Iodonium and Sulfonium Photoacid Generators in Hybrid Radical–Cationic Systems

Hybrid radical–cationic photopolymer formulations combine reaction channels that differ in initiation, propagation, inhibition, termination and post-irradiation behaviour. Iodonium or sulfonium photoacid generators may participate directly or through a sensitiser, while a radical initiator or sensitiser-derived radical supports the radical channel. Nominal component identity does not establish which channel was activated, how efficiently acid was generated, whether conversion continued after irradiation, or which ionic and neutral photolysis products remained in the cured material. This review presents a qualification architecture that separates spectral access, excited-state interaction, acid generation, channel-specific conversion, latency, dark cure and residual-species fate. Primary studies demonstrate visible and near-infrared sensitisation of iodonium systems, one-component iodonium photoacid generators, sulfonium-enabled hybrid polymerisation, and formulation-level validation by spectroscopy, photorheology and additive manufacturing. Their results remain specific to the sensitiser, salt, counterion, monomer, atmosphere, irradiation history and analytical method. A defensible engineering decision therefore requires independent evidence for each reaction channel and for the cured material's residual ionic state. No universal PAG, sensitiser, loading, wavelength, exposure or dark-cure schedule follows from the available evidence.