A photoinitiator is a molecule that creates reactive species (free radicals, cations or anions) when exposed to radiation (UV or visible). They initiate the polymerization process by adding themselves to the monomer or oligomer.
Photoinitiators are used extensively together in combination with cross-linkable monomers and oligomers in ultra-violet-curable inks and coatings, adhesives and many other products.
Photoinitiators can be divided into two classes radical photoinitiators and cationic photoinitiators. Radical photoinitiators can react differently under UV-irradiation. Depending on their reaction they are classified as Norrish Type I or Norrish Type II Photoinitiators.