Photoinitiators-Sulfonic Acids

A large number of cationic photoinitiators are known. The most significant from the commercial point of view are aryldiazonium, diaryliodon-ium, triarylsulfonium, and ferrocenium salts. These salts possess anions of very low nucleophilicity which do not terminate the polymerization process. Nonionic cationic photoinitiators such as organosilanes, latent sulfonic acids, and some other miscellaneous compounds are also used. 

A typical acid photogenerator is triphenyl sulfonium triflate (Figure 6.13), which generates trifluoromethane sulfonic acid. Onium salt photoinitiators of this type can induce cationic photopolymerisation under photoirradiation. The reaction is complex and several products of this type are used with Lewis acids, such as AsPF ", AsF " and BF4". See, for example, [44]. 

In addition there is a report of a water soluble polymeric (copolymer) photoinitiators. These initiators were synthesized by copolymerizing 2-acryloxy and 2-acrylamido anthraquinone monomers with three water-soluble comonomers acrylamide, 2-acrylamido-2-methylpropane sulfonic acid and 2-acryloxyethyl trimethylammonium iodide. The polymerization activity correlates well with their measured photo reduction quantum yields in water... 

Sulfonic (-SO3H) and Sulfinic (-SO2H) Acid-Based Nonsalt Photoinitiators Photosensitive tosylate esters of nitrobenzyl [65] and benzoin [73], sulfonyl ketones [67,68], and diphenyl disulfones [68] can be employed in cationic photopolymerization, especially in photolithography and chemical amplification processes. The advantages of these photoinitiators are the ease of synthesis, the absorption wavelengths suitable for deep-UV curing and the high yield of acid formation [65]. 

TABLE 11.2 Sulfonic and Sulfinic Acid-Based Photoinitiators and Corresponding Initiating Species Generated upon Photolysis. 

---