Cationic photopolymerizations advantages

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]. 

An advantage of this type of photopolymerizations is that as they are non-radical chain polymerizations, they are insensitive to oxygen. In addition, as the cation is relatively stable, the reaction is able to continue in the dark. Applications of this chemistry may be found in the fields of coatings, adhesives, printing inks, and also for photocurable composites and microelectronic photoresists. 

An usual way to generate a strong acid as an initiator of cationic polymerizations is by the UV decomposition of a complex aromatic salt of a Lewis acid. Cycloaliphatic epoxy monomers are used in this reaction because they exhibit higher reactivities than those of glycidylether epoxies such as DGEBA. These formulations are used in photopolymerization processes whose main advantage apart from the fast reaction rate is the insensitivity to oxygen (contrary to free-radical polymerizations). 

The cationic photoinduce process present some advantages compared to the radical one [10] in particular lack of inhibition by oxygen, low shrinkage and good adhesion and mechanical properties of the UV-cured materials. Moreover, the monomers employed are generally characterized by being less toxic and irritant with respect to acrylates and methacrylates, largely employed in radical photopolymerization. 

---