Photopolymerization initiators are used in many fields to generate photocurable composites. These composites are polymerized by irradiation with UV light and electron beam which leads to altered physical properties of the composites such as solubility, viscosity and adhesiveness. In particular, the phenomenon in which a liquid changes into a solid is most useful and is applied to surface-treating techniques in fields including paints, printing inks, dental materials, lithography, photoresist, etc. A growing number of intensive studies have been made for the development of improved properties for novel photopolymerization initiators.

Photopolymerization initiators can be divided roughly into three groups with the generated active species, as shown in the upper figure. The conventional photopolymerization initiators generate free radicals upon light irradiation, and the resulting radical starts the polymerization process. Typical initiators are represented by benzoin derivatives. Photo-acid generators which produce cations (acid) by light irradiation, were put to practical use in the late 1990s. Photo-base generators, which produce anions (base) by light irradiation, are of current interest in research.
Photopolymerization initiators are divided roughly into three groups as shown above. For a photopolymerization initiator to be an excellent initiator the following properties must be considered;
(1) High quantum yields in the generation of radicals, cations and anions
(2) Overlap in absorption wavelength of the photopolymerization initiators and emission wavelength of the light source
(3) High thermal stability and high stability in darkness
(4) Highly soluble in monomers and oligomers
In addition to those just developed, we have a wide assortment of initiators, such as benzoin derivatives which are already in use.