Free radical polymerization photopolymers

Photopolymerization reactions are widely used for printing and photoresist appHcations (55). Spectral sensitization of cationic polymerization has utilized electron transfer from heteroaromatics, ketones, or dyes to initiators like iodonium or sulfonium salts (60). However, sensitized free-radical polymerization has been the main technology of choice (55). Spectral sensitizers over the wavelength region 300—700 nm are effective. AcryUc monomer polymerization, for example, is sensitized by xanthene, thiazine, acridine, cyanine, and merocyanine dyes. The required free-radical formation via these dyes may be achieved by hydrogen atom-transfer, electron-transfer, or exciplex formation with other initiator components of the photopolymer system. 

In another commercial application of free-radical polymerization, polymerizations may be carried out in industrial coatings in the presence of air to yield a variety of coatings and structures of commercial import. This development is possible. In part, because certain vinyl monomers, particularly the acrylates, are less sensitive to retardation by oxygen compared with other monomers. It is therefore possible to produce radiation-cured coatings. UV-cured printing inks and the photopolymers are important in imaging for printing, photoresist, and related applications. 

It was not until the invention of iodonium and sulfonium salts as photo-initiators by Crivello (1975) that cationic photo-polymerization became practical (see Crivello et al., 1977,1990,2000). Upon irradiation of these Crivello salts, acids are generated. Another significant difference between free radical and cationic polymerizations is the latter process is a living polymerization— once the acid species is formed, it remains active even after the irradiation is stopped. In contrast to this behavior free radicals die soon after irradiation is stopped. Also, unhke free radical polymerizations cationic reactions are not inhibited by oxygen. Quite often the dark reaction following irradiation can play an important role in enabhng a cationic system to develop its full properties and this leads manufacturers of commercial cationic photopolymers to often recommend a thermal (dark) postcure after carrying out the photo-irradiation process. 

Photospeeds reported were similar to those encountered in photo-reducible dye sensitized polymerizations, and similar photopolymer functionality was demonstrated (e.g., washout development). Reductive decomposition of diazonium salts is well known to produce free radicals (98). 

Recendy, photopolymer systems have aroused increased interest because of their manifold applications in several high technologies [1-3]. Among such systems, those derived from photoinduced polymerization play an important role. The fundamental principles of these systems are based on the production of species X by photoreactions, which then initiates thermal reactions of low-molecular products leading to polymer or network formation see Eq. (1). In general, these thermal reactions are associated with low activation energies (about 60 kJ mol 1 for free radical chain polymerization). Therefore, such processes can also occur suffidentiy fast at room temperature. 

Photopolymerization is traditionally initiated by direct photolysis of a precursor to provide free radicals via bond homolysis. Examples of such initiators include benzoin, and benzoin ethers, disulfides, and azoalkanes or dialkylperoxides. Hydrogen abstraction chemistry, typified by benzophenone photochemistry, is also recognized as extremely useful. However, a number of viable commercial photopolymer imaging systems are based upon ionic (especially cationic) polymerization. These systems will be discussed next. 

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