Céline Dietlin

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.

Depth characterization of photopolymerized films by confocal Raman microscopy using an immersion objective.

The depth characterization of photopolymer films by confocal Raman microscopy is often troublesome due to refraction effects. To minimize these effects, we used an oil immersion objective and a method was developed to avoid penetration of the oil without damaging the sample surface. Since the surface may be sticky if oxygen in the air inhibits the photopolymerization, a protective layer could not be put onto the film. Therefore, the method consists in using a thin polypropylene foil as substrate for the coating and placing the sample upside down under the objective. In this manner, the immersion oil could be deposited on top of the polypropylene. The advantage of this setup is that the oil, polypropylene substrate, and photopolymer film have close refractive indices. Basic calculations showed that the depth resolution is hardly affected in that configuration and double-bond conversion profiles could be plotted as a function of reliable nominal depth. The validity of the methodology was confirmed by experiments carried out with a dry metallurgical objective on the sample surface, face up, where refraction effects are still minor. In addition, infrared spectroscopy, which was used to follow the photopolymerization, corroborated the Raman conversion of the films over their thickness. The confocal Raman microscopy method can be applied to various photopolymerized systems to characterize their behavior towards oxygen inhibition and other heterogeneities in conversion arising from inner filter effects or interactions between additives for instance.

Macroporous organosilica films via a template-free photoinduced sol–gel process

In the present work, we have designed a simple and original procedure for the spontaneous formation of macroporous organosilica films by photopolymerization-induced phase separation, avoiding the intervention of a template. The simple mixture of 3-(glycidyloxypropyl)trimethoxysilane (CH3O)3Si(CH2)3OCH2-CHCH2O (GPTMS), and poly(diethoxysiloxane) (C2H5O)2SiO)n (PDEOS, n ≈ 5), without solvent and water followed by UV illumination in the presence of a diaryl iodonium salt (photoacid generator) afforded macroporous hybrid films. This method relies on a single-step photoinduced sol–gel process with photogenerated Bronsted acids, resulting from the iodonium salt photolysis, which catalyse the hydrolysis–condensation reactions of both precursors. The formation of macropores was found to be strongly dependent on the concentration of PDEOS. It has been indeed proven that an increase in the fraction of PDEOS yielded macropores of increased diameters from 100 nm to 50 μm. In this study, our efforts have been focused on two essential aspects. On the one hand, the characterization of the macroporous films with regard to morphology, organic–inorganic microstructure and optical properties was commented upon thoroughly; on the other hand a general mechanism leading to the development of macropores was proposed and discussed.

Novel naphthalimide–amine based photoinitiators operating under violet and blue LEDs and usable for various polymerization reactions and synthesis of hydrogels

A series of naphthalimide derivatives containing tertiary amine groups (DNNDs) have been prepared. Some of these derivatives (e.g. DNND3, DNND4 and DNND5) exhibit interesting shifted absorption so that they can be utilized as versatile photoinitiators upon exposure to various violet and blue LEDs (385, 405, 455 and 470 nm). They are particularly efficient for the cationic photopolymerization of an epoxide and the free radical photopolymerization of an acrylate. The thiol–ene photopolymerization, as well as the synthesis of interpenetrating polymer networks (of epoxide/acrylate blend) IPNs, is feasible. Remarkably, the production of hydrogels can also be easily achieved using a DNND derivative after inclusion in a cyclodextrin cavity. The photochemical mechanisms have been comprehensively investigated by steady state photolysis, Electron Spin Resonance (ESR), fluorescence, electrochemistry and laser flash photolysis and discussed in detail.

New copper(I) complex based initiating systems in redox polymerization and comparison with the amine/benzoyl peroxide reference

Novel copper complex based initiating systems for redox free radical polymerization (FRP) of methacrylate resins under mild conditions are proposed. Remarkably, the FRP performance of the Cu(I)/ascorbic acid (Vitamin C – VitC)/benzoyl peroxide (BPO) redox system is noticeably more significant than that of the amine/BPO reference especially at the top surface where the oxygen inhibition is particularly deleterious. The Cu(I)/BPO interaction leads to a decomposition of the peroxide and generates a benzoyl radical and Cu(II) as a byproduct. The ascorbic acid acts as a reducing agent and allows the regeneration of Cu(I) from Cu(II). Hydroperoxides formed at the surface through the radical/O2 interaction are also decomposed by the new proposed Cu complex leading to additional polymerization initiating species. The structure/reactivity relationship of the new proposed Cu(I) complexes in the Cu(I)/VitC/BPO systems is studied. The use of Cu(I)/VitC/BPO/iodonium salt systems in redox photoactivated FRP is also explored.

Mechanosynthesized copper(I) complex based initiating systems for redox polymerization: towards upgraded oxidizing and reducing agents

Significant improvements of the recently proposed Cu(I)/vitamin C + water/dibenzoyl peroxide (BPO) system for the redox free radical polymerization of methacrylates in air are presented here, the goal being to avoid the presence of water and to eliminate the unstable BPO compound. Additionally, the use of mechanosynthesized Cu(I) complexes bearing cheap triphenylphosphine and methylpyridine ligands, allowed us to reduce the economical and synthesis costs significantly. Water-free reducing agents such as 6-O-palmitoyl-L-ascorbic acid, organotin or hydrazine derivatives are evaluated. Cumene hydroperoxide and tert-butyl perbenzoate are proposed as a replacement for less stable BPO. Most of these novel combinations are more efficient in air than the usual amine/benzoylperoxide system (4-N,N-TMA/BPO) reference especially at the sample surface. Among them, Cu(I)/organotin/cumene hydroperoxide appears as an efficient and competitive alternative to the conventional amine/BPO couple.

