Irradiation of Thermal and Redox Initiators

Although both aliphatic and aromatic ketones have been studied, the aromatic ketones are more useful in commercial practice, since their absorptions occur at longer wavelength (lower energy) and their quantum yields are higher [Ledwith, 1977 Pappas, 1988]. Ketones and their derivatives undergo homolysis by one or both (often simultaneously) of two processes a-scission and electron transfer [Padon and Scranton, 1999]. a-Scission involves 

Tertiary amines with an a-hydrogen are among the most effective electron donors other electron donors include alcohols, amides, amino acids, and ethers. A third process, direct hydrogen atom transfer from RH to the ketone, is not common hut does occur with some photoinitiators. The overall result is the same as the electron-transfer process. Although two radicals are produced by photolysis of the photoinitiator, only one of the radicals is typically active in initiation—the aroyl and amine radicals in Eqs. 3-48 and 3-49, respectively. The other radical may or may not initiate polymerization, hut is active in termination. The decrease in photoinitiator concentration during polymerization is referred to as photo-bleaching. 

Similar photolytic reactions occur with benzoin (VIII), benzyl ketals (IX), aroylpho-sphine oxides (X), and a-aminoalkylphenones (XI). These and related compounds are used 

An expression for the polymerization rate is obtained by combining Eqs. 3-50 and 3-25 to yield 

The absorbed light intensity la is obtained from a consideration of the Beer-Lambert law in the form 

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