Photolysis, aryldiazonium salts

Fig. 2. Principle of Lewis acid formation by photolysis of aryldiazonium salts, MX = PFj, BFj, SbFs etc.

However, several inherent drawbacks limit the utility of aryldiazonium salts as photoinitiators in a number of practical applications for epoxy curing. Nitrogen evolution during photolysis of the initiator causes bubbles and pinholes in coatings. Other problems arise from the poor thermal stability of aryldiazonium compounds and from their inherent sensitivity to moisture. The addition of stabilizing additives such as nitriles amides sulfoxides and poly(vinylpyrrolidone) has proven effective in extending the solution stability of aryldiazonium salt/epoxy mixtures. 

Figure 2. Photolysis of aryldiazonium salts. Photochemistry of these salts is dependent on the nucleophilicity of the anion.

The acid generating photoinitiators that provide the basis for our studies are onium salts that have been described in the work of Schlesinger (6,7) and Watt ( 8) and by Crivello (9) who pioneered their use as initiators for photocuring of coatings. The initiators include aryldiazonium salts that generate Lewis acids upon photolysis and diaryliodonium and triarylsulfonium salts that generate strong Bronsted acids... 

For imaging applications, a photochemical means of producing the Lewis or Brpnsted acid photoinitiator is required. Aryldiazonium salts (XLI) were found to produce Lewis acids on photolysis ... 

A paper on new approaches to the generation of arylphosphinidenes has been published. The stable bis-azide precursor (133) upon photolysis, or vapour phase thermolysis, gave (134), obviously via the phosphinidene (135) the same product was obtained by photolysis of the phosphaketene (136). The reduction of aryldiazonium salts to arenes with triethyl phosphite or triphenylphosphine is shown to proceed by a radical-chain mechanism. The previously described photo-Arbuzov rearrangement of benzyl phosphites has been used to prepare several acyclic phosphonate nucleotide analogues, e.g. (137). ... 

Over the past several years, there have been developed several new classes of onium salt photoinitiators capable of initiating cationic polymerization. The most significant of these are aryldiazonium salts, diaryliodonium salts, triarylsulfonium salts, and dialkylphenacyl-sulfonium salts. The mechanisms involved in the photolysis of these compounds have been elucidated and will be discussed. In general, on irradiation acidic species are generated which interact with the monomer to initiate polymerization. Using photosensitive onium salts, it is possible to carryout the polymerization of virtually all known cationically polymerizable monomers. A discussion of the various structurally related and experimental parameters will be presented and illustrated with several monomer systems. Lastly, some new developments which make possible the combined radical and cationic polymerization to generate interpenetrating networks will be described. 

Photoinitiated cationic polymerization has been the subject of numerous reviews. Cationic polymerization initiated by photolysis of diaryliodonium and triarylsulfonium salts was reviewed by Crivello [25] in 1984. The same author also reviewed cationic photopolymerization, including mechanisms, in 1984 [115]. Lohse et al. [116], reviewed the use of aryldiazonium, diphenyliodonium, and triarylsufonium salts as well as iron arene complexes as photoinitiators for cationic ring opening polymerization of epoxides. Yagci and Schnabel [117] reviewed mechanistic studies of the photoinitiation of cationic polymerization by diaryliodonium and triarylsulfonium salts in 1988. Use of diaryliodonium and sulfonium salts as the photoinitiators of cationic polymerization and depolymerization was again reviewed by Crivello [118] in 1989 and by Timpe [10b] in 1990. 

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