redox reaction mediated via radicals in depth, where the light does not penetrate anymore. In such case, the possible on-demand curing afforded by the photochemistry is an additional and attractive benefit. Indeed, these systems open new opportunities in industry where the cycle time becomes a more and more challenging task. In addition, the versatility of dual-cure initiating systems is intended to provide greater flexibility in the industrial processes.Recently, pyrylium salts have been reported as a new photothermal initiator for epoxide monomers under irradiation or by thermal heating. [15,16] In this paper, the versatility of pyrylium-based initiating systems combined with vinyl ethers as coinitiators is discussed. It is shown that 2,4,6-triphenylpyrylium tetrafluoroborate (TPP + ) behaves as a very effective cationic thermal initiator in the presence of small amounts of vinyl ether. Similarly, the photoinitiation ability of TPP + is slightly increased in the presence of vinyl ethers. These two features open the possibility to use this system as a dual-cure initiating system for cycloaliphatic epoxides. Moreover, a kickstarting effect is observed, which enables to tune the thermal reactivity of the initiating system. These results Pyrylium salts combined with vinyl ethers are shown to act as new versatile dual-cure initiating systems for both photochemical and thermal initiation of oxirane monomers. The combination of both possibilities allows the curing of thick samples through photoinduced frontal polymerization. On the basis of quantum calculations and photochemical experiments, some clues are given about the reaction mechanisms involved. Interestingly, a sequential kick-starting effect is observed in the presence of vinyl ether enabling the curing of oxetane monomers. Thereby, this communication presents a short overview of potential of pyrylium salts in cationic polymerization of oxiranes.
An initiating system based on pyrylium salts and the hydroperoxide group is a promising method to perform a dual-cure of epoxides via cationic polymerization.
Pyrylium salts are found to be effective initiators for both photochemical and thermal cationic polymerization of epoxy resin. The photopolymerization results show that triphenylpyrylium salt derivatives are the most efficient structures. These compounds also exhibit some thermal reactivity at room temperature in the absence of light. However in such case, the gel time of the resin is quite high. Therefore, to speed up the thermal reaction, nucleophilic compounds are added as coinitiators, these compounds being known to yield a fast decomposition of pyrylium salts. This indeed increases the polymerization of epoxy resin at room temperature, opening the way to the development of quite efficient dual‐cure photochemical/thermal initiating system for cationic polymerization.
Front Cover: Nowaday, pyrylium salts could be considerate to be effective initiators for both photochemical and thermal cationic polymerization of epoxy resin. Triphenylpyrylium salt derivatives are the most efficient structures to photopolymerize the resin. Beside, these compounds also exhibit some thermal reactivity at room temperature in the absence of light, a fact opening the way to the development of new dual cure photochemical/thermal initiating system. Further details can be found in the article by David Maréchal, Xavier Allonas,* Maxime Lecompère, Adrien Criqui page 1169.
The development of new initiating systems for epoxy resin curing remains a highly interesting topic, especially for composites and coatings applications. Among all the possibilities existing to cure an epoxy resin, recent works have highlighted novel photo-thermal cationic initiators based on pyrylium salts. Interestingly, absorption spectra of trisubstituted pyrylium salts extend up to the visible region, a feature that was not exploited yet. In this paper, the pyrylium salts absorption spectra were predicted using Density Functional Theory. It is shown that accurate prediction of UV-Vis spectra can be made at relatively moderate level of theory. Then, visible photopolymerization of epoxy resin was performed under mercury lamp and 395 nm LED. Finally, a photoinduced thermal frontal polymerization of a thick sample of epoxy resin was performed by prompting the reaction at 395 nm. This work opens new opportunity in the polymerization of thick materials such as for composites.
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