Phenacyl pyridinium salt-benzoxazine: a photocation generator for benzoxazine ring opening polymerization

This study reports a novel photoactive benzoxazine monomer containing a phenacyl pyridinium salt moiety (P-Pyr_PA-SbF₆), which is designed to initiate cationic ring-opening of benzoxazines upon light exposure. For the purpose, the synthesis of the precursor pyridine-functionalized benzoxazine (P-Pyr) and its subsequent conversion to phenacyl pyridinium salts were successfully achieved. The chemical structures of benzoxazines were confirmed by spectroscopic analyses (¹H NMR, ¹³C NMR, FTIR). Photophysical studies demonstrated the photoactivity of P-Pyr_PA-SbF₆, evidenced by UV photobleaching and NMR analysis. Differential Scanning Calorimetry (DSC) showed that UV irradiation (360 nm) significantly reduced the self-curing temperature and enthalpy of P-Pyr_PA-SbF₆ itself. Furthermore, P-Pyr_PA-SbF₆ exhibited effective thermal and photo-assisted catalytic activity for polymerizing a conventional monofunctional benzoxazine monomer (P-a). Light pre-treatment of P-a blends containing P-Pyr_PA-SbF₆ substantially lowered the curing onset temperatures (e.g., from 222 °C for neat P-a to 146 °C with 10 mol% salt) and reduced the activation energy (E_a) for P-a polymerization. These findings reveal P-Pyr_PA-SbF₆ as a promising system for achieving controlled, lower-energy curing of polybenzoxazines via light induction.