Bernadette Graff

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.

Naphthalimide based methacrylated photoinitiators in radical and cationic photopolymerization under visible light

The abilities of two naphthalimide derivatives with a methacryloyl group to initiate, when incorporated in multi-component systems, a ring-opening polymerization of epoxides and a radical polymerization of acrylates under different irradiation sources (e.g. very soft halogen lamp irradiation, laser diode at 457 nm, laser diode at 405 nm and blue LED bulb at 462 nm) have been investigated. One of them is particularly efficient for the cationic and radical photopolymerization of an epoxide/acrylate blend in a one-step hybrid cure and leads to the formation of an interpenetrated polymer network IPN (30 s for getting tack free coatings). The migration stability of one of these naphthalimide derivatives is found to be excellent in the cured polyacrylates and IPNs. The photochemical mechanisms are studied by steady state photolysis, fluorescence, cyclic voltammetry, electron spin resonance spin trapping and laser flash photolysis techniques.

A dinuclear gold(I) complex as a novel photoredox catalyst for light-induced atom transfer radical polymerization

Controlled/living atom transfer radical polymerization of methacrylates and acrylates initiated by ethyl α-bromophenylacetate (EBPA) as the initiator in the presence of low loadings (1.25 mol% vs. initiator) of a dinuclear gold(I) complex based photocatalyst [Au2(μ-dppm)2]Cl2 has been accomplished in solution and in laminate under UVA and visible-light photoreductive conditions. In solution, the linear increase of molecular weights with methyl methacrylate (MMA) conversion and the low dispersity are consistent with a controlled/living process. In a film, trimethylolpropane triacrylate (TMPTA) was polymerized and the ethyl α-bromophenylacetate (EBPA)/[Au2(μ-dppm)2]Cl2 system resulted in a faster rate of polymerization compared to EBPA/Ir(ppy)3. Chain extensions of polymers were successfully conducted leading to block copolymers, which also confirms the living character of this new system. Photophysical experiments support a conventional photoredox-catalyzed ATRP mechanism. Finally, this approach utilizes a gold catalyst featuring better solubility and lower cost than the well-known Ir(ppy)3 complex.

Light‐Harvesting Organic Photoinitiators of Polymerization

Two new photoinitiators with unprecedented light absorption properties are proposed on the basis of a suitable truxene skeleton where several UV photoinitiators PI units such as benzophenone and thioxanthone are introduced at the periphery and whose molecular orbitals MO can be coupled with those of the PI units: a red-shifted absorption and a strong increase of the molecular extinction coefficients (by a ≈ 20-1000 fold factor) are found. These compounds are highly efficient light-harvesting photoinitiators. The scope and practicality of these photoinitiators of polymerization can be dramatically expanded, that is, both radical and cationic polymerization processes are accessible upon very soft irradiation conditions (halogen lamp, LED…︁) thanks to the unique light absorption properties of the new proposed structures.

Excited state processes in polymerization photoinitiators

The present paper is devoted to the investigation of excited state processes efficiency/reactivity reaationships of photoinitiators of polymerization. The reactivity of a substituted benzophenone depends upon the nature and position of the substituent. For 4-methoxy-benzophenone, the lifetime of the triplet state is 800 ns; replacement of methoxy by methylthio changes the lifetime to 5 μs. The rate constant for deactivation by methyl methacrylate (MMA) falls by a factor of 60; the rate of polymerization of a solution of MMA (7 mol 1−1) irradiated in the presence of these compounds increases by a factor of 1.5. The behavior of some hydroxyalkylphenylketones bearing an ether substituent at the para position of the phenyl ring is special: their triplet states are short lived (8.5–15 ns in deaerated solutions) but are longer lived than the parent unsubstituted compound (1.4 ns). Introduction of thio and dialkyl amino groups leads to an increase of the lifetimes (3–4 μs). These substituents change the reactivity of the triplet states in the presence of oxygen, monomer and amine but have a weak influence on the rate of polymerization of MMA in organic solvent initiated by light irradiation of these compounds. Irradiation of morpholino- and amino-ketones substituted by ether, thioalkyl and dialkylamino groups leads to the generation of triplet states with main characteristics (spectra, lifetimes, rate constants of interaction with O2, amines and monomers) strongly dependent on the substitution (e.g. the introduction of a para dimethylamino group on a morpholino ketone increases the triplet state lifetime to 2 μs, from 1 ns for the parent compound). Interaction between these compounds and thioxanthones as photosensitizers is indicated by using a dye laser operating at λ = 430 nm. Results on the behavior of these compounds in solution photopolymerization experiments are compared to those obtained through laser spectroscopy. Polymers obtained from photosensitive copolymerizable systems show outstanding reactivity and durability. Excited state properties of photoinitiators containing unsaturated ethylenic groups have been studied, with respect to triplet state lifetimes, rate constants of interaction with the environment (oxygen, amines, monomers). Measurements of rates of polymerization (Rp) in solution (at various monomer concentrations [M]) allowed comparison of the theoretical dependence of Rp on [M] with the experimental results.

