Jean-Pierre Fouassier

Photopolymerization reactions under visible lights: principle, mechanisms and examples of applications

A general overview of visible light photoinduced polymerization reactions is presented. Reaction mechanisms as well as practical efficiency in industrial applications are discussed. Several points are investigated in detail: photochemical reactivity of photoinitiating system (PIS), short overview of available photoinitiators (PIs) and photosensitizers (PSs), mechanisms involved in selected examples of dye sensitized polymerization reactions, examples of applications in pigmented coatings usable as paints, textile printing, glass reinforced fibers, sunlight curing of waterborne latex paints, curing of inks, laser-induced polymerization reactions, high speed photopolymers for laser imaging, PISs for computer-to-plate systems.

Camphorquinone -amines photoinitating systems for the initiation of free radical polymerization

Results of the camphorquinone/hindered piperidines, visible-light photoinduced polymerization of triethyleneglycol dimethacrylate are presented. The effectiveness of piperidines as a coinitiator is compared with a few aliphatic amines and aromatic amines. The main objective in this research was to study the mechanism of photoinitiation of polymerization. Reactive radicals that initiate the polymerization are formed by a mechanism of hydrogen atom abstraction by the triplet state of camphorquinone, mediated by photoinduced electron transfer. The different efficiencies of the aliphatic amines and of the aromatic amines affecting photopolymerization are explained on the basis of the different quenching reactivities of the excited states of camphorquinone.

Photopolymerization upon LEDs: new photoinitiating systems and strategies

This paper is focused on: (i) the recent developments in LED technology, allowing the design of novel and efficient light sources for the free radical and cationic photopolymerization of various monomers, the synthesis of interpenetrating polymer networks (IPNs) or thiol–ene photopolymerization; (ii) the existing and emerging applications in these areas; and (iii) the development of novel photoinitiators and photoinitiating systems specifically adapted for LED excitation.

Visible light induced photopolymerization : Speeding up the rate of polymerization by using co-initiators in dye/amine photoinitiating systems

The activity of six newly designed three-component systems (containing a dye, an amine and a triazine derivative) for the initiation of the photopolymerization of multifunctional acrylates under visible light has been evaluated. The selection of the dyes was based on thermodynamic considerations. A discussion of the photochemical reactivity of these systems reveals the role played by thermodynamics and outlines different aspects concerned with kinetics.

Synthesis, reactivity, and properties of new diaryliodonium salts as photoinitiators for the cationic polymerization of epoxy silicones

Diaryliodonium tetrakis (pentafluorophenyl) borate salts generate a higher reactivity than any other known diaryliodonium salt. The photochemical properties of diaryliodonium tetrakis (pentafluorophenyl) borate salts were compared to those of the diaryliodonium hexafluoroantimonate salt. The results show that these new salts are the most reactive photoinitiators in this family. In addition, diaryliodonium tetrakis (pentafluorophenyl) borate salts are soluble in low polarity media, such as epoxy silicone oils, which are rich in epoxy groups and insensitive to humidity. These salts have the advantage not to contain a heavy metal (such as antimony). The new properties generated by the use of the tetrakis (pentafluoropheyl) borate anion make the future of the cationic photopolymerization promising.

Triplet-triplet absorption of 2-(2′-hydroxyphenyl) benzoxazole (HBO) in polar solvents

Abstract The transient TT absorption spectra of 2-(2′-hydroxyphenyl) benzoxazole (HBO) have been measured in liquid polar solvents and in polymethylthacrylate. In non-protonic polar solvents the main TT absorption is from the keto form of the compound ( 3 K ∗ ). With a triplet sensitizer the compound just gives a TT absorption of its enol from ( 3 E ∗ ) after triplet energy transfer from the sensitizer to HBO. The excited state intramolecular proton transfer (ESIPT) from the enol form to keto form occurs just in the singlet state. The protonic solvent can break the intramolecular hydrogen bond of HBO and decrease the ESIPT process. In solid solution both 3 E ∗ and 3 K ∗ can be formed and the TT absorptions were detected because of the inhibition of the rotational conformation transfer.

Spin—orbit-coupling-induced triplet formation of triphenylpyrylium ion: a flash photolysis study

Abstract The excited singlet of the 2,4,6-triphenylpyrylium ion (TPP + ) reacts with iodobenzene (PhI) by electron transfer leading to radicals and the triplet state of TPP + . The induced formation of 3 TPP + by spin—orbit coupling has been observed. An encounter complex intermediate is proposed for the quenching of 3 TPP +* by PhI. Homogeneous recombination of radicals is also observed in the millisecond time domain. Flash photolysis has been used to study the excited state reactions of TPP + .

Time-resolved laser-pumped dye laser spectroscopy: energy and electron transfer in ketones

Abstract The synergic effects which are generally invoked to account for the specific features of a system of two ketones used as polymerization photoinitiators are reconsidered. The increase in reactivity observed when mixing these two initiators is reinterpreted in terms of a simultaneous energy and electron transfer in the pair. The relative efficiencies of these processes depend on the energy gap between the triplet states involved, which is known to be influenced by the polarity of the medium.

Two and three component photoinitiating systems based on coumarin derivatives

This study shows that several coumarin or ketocoumarin/additives combinations (bisimidazole derivative, mercaptobenzoxazole, titanocene, oxime ester) are able to initiate quite efficiently a radical polymerization reaction. The interactions between the excited states of coumarins or a ketocoumarin and various additives have also been studied by laser absorption spectroscopy, time resolved fluorescence and photoconductivity; the redox potentials of these different compounds have been measured and allowed the calculation of free enthalpy variations for a possible electron transfer reaction. The whole results explain the interaction mechanism and show that the coumarins are able to form radicals through an electron transfer reaction with the different additives whereas the ketocoumarin leads to an energy transfer with bisimidazole and to an hydrogen abstraction with the benzoxazole derivative.

Excited-State Properties of Camphorquinone Based Monomeric and Polymeric Photoinitiators

The excited-state properties of a new polymeric photoinitiator-bearing camphorquinone or/and amine moieties were studied and compared to the behaviour of the precursor molecules. The triplet state of the polymeric system was extremely short-lived, due to the close vicinity of the amino group. In addition, the singlet state also reacted with the amino group. A study on camphorquinone and methyldiethanolamine (=2,2′-(methylimino)bis[ethanol]) revealed that both these pathways led to the formation of a ketyl radical and an amine-derived radical. Therefore, high efficiency of the radical generation was expected. However, the radical photopolymerization of a polyfunctional mixture of acrylic monomers with various combinations of monomeric and polymeric photoinitiators gave evidence that the polymeric structure of the photoinitiating system may differently affect the overall cure rate of the formulation.