Triaryl iodonium salt

While electron beams can produce cations, they are not effective at producing cationic cure in the absence of suitable photoinitiators. The same cationic photoinitiators used for UV cure are often also e-beam sensitive. Examples are triaryl sulfonium or diaryl iodonium salts [41]. 

Compounds with a nucleophilic sulphur are readily arylated by iodonium salts in their anionic form, and less readily in other cases, notably in sulphides triaryl sulphonium salts are useful photoinitiators in cationic polymerizations. A list of chalcogen compounds arylated by iodonium salts appears in Table 8.7. 

It should be pointed out that the photolysis of the diaryliodonium and triaryl-sulfonium salts proceeds differently from the photolysis of the diazonium systems. The reaction mechanism of the photolysis of iodonium salts is described by Reactions [6.46] and [6.47]. 

Diaryl iodonium and triaryl sulfonium salts are among the most versatile photoinitiators. These compounds have been thoroughly investigated by Crivello... 

Few photoinitiators are available for cationic systems. The most widely used are diaryl iodonium salts such as diaryliodonium hexafluoro-antimonate, triaryl salts such as triphenyl sulfonium hexafluoro-phosphate, and mixed triphenyl sulfonium salts. These photoinitiators are decomposed by UV light by a homolytic cleavage to produce a radical anion and a radical cation. The latter abstracts hydrogen from surrounding molecules and generates a proton, which is the initiating species ... 

Cationic photoinitiators such as onium salts yield cationic species, which are capable of initiating cationic polymerization, upon irradiation. Radical cations and Br0nsted acids are generated via irreversible photofragmentation of diaryl iodonium, triaryl sulphonium, and /V-alkoxy pyridinium salts [ 13,14] as depicted in Scheme 13.3. 

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