Versatility of Pyrylium Salt/Vinyl Ether Initiating System for Epoxide Dual‐Cure Polymerization: Kick‐Starting Effect of the Coinitiator

Abstract

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.

Dual-cure Photo-thermal Initiating System for Cationic Polymerization of Epoxy under LED Visible Light

“…It has been established that TPP + is an efficient cationic photo and thermal initiator [1][2][3][4]. Thus, it is a quite suitable compound for designing a dual-cure initiating system.…”

“…It was reported that hydrogen peroxide at 1 mol% is sufficient to induce a front of polymerization [3]. It has been also shown that at 3 wt% of isobutylvinylether can be use in addition to an oxetane monomer to speed up the thermal polymerization of the epoxy resin [1].…”

“…As example, the decrease of light penetration with increasing thickness represents a serious drawback for thick, pigmented coatings and fiber-reinforced composites. Therefore, the concept of dual-cure initiating systems based on both photo-thermal curing appears as a promising alternative to solve the issue [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. Indeed, the combination of a photochemical polymerization promoting a thermal curing has rendered possible the curing of thick samples.…”

“…Moreover, the thermal process can also be used to complete the curing of the epoxide resin in shadowed areas, typically in the case of 3D parts. In the best cases, the photochemical process would also speed up the thermal curing to perform a photoinduced thermal frontal polymerization [1][2][3][4][5][6][7][8][9][10][11][12][13][14].…”

“…Recently, pyrylium salts have been reported as new photo/thermal initiators for epoxy monomers under UV irradiation or under heat [1][2][3][4]. In these works, the versatility of dual-cure initiating systems based on pyrylium salts has been demonstrated.…”