Benzylpyrazinium Salts as Photo-Initiators in the Polymerization of Epoxide Monomers

Abstract

In order to study the capability of pyrazinium salt derivatives to act as photo-initiators of epoxide monomers, benzyl pyrazinium hexafluoroantimonate (BPH), benzyl 3,5-dimethyl pyrazine hexafluoroantimonate (BDH) and benzyl quinoxalinium hexafluoroantimonate (BQH) were synthesized by the Menschutkin reaction of benzyl bromide with pyrazine, 2,6-dimethyl pyrazine, and quinoxaline, followed by exchanging with hexafluoroantimonate (SbF6). BPH, BDH, and BQH exhibited characteristic ultraviolet (UV) absorbance as well as exothermic peaks as a function of irradiation time in a differential photo-calorimeter (DPC). In the absence of photo-irradiation, cyclohexene oxide (CHO) underwent slow polymerization at 25 °C using BPH derivatives, but quantitative conversion was achieved even after a 5-min photo-irradiation. In addition, photo-irradiation was required for the photo-polymerization of CHO and styrene oxide (STO), which was characterized by a short induction period followed by a very rapid and exothermic polymerization. While glycidyl methyl ether (GME) required long induction periods, glycidyl phenyl ether (GPE) underwent rather slow and/or no photo-polymerization. The reactivity order of the monomers was CHO > STO >> GME >>> GPE, and the reactivity order for the photo-polymerization of CHO was BPH > BQH > BDH. It was found that BPH, BDH, and BQH could serve as photo-latent initiators for CHO, STO and GME, respectively.

Recent Advances on Quinoxaline-Based Photoinitiators of Polymerization

“…In 2014, a benzyl quinoxalinium hexafluoroantimonate (BQH) was reported for the first time as a Type I photo-latent initiator (See Figure 35) [245]. Upon irradiation with a The higher monomer conversions obtained by using the two-component dye/EDB systems were assigned to synergistic effects resulting from the photocleavage of the thioether groups and the hydrogen abstraction mechanism of the quinoxaline groups, producing, in turn, more free radicals due to the concomitant occurrence of the Type I and Type II mechanism.…”

“…In 2014, a benzyl quinoxalinium hexafluoroantimonate (BQH) was reported for the first time as a Type I photo-latent initiator (See Figure 35) [245]. Upon irradiation with a UV light, the salt cleaved, generating a benzyl cation acting as the initiation species.…”