Photochemical and Radiation Initiation

Photochemical and Radialion Initiatioii.— The topics of photochemical initiation of cationic polymerizations, radiation-induced initiation, and their inter-relationships have been reviewed recently. 

Factors influencing the reactivity of triarylsulphonium salts include counterion type and the nature of ring substituents. For a series of triphenyl sulphonium salts, reactivity was found to vary with the counterion in the order SbF AsF PFj BFj . A reaction scheme proposed to account for the activity of these salts as initiators of both free-radical and cationic polymerizations is outlined in reactions (D—(3), 

The use of free-radical sources that undergo a one-electron transfer oxidation in conjunction with cationic photo-initiators has been examined. - Regeneration of potential initiators leads to high yields of active centres from each photon absorbed by the system. 

Cationic polymerization of isobutylene induced by radiation from tritium resulted in high-molecular-weight polymer at —78 The process involved diffusion of cations from the gas phase into the liquid phase and the rate of tritium decay was found to be independent of temperature. 

Polymerizations induced by y radiation proceed by both cationic and free-radical mechanisms. Predominance of the cationic process can be assured by exhaustive purification of monomers and solvents or by employing high radiation dose rates (from electron beam sources). The rate of cationic polymerization has been shown to be strongly influenced by the dielectric constant of the medium, giving rise to discrepancies between radiation-induced bulk polymerizations and chemically initiated solution polymerizations. These discrepancies have been attributed to specific solvation of propagating ions by monomer and polymer. 

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Free-radical polymerization processes are used to produce virtually all commercial methacrylic polymers. Usually free-radical initiators (qv) such as azo compounds or peroxides are used to initiate the polymerizations. Photochemical and radiation-initiated polymerizations are also well known. At a constant temperature, the initial rate of the bulk or solution radical polymerization of methacrylic monomers is first-order with respect to monomer concentration, and one-half order with respect to the initiator concentration. Rate data for polymerization of several common methacrylic monomers initiated with 2,2 -azobisisobutyronitrile [78-67-1] (AIBN) have been deterrnined and are shown in Table 8. 

Usually, free-radical initiators such as azo compounds or peroxides are used to initiate the polymerization of acrylic monomers. Photochemical and radiation-initiated polymerizations are also well known. Methods of radical polymerization include bulk, solution, emulsion, suspension, graft copolymerization, radiation-induced, and ionic with emulsion being the most important. 

In the case of photochemically and radiation initiated poljnnerization E. = 0. Therefore, the pol nnerization energy E < 0 and the degree of pol nnerization increases with increasing temperature. 

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