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Photoinitiated free radical polymerization has been widely used in research and industrial applications during the past few decades. Photopolymerization offers compelling advantages over traditional thermal polymerization, including low energy consumption, room temperature curing, spatial and temporal control of initiation, and solvent-free polymerization. These advantages have lead to tremendous growth in the use of photopolymerization in a variety of applications, including coatings on a variety of substrates, adhesives, flexographic printing plates, soft contact lenses, and dental materials. Photopolymerizations are simply polymerization reactions initiated by light, typically in the ultraviolet or visible region of the light spectrum. Photopolymerizations are initiated by certain types of compounds which are capable of absorbing light of a particular wavelength. The wavelength or range of wavelengths of the initiating source is determined by the reactive system including the monomer(s), the initiator(s), and any photosensitizers, pigments or dyes which may be present. Photoinitiated radical polymerization may be initiated by both cleavage (Type I) and H-abstraction type (Type II) initiators. Because of their vital role in photopolymerization, photoinitiators are the subject of particularly extensive research. Most of this research has focused on type I photoinitiators, which upon irradiation which undergo an a- cleavage process to form two radical species. Type II photoinitiators are a second class of photoinitiators and are based on compounds whose triplet excited states are reacted with hydrogen donors thereby producing an initiating radical. Because the initiation is based on bimolecular reaction, they are generally slower than type I photoinitiators which are based on unimolecular formation of radicals. On the other hand, type II photoinitiators possess better optical absorption properties in the near-UV spectral region. Moreover type I compounds give raise to volatile photodecomposition products due to the cleavage mechanism adding to migration the problem of release of odour. In this respect the type II photoinitiators have a more favourable profile because the ketyl radical either is re-oxidised back to the ketone or gives rise to recombination products with formation of higher molecular weight derivatives with a lower volatility than parent compounds. Typical type II photoinitiators include benzophenone, thioxanthones, benzil, and quionones while alcohols, ethers, amines and thiols are used as hydrogen donors.… [ABSTRACT FROM AUTHOR]