Photoinitiating systems for cationic photopolymerization: Ongoing push toward long wavelengths and low light intensities

Recently, long-wavelength photoinitiating systems for cationic photopolymerization (LWCPIs) have attracted growing interest due to the wide applications of visible light emission sources. The approaches used to expand the wavelength range of the cationic photoinitiating (CPI) system range from designing new onium salts with long-wavelength absorbing chromophores to photosensizing conventional onium salts with various visible light photosenzitizers or free radical sources. In this review, the initiating mechanisms and characteristics of each approach are introduced, and the latest developments are highlighted. Based on analysis and comparison of recent progress, forthcoming developments are proposed as well.