Green light sensitive diketopyrrolopyrrole derivatives used in versatile photoinitiating systems for photopolymerizations

The monomeric (DPPDT) and polymeric (PDQT) diketopyrrolopyrrole–thiophene derivatives combined with an iodonium salt or an amine (and optionally an additive) are used as photoinitiating systems to initiate the cationic polymerization of epoxides or divinyl ethers, radical polymerization of acrylates and thiol–ene polymerization under different irradiation sources (i.e. very soft halogen lamp, laser diodes at 473 nm, 532 nm and 808 nm). The DPPDT based systems are very efficient upon the green light exposure. Remarkably, these systems are characterized by a higher efficiency than references based on camphorquinone, a well known photosensitizer for visible light. The photochemical mechanisms are studied by steady state photolysis, fluorescence, cyclic voltammetry, laser flash photolysis and electron spin resonance spin trapping techniques.

Chalcone derivatives as highly versatile photoinitiators for radical, cationic, thiol–ene and IPN polymerization reactions upon exposure to visible light

Chalcone derivatives are proposed as novel photoinitiators for polymerization upon exposure to visible light (even under soft irradiation conditions: blue LED bulb at 462 nm, halogen lamp, and sunlight). Remarkably, these structures are highly versatile allowing access to photoinitiating systems for (i) the free radical polymerization of acrylates, (ii) the cationic polymerization of epoxides and vinyl ethers, (iii) the synthesis of interpenetrated polymer networks (IPNs) and (iv) the thiol–ene processes. Excellent polymerization profiles are obtained. The initiation mechanisms are analyzed through ESR, steady state photolysis, cyclic voltammetry, laser flash photolysis and fluorescence experiments.

Push–pull (thio)barbituric acid derivatives in dye photosensitized radical and cationic polymerization reactions under 457/473 nm laser beams or blue LEDs

A series of new dyes based on (thio)barbituric acid derivatives were synthesized and used as photoinitiators of polymerization upon very soft irradiation (laser diodes at 457 nm and 473 nm; blue LED bulbs or even a halogen lamp). These dyes can work according to an oxidative cycle (dye/diphenyl iodonium salt/N-vinylcarbazole system) or a reductive cycle (dye/amine/alkyl halide system) and partly behave as organic photocatalysts. Ring-opening polymerization (ROP) of epoxides and radical polymerization (RP) of acrylates are easily promoted. Excellent RP and/or ROP polymerization profiles are obtained. The contrasting behavior of the thiobarbituric vs. barbituric acid derivative is discussed. The initiation mechanisms, analyzed by Electron Spin Resonance (ESR) and steady state photolysis experiments, are detailed.

Mechanistic and Preparative Studies of Radical Chain Homolytic Substitution Reactions of N-Heterocyclic Carbene Boranes and Disulfides

Reactions of 1,3-dimethylimidazol-2-ylidene-borane (diMe-Imd-BH3) and related NHC-boranes with diaryl and diheteroaryl disulfides provide diverse NHC-boryl monosulfides (diMe-Imd-BH2SAr) and NHC-boryl disulfides (diMe-Imd-BH(SAr)2). Heating in the dark with 1 equiv of disulfide favors monosulfide formation, while irradiation with 2 equiv disulfide favors disulfide formation. With heteroaryl disulfides, the NHC-borane in the primary NHC-boryl sulfide product migrates from sulfur to nitrogen to give new products with a thioamide substructure. Most substitution reactions are thought to proceed through radical chains in which homolytic substitution of a disulfide by an NHC-boryl radical is a key step. However, with electrophilic disulfides under dark conditions, a competing ionic path may also be possible.

New insights into radical and cationic polymerizations upon visible light exposure: role of novel photoinitiator systems based on the pyrene chromophore

New aromatic derivatives PyD based on a pyrene moiety are proposed here as new photoinitiators (PIs) in multi-component systems (with an iodonium salt, a sulfonium salt and optionally a silane) for the radical polymerization of acrylates and the cationic polymerization of epoxy, epoxy-silicone and vinyl ethers. These new photoinitiators allow the use of a long wavelength excitation (using visible light delivered by a halogen lamp, a laser diode at 457 nm or a Xe–Hg lamp). Excellent polymerization profiles are obtained. For polymerization processes using pyrene, very low conversions are reached in agreement with the lack of absorption of this compound for λ > 380 nm i.e.PyDs are attractive compounds for visible light. Acrylate–epoxide, acrylate–vinyl ether and vinyl ether–epoxide blends are also easily polymerized i.e. for the synthesis of interpenetrating or crosslinked polymer networks. The initiation step mechanisms analyzed by ESR, fluorescence, steady state photolysis and laser flash photolysis experiments are discussed.