Cationic Ring-Opening Photopolymerisation in VPP: Photoacid Generation, Dark Cure, Humidity and Epoxy/Oxetane Network Formation
Segurola, Juan
2026-08-27 · Report · Version 1.1
Vat photopolymerisation is dominated by free-radical acrylate and methacrylate chemistry. Cationic photopolymerisation provides a distinct route in which light generates a strong acid or an acid-forming species that initiates ring opening of epoxides and oxetanes or chain growth of vinyl ethers. The active ionic centres can persist after illumination, enabling continued dark reaction and conversion in regions no longer receiving light. This persistence is simultaneously an opportunity and a confinement problem. Continued reaction can improve through-depth conversion and reduce dependence on oxygen at the propagation stage, but it can also enlarge features after exposure, shift dimensions with waiting time and weaken the direct relation between projected optical boundary and final polymer boundary. Water, alcohols, amines, basic fillers, pigments, counterion chemistry and temperature can alter acid availability, propagation, transfer and termination. Recent cationic VPP research spans visible-light one-component iodonium salts, carbon-dot and metalloporphyrin sensitisation, oxetane-accelerated epoxy resins, hot lithography, cationic ceramic slurries, dual-wavelength acrylate-epoxy systems and cationic photoinhibition lithography. These studies demonstrate printing at 365, 405 and longer wavelengths, but also show that spectral absorption alone is insufficient: acid- generation quantum yield, monomer class, formulation basicity, temperature, humidity and post-exposure time jointly determine the realised material state. This review develops a VPP-specific qualification framework for cationic ring-opening systems. It separates photoacid generation from monomer propagation, distinguishes dark cure from thermal secondary cure, and requires post-exposure conversion, humidity response, dimensional evolution and chemical-specific conversion to be reported. Hybrid radical/cationic systems are included only where they clarify cationic behaviour; their cure-sequence architecture remains the scope of ER-59.