Photopolymerization processes of thick films and in shadow areas: a review for the access to composites

The photopolymerization processes are currently associated with thin samples for which the light penetration is good enough to activate the photoinitiator or the photoinitiating system for the entre samples thickness. The photopolymerization of very thick films and in shadow areas where the light penetration is inhibited (e.g. in filled, pigmented, and dispersed samples) remains a huge challenge (e.g. for the access to composites). In the present paper, an overview of the different strategies for the photopolymerization of thick samples is reported. First, strategies based on the optimization of the photonic (light intensity, excitation wavelength, etc.) or chemical (efficiency/reactivity/bleaching of the photoinitiating systems, etc.) parameters are presented that result in a full temporal and spatial control. Then, the main strategies based on propagation/diffusion mechanisms of latent species for the curing beyond the irradiated areas are given (partial loss of spatial resolution and access to shadow areas). Also, dual systems (thermal/photochemical or photochemical/redox) are described. The state of the art for the access to thick samples by photopolymerization processes as well as some perspectives are provided.

A novel naphthalimide scaffold based iodonium salt as a one-component photoacid/photoinitiator for cationic and radical polymerization under LED exposure

A strong drawback of the photoinitiators of cationic polymerization or photoacids is the photosensitivity for short and energetic wavelengths preventing their general use (specialized photochemical equipment with safety concerns must be used). In the present paper, a novel iodonium salt bearing a naphthalimide moiety (naphthalimide-Ph-I+-Ph) is proposed as a one-component photoinitiator/photoacid operating at longer and safer wavelengths (i.e. violet light emitting diodes at 365, 385 nm and 395 nm). It allows the polymerization of various formulations (methacrylates, epoxides, vinyl ethers). A high reactive function conversion for multifunctional monomers can be achieved: e.g. 50% for a diepoxide under air, >90% for a divinylether (with a very high rate of polymerization Rp), almost 100% for an epoxide/vinyl ether blend (very high Rp) under air, and 85% for methacrylates (high Rp) in laminate (43% under air). These results are above the ones obtained with a thianthrenium salt chosen as a reference e.g. a lower epoxy conversion ∼25% and a clearly lower Rp for the diepoxide polymerization. ESR-spin trapping, laser flash photolysis, steady state photolysis and molecular orbitals calculations support the formation of Ph˙ and naphthalimide-Ph-I˙+ as well as the generation of H+, thereby explaining the photoinitiation step mechanism.

Photoinduced self-assembly of carboxylic acid-terminated lamellar silsesquioxane: highly functional films for attaching and patterning amino-based ligands

Recently, long n-alkyltrimethoxysilanes (H3C(CH2)nSi(OCH3)3) have proven to self-assemble into mesoscopically ordered passive lamellar films through an efficient solvent-free photoacid-catalysed sol–gel process. By using an analogue precursor architecture presenting a terminal ester group (H3COC(O)(CH2)10Si(OCH3)3), both functional and tuneable nanostructured organosilica films were synthesized, while keeping all the processing advantages of light-induced self-assembly. The subsequent attachment of a fluorescent amino-based ligand (Safranin O) was performed using a two-step procedure. The ester end groups were first hydrolysed in reactive carboxylic acid using standard methods, and activated with an amino ligand to form amide bonds. Hydrolysis and ligand coupling were assessed through infrared and solid-state 1H NMR spectroscopy. Direct patterning of the fluorescent ligand-functionalized silsesquioxane film was performed by exposure to deep UV light under a mask to cause the local degradation of the dye. The resultant photopatterned film was detected using fluorescence microscopy. This UV method could represent an effective and general approach for attaching and patterning amino-based ligands, with less restriction on substrate and surface preparation than self-assembled monolayers.

Metal Acetylacetonate–Bidentate Ligand Interaction (MABLI) as highly efficient free radical generating systems for polymer synthesis

Metal Acetylacetonate–Bidentate Ligand Interaction (MABLI) is presented here as a new chemical mechanism for the highly efficient generation of free radicals for polymer synthesis. This MABLI process involves simultaneous ligand exchange and a change of the metal oxidation degree and is associated with the efficient release of free radicals. In conventional redox two-component radical generating systems, two criteria are required to be efficient: (1) oxidizing agents must exhibit a low bond dissociation energy (BDE) i.e. they are usually unstable (e.g. peroxides) and (2) a small difference must exist between the oxidation potential of the reducing agent and the reduction potential of the oxidation agent. In contrast, here, the criteria for efficient MABLI radical generation were energetic and geometric for both bidentate ligands and metal acetylacetonates. The strength of this approach is to use stable compounds in 2-components free radical initiating systems and to generate carbon centered radicals. Mechanistic investigations demonstrated the formation of new metal adducts by means of high-resolution mass spectroscopy (HR-ESI-MS) as well as UV-vis spectrometry. As a result of its high radical generating rate, the potential of MABLI was illustrated on the methacrylate free radical polymerization under mild conditions (room temperature, in air) and initiated with a small amount of metal acetylacetonate though it opens new perspectives for acac-like additions in organic chemistry.