International audienceA short review on the role of photocatalysts in photopolymerization reactions is presented. A special emphasis is done on photoredox catalysts leading to i) high performance initiating systems for polymerization upon low light intensity for the use of light emitting diodes (LEDs) and/or visible light or ii) Controlled Radical PhotoPolymerization (CRP2) processes with formation/reactivation of dormant species triggered by light
Silyl glyoxylates are proposed here
as a new class of high performance
type I photoinitiators for free radical polymerization under air or
in laminate (e.g., (meth)acrylates) upon exposure to different near-UV
(at 395 nm; at 405 nm) and blue (at 477 nm) LEDs. The new proposed
photoinitiators can also be used in the presence of additives that
can enhance their initiating ability: an iodonium salt and an amine
or a phosphine. The silyl glyoxylate-based photoinitiating systems
exhibit excellent polymerization performances upon blue LED light
(at 477 nm) with exceptional bleaching properties compared to camphorquinone (CQ)-based systems.
This can be highly worthwhile for the preparation of colorless polymers
upon visible light. Real-time Fourier transform infrared spectroscopy
(RT-FTIR) experiments are used to monitor the polymerization profiles.
The involved chemical mechanisms are investigated by fluorescence,
laser flash photolysis, electron spin resonance (ESR), and steady
state photolysis experiments. Molecular orbital calculations are also
carried out. The overall excited state processes and the chemical
mechanisms involved in the initiation step are detailed.
Diaryliodonium salts are well-established compounds in free radical chemistry and are already used as photoinitiators (free radical or cationic polymerization), but the presence of counter anions is a strong drawback. Indeed, a counter anion is always required (e.g., SbF6−) leading to potential toxicity issues or release of HF. In the present paper, counter anion-free and fluoride-free aryliodonium salts are proposed, that is, aryliodonium ylides (AY) are studied here as new and efficient additives for radical chemistry and an example is provided for the camphorquinone (CQ)/amine based photoinitiating systems (PISs) for the polymerization of thick (1.4 mm) and thin (20–13 µm) methacrylates under air and blue light irradiation. The newly proposed PISs, for example, CQ/amine/AY, presented excellent polymerization performances and good bleaching properties were obtained after polymerization. Real-time Fourier transform infrared spectroscopy (RT-FTIR) was used to monitor the photopolymerization profiles. The chemical mechanisms involved were investigated using electron spin resonance (ESR).
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