Photoinitiation by Onium Salts

Aryldiazonium (ArNjZ-), diaryliodonium (Ar2I+Z ), and triarylsulfonium (Ar3S+Z ) salts, where Z is a nonnucleophilic and photostable anion such as tetrafluoroborate (BF4), hexafluoroantimonate (SbFg), and tetraperfluorophenylborate [(CgFsXB-], and 

Diaryliodonium and triarylsulfonium salts act as photoinitiators of cationic polymerization. Photolytic celeavage of an Ar—I or Ar—S bond yields a radical-cation (Eq. 5-8) that reacts with HY to yield an initiator-coinitiator complex that acts as a proton donor to initiate 

Cationic photoinitiators are used in coatings, printing inks, adhesives, sealants, and photoresist applications. Most of the applications involve vinyl ether polymerizations or ringopening polymerizations of epoxy monomers (Sec. 7-2b). 

Quantitative aspects of photopolymerization have been described in Sec. 3-4c. There are some differences between radical and cationic photopolymerizations. The dependence of Rp on light intensify is half-order for radical polymerization, but first-order for cationic polymerization. Radical photopolymerizations stop immediately on cessation of irradiation. Most cationic photopolymerizations, once initiated, continue in the absence of light because most of the reaction systems chosen are living polymerizations (Sec. 5-2g). 

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Although in photocurable formulations, difunctional oxirane derivatives are employed for mechanistic studies, monofunctional oxiranes are used including cyclohexene oxide, styrene oxide, or phenyl glycidyl ether. These studies indicate that the cationic polymerizations proceeding as a result of photoinitiation by onium salts have typical characteristics of polymerizations initiated by strong protonic adds. Thus, initiation involves protonation of oxirane ring while propagation proceeds on tertiary oxonium ions as active species, that is, by the ACE mechanism. 

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