Diazonium cationic photoinitiators

Cationic photoinitiators are compounds that, under the influence of UV or visible radiation, release an acid, which in turn catalyzes the desired polymerization process. Initially, diazonium salts were used, but they were replaced by more thermally stable iodonium and sulfonium salts. Examples of cationic initiators are in Table 4.3. 

The cationic photoinitiator triarylsulfonium, diaryliodonium, and related aryl diazonium salts containing nonnucleophilic counterions, were used by Konarski et al. (2) to prepare adhesives. 

Cationic photoinitiation is based on the ring opening of the oxirane group. The photoinitiators of practical importance belong to three main classes of compounds diazonium salts, onium (e.g. iodonium and sulfonium) salts, and organometallic complexes,55 which upon irradiation by UV light decompose and yield an acid catalyst. 

Cationic photoinitiation is generally based on a ring-opening reaction of an oxirane group (see Scheme 6). There are three main classes of cationic photoinitiator molecules, namely diazonium salts, onium salts, and organometallic complexes. When photopolymerization of epoxides is considered, the use of diazonium salts is limited due to stability issues. In addition, they release nitrogen gas that forms bubbles and pinholes in the coatings. Onium salts... 

Muller, U, Utterodt, A., Morke, W., Deuber, B., and Herzig, C, New insights about diazonium salts as cationic photoinitiators, /. Photochem. Photobiol. A Chem., 140, 53, 2001. 

Cyclic ethers, cyclic acetals, and some vinyl compounds can be polymerized by cationic processes. Photoinitiation of these polymerizations by ultraviolet light are known (12,106). Some extension of direct photolytic sensitization of cationic processes to visible wavelengths is obtained by the use of colored diazonium salts as initiators. For example, Schlesinger (11a) used diazonium salts substituted in the para position with electron withdrawing groups, but sensitivity was limited to the blue to green regions of the spectrum. 

The photoinitiators for cationic polymerizations are mainly of two types. One is based on aryldiazonium salts and the other on diaryliodonium, triarylsulfonium, or triaiylselenonium salts. The diazonium salts photodecompose to yield both Lewis acids and strong protonic acids ... 

This is also probably the case with the other classes of photoinitiators, with the exception of the diazonium and ferrocenium salts. The photolysis of diazonium salts is well known to generate Lewis acids which initiate polymerization either by themselves or in combination with a protogen. Similarly, the photolysis of ferrocenium salts proportedly proceeds by the mechanism shown in equation (24), which involves the formation of the Lewis acid shown. Apparently, this iron-containing Lewis acid is strong enough to initiate many types of cationic polymerization, including those of vinyl ethers. 

Diazonium salts, relatively stable to heat, have been used as photoinitiators in polymerization processes. Both phenyl cations and radicals generated during the illumination stage will initiate polymerization of vinyl ethers and the crosshnking of low-molecular-weight polymers with reactive alkene substituents. ... 

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