Bismuthonium- and pyrylium-based radical induced cationic frontal polymerization of epoxides

Abstract

Cationic polymerization is a powerful tool when it comes to adhesive, coating, composite, and bulk material production. A variety of monomers is suitable for this technique, however epoxy-based systems are used in many applications, due to their high reactivity and versatility. With the introduction of the radical induced cationic frontal polymerization (RICFP), an even more efficient pathway to cure epoxides and with it a variety of new applications was obtained. The prevailing initiator class for RICFP applications is the iodonium salt. Decades of research and fine-tuning of the formulations lead to a highly effective and versatile initiator system. With the introduction of bismuth- and oxygen-based onium salts for frontal polymerization, the well-known iodonium salt is challenged. Bismuthonium hexafluoroantimonates shows high rate of polymerization and conversions of 84% in an epoxy system. The advantage of bismuthonium-based systems is the pot life of the formulations, which can be cured at the press of a button. In this study, iodonium salts last around 3 days before a significant viscosity increase is measured, while the bismuthonium initiator lasts over 10 times as long with no significant drop in reactivity or frontal velocity.