Direct photolysis of the initiator

Reactive cations can be generated via three different routes (a) by direct photolysis of the initiator, (b) by sensitized photolysis of the initiator, and (c) by free radical mediation. These routes are described below. 

Crivello s pioneering work on onium salt-type photoinitiators (sulfonium and io-donium salts) gave great impetus to investigations of cationic polymerizations [5, 6]. A common feature of mechanisms proposed in relation to onium salt-type initiators of the general structure is the generation of 

Bronsted acids (superacids) of the structure based on non-nucleophilic 

Chart 10.5 Typical electron-transfer photosensitizers that may be applied in conjunction with onium salts [54]. 

Derivatives of anthracene and carbazole are typical electron-transfer photosensitizers. Of practical interest are derivatives containing cationically polymerizable epoxide groups (see Chart 10.5) [54]. During the ring-opening photopolymerization of epoxides, these sensitizers are covalently incorporated into the polymeric network and cannot be removed by extraction. Therefore, the potential risk of toxic effects of the sensitizers is strongly diminished. 

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