Initiators are often used in chain-growth polymerization such as radical polymerization to regulate initiation by heat or light. Thermal polymerization initiators are compounds that generate radicals or cations upon exposure to heat. For example, azo compounds such as 2,2'-azobis(isobutyronitrile) (AIBN) and organic peroxides such as benzoyl peroxide (BPO) are well-known thermal radical initiators, and benzenesulfonic acid esters and alkylsulfonium salts have been developed as thermal cation initiators. Photopolymerization initiators are roughly categorized into three groups depending on the generated active species (radicals, cations, anions). Conventional photopolymerization initiators such as benzoin derivatives generate free radicals upon light irradiation. Photo-acid generators which produce cations (acid) upon light irradiation found practical use in the late 1990s. Photo-base generators, which produce anions (base) upon light irradiation, are of current research interest.