Start by Low-Energy Radiation

If the energy-rich radiation is absorbed as visible or ultraviolet light, then the energy is immediately assimilated and the molecule is converted into the excited state. The excited molecule either dissociates into free radicals or dissipates the energy by fluorescence, phosphorescence, or thermal collision deactivation. 

In the simplest case, the monomer molecule itself is converted to the excited state. The free radicals produced by dissociation then start the polymerization of the rest of the monomer molecules. 

If the monomer itself cannot form free radicals by this process, then a photoinitiator must be added. Bisulfides form two free radicals RS on irradiation. The azo group of azobisisobutyronitrile absorbs light at 350 nm and then forms free radicals [reaction (20-3)]. Certain aliphatic ketones decompose according to a Norrish type I mechanism into two free radicals  

Aliphatic ketones, which decompose according to a Norrish type II mechanism, however, do not yield free radicals  

Photosensitizers can act in two ways. For example, benzophenone is raised to the excited state by UV light. It then transfers its energy to monomers. Other photosensitizers react in the excited state with the monomer to form the polymerization-starting species. 

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