Xavier Allonas

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.

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.

Dyes as Photoinitiators or Photosensitizers of Polymerization Reactions

A short but up-to-date review on the role of dyes in the photoinitiation processes of polymerization reactions is presented. Radical and cationic reactions are largely encountered in the radiation curing and the imaging areas for use in traditional coating applications as well as in high tech sectors such as nanofabrication, computer-to-plate processing, laser direct imaging, manufacture of optical elements, etc. Recent promising developments concerned with the introduction of the silyl radical chemistry that enhances the polymerization efficiency are also discussed.

Radical polymerization activity and mechanistic approach in a new three‐component photoinitiating system

A new three-component photoinitiating system (based on isopropylthioxanthone ITX, amine AH, and a bifunctional benzophenone–ketosulfone BP-SK photoinitiator) for acrylate polymerization reactions was investigated through steady-state photolysis (photodegradation, redox potentials, and acidity release determinations) and time-resolved laser spectroscopy. The photopolymerization activity has been checked. It is shown that addition of ITX to BP-SK/AH clearly enhances the efficiency of the photopolymerization of clear or pigmented coatings. This is explained (although, a direct interaction between the triplet state of ITX and BP-SK occurs to some extents) on the basis of the interaction of BP-SK with the ketyl radical of ITX.

Investigation of the triplet quantum yield of thioxanthone by time-resolved thermal lens spectroscopy: solvent and population lens effects

Abstract The triplet quantum yield Φ T of thioxanthone (TX) was measured in different solvents by means of time-resolved thermal lens spectroscopy. It was observed that the experimental signal depends on the probe light wavelength. This effect which was mainly ascribed to the population lens (PL) arises from the change in solution composition after excitation, and adds its contribution to the thermal lens signal. From the transient absorption spectrum of the triplet state of TX, the change in refractive index associated with this PL was calculated as a function of the wavelength. Therefore, probing the photolens at an appropriate wavelength allowed accurate measurement of Φ T . It was found that besides departing seriously from unity, Φ T decreases from 0.85 in hexane to 0.56 in methanol.

Investigation of cleavage processes in photoinitiators: from experiments to molecular modeling

Abstract The present paper shows an original approach combining laser induced photoacoustic calorimetry (PAC) and molecular modeling for the investigation of the cleavage process in radical photoinitiating systems. The bond dissociation enthalpies (BDE) and the enthalpies for the photodissociation reaction ΔHr diss of the photoinitiators are calculated and the quantum yields of dissociation φ diss from the excited state are determined. It appears that these quantum yields are not unity and do not correlate either with BDEs or ΔHr diss .

Investigation of the photochemical properties of an important class of photobase generators: the O-acyloximes

Abstract The influence of the molecular structure and the required conditions for bond breaking in the triplet state of 10 carefully selected O -acyloximes (eight of them are original) usable as photobase generators are deeply investigated for the first time. Ground state absorption and fluorescence spectra revealed that the transitions involved exhibit a ππ ∗ character. A clear nonvertical character in triplet–triplet energy transfer was found in compounds exhibiting a flexible arylCNO moiety. An important and new point is concerned with this effect which strongly affects the triplet sensitized decomposition process, due to the possible formation of a highly vibrationally excited triplet state from which dissociation can occur. Laser experiments enabled to detect the primary free radicals formed from both direct and sensitized dissociation and to measure the relative quantum yields in iminyl radicals. Comparison of these laser experiments with steady state quantum yields of photolysis demonstrates the involvement of in-cage reactions that influence the overall efficiency of base generation, depending on the acyl moiety. All these facts were rationalized in a global mechanism of photodissociation and found to explain well the results of photoinitiated thermal crosslinking experiments performed in a model media with the starting oxime esters.

Photochemistry of Naphthalimide Photoacid Generators

The photophysical properties of a series of 1,8-naphthalimide photoacid generators were studied by steady state fluorescence and phosphorescence spectroscopy. Emission and excitation anisotropies, triplet quantum yields in polar and nonpolar solvent and photoacid generation were evaluated. The singlet excited state exhibits a low polarity and is strongly deactivated by an efficient intersystem crossing process. In protic solvent, a homolytic singlet cleavage of the N-O bond occurs and leads to the acid production. The existence of a triplet state close to the singlet state was clearly evidenced. The presence of close singlet excited states is supported by fluorescence anisotropy and picosecond laser spectroscopy experiments. Results of DFT calculations well confirm the experimental contentions and yield important information about the cleavage process involved in such compounds.