3D printing, the high productivity for the curing of inks, coatings, paints, composites, the ability to photopolymerize pigmented or highly filled thick samples for composites with improved mechanical properties, and the working out in hydrogels, eco-friendly water-based formulations, or self-assembled systems; (iii) systems operating under safe irradiation devices and visible, red, and near-infrared (NIR) light sources; and (iv) systems for new photopolymerization processes, e.g. controlled polymerization, hybrid polymerization to reduce polymerization stress and shrinkage, dual cure polymerization combining redox and light activation for reactions in shadow areas, polymerization of biosourced monomers, two-photon photopolymerization, and emulsion photopolymerization upon visible lights.

The following picture schematically depicts 60?years of evolution of the research and development in this area. Originally, the screening of the available chemicals, the synthesis of suitable derivatives, and the large use of trial and error experiments allowed to satisfactorily photocure monomer/oligomer formulations according to the rather undemanding requested conditions at this period. Then, the investigation of the excited state processes and the proposal of mechanistic schemes helped to design more powerful systems for an improved performance. Finally, the molecular orbital (MO) calculations provided new solutions for the construction of efficient molecules and the interpretation of the chemical reactivity. Questions arising from end users or researchers for the development of novel applications lead to a demand of novel "five-legged" photoinitiators that, along time, are more and more sophisticated. Moreover, good fortune, close attention, or brilliant ideas, even without any heavy experimental or theoretical approach, have also played a key role in the discovery of novel structures somewhere on our planet.

The above considerations prompted us to write today a new book on photoinitiators in which the most recent developments have been stressed. Here, we intend (i) to give the best up-to-date situation of the subject and take time to briefly define a lot of basic principles and concepts, mechanistic reaction schemes, and examples of studies that remain true and are not submitted to a significant aging on a 10?year timescale, (ii) to keep a complete presentation of the encountered PISs together with a discussion of the structure/excited state processes/reactivity/efficiency relationships, (iii) to focus on the involved mechanisms (the role of the basic research toward the progress of the applied research being absolutely necessary), and (iv) to outline the latest developments and trends in the design of novel tailor-made photoinitiators/PISs (based on experimental and theoretical approaches) as well as the corresponding existing, emerging, promising, or challenging applications where current or new systems are employed. To our opinion, such an extensive and complete book within this philosophy remains totally original today.

The book is divided into four parts including 19 chapters. As mentioned before, we decided to provide a basic thorough description of the processes and mechanisms together with an in-depth treatment of the design of PIs and PISs and a general presentation of the current applications.

In Part I, we deliver a general but concise presentation of the basic principles of photopolymerization reactions with a description of the available light sources, the different monomers, the characteristics, kinetics, and monitoring of the reactions together with a few words on the application areas (Chapter 1). As it specially concerns the polymer science point of view, this chapter will only focus on what is necessary to clearly understand the following chapters. The characteristics, the role, the different basic processes, and the reactivity/efficiency of the PISs are described in Chapter 2 together with a few photochemistry checks.

Part II is devoted to the structures, excited state processes, reactivity, and efficiency of photoinitiators. In a first step, we provide (i) the most exhaustive presentation of the commercially or academically used or potentially interesting PISs developed in the literature so far (photoinitiators, co-initiators, and photosensitizers), (ii) the characteristics of the excited states, (iii) the involved reaction mechanisms, and...