Regulating Cationic Polymerization of Difunctional Epoxy Resin through Structural Variations of the Thermal Initiator

The cationic polymerization of epoxides facilitates the rapid cross-linking of epoxy resins, increasing their effectiveness for packaging and adhesive applications and efficiency of their manufacturing. However, the reaction conditions, such as temperature, should be adjusted, and the polymerization behaviors should be modulated to meet the demands for resin products and, consequently, broaden the application of epoxy resins. This study investigated the influence of structural variations in sulfonium-type initiators on thermal cationic polymerization. We prepared 47 different thermally activatable initiators by combining four designed sulfonium-type cations with varying aromatic ring substituents and 13 anions categorized by their central charge-bearing atoms. We evaluated the polymerization and cross-linking activities of the initiators with bis[4-(glycidyloxy)phenyl]methane. The structural variations of the anions had a larger influence on polymerization than that of the cations. Specifically, polymerization required higher thermal energy and reaction temperatures with stronger ionic interactions between the anion and cation. The delocalization of negative or positive charges by the substituted groups on both ions and steric hindrance affected the ionic interactions. Finally, various initiators facilitated fine-tuning of the initiation temperature to 53–139 °C for epoxy resin production. The thermal behaviors were analyzed using differential scanning calorimetry, including the kick-off temperature, peak temperature, and reaction enthalpy. Therefore, we elucidated cationic polymerization based on the chemical structure of the initiator, which can be used to achieve a high-performance, on-demand curable epoxy-based system.