Arylsulfonium salts

The formation of the benzyl radical, as in Eq. (38), is made likely by the finding of a small yield of 1,2-diphenylethane [111,112] and that 4-nitrobenzyldimethylsulfonium salts give good yields of 4,4-dinitrobibenzyl on reduction at a mercury cathode [113]. Arylsulfonium salts are cleaved more slowly than alkylsulfonium salts in some cases a reversible ET is indicated by CV [114]. 

Onium salts have many of the characteristics required of photosensitive initiators for light-curable coatings. Aryl-diazonium, aryliodonium and arylsulfonium salts all initiate rapid cure. Storage life of photosensitized formulations ranges from a few hours to more than six months, depending upon structure of the photoinitiator, composition of the coating and storage conditions. 

Ledwith and coworkers have studied the cationic polymerization of THF initiated by onium salts in combination with free radical proginators.(20-23) When 2-2-dimethoxy-2-phenylacetophenone (IRGACURE 651) was irradiated in THF, the experiment containing an aryliodonium salt had a seven fold greater polymerization rate than a comparable experiment containing an arylsulfonium salt (21). 

The sulfonium salt was shown to spontaneously oxidize highly stabilized free radicals such as the triphenylmethyl radical to form the corresponding triphenylmethyl carbonium ion.(20) It would also appear that the dimethoxybenzylic free radical (a Norrish Type I photocleavage product of 2,2-dimethoxy-2-phenyl-acetophenone) is similarily oxidized by the arylsulfonium salt ( ). 

Figure 9. Photoreduction of onium salts by free radicals. Photoinitiators which directly form easily oxidized free radicals can efficiently sensitize aryliodonium salts, resulting in the formation of acid (HAsP ). Having higher reduction potentials, the arylsulfonium salts do not oxidize most free radicals.

Sensitization of aryliodonium and arylsulfonium salts having nonnucleophilic anions can greatly increase the quantum efficiency of photolysis. This results in the more rapid curing of epoxy based coatings. 

Crivello et al. [32], described arylsulfonium salts with two or three photoactive groups in the same molecule. These were prepared via arylation of thiophenoxyaryl sulfides by the thermal decomposition of diaryliodonium salts. The extended conjugation of the chromophores introduced long wavelength absorption bands. 

Bayer AG [35] patented the preparation of arylsulfonium salts by reaction of aromatic compounds with a sulfoxide in the presence of hydrofluoric acid. 

This suggests that the mechanism for the formation of arylsulfonium salt 7, R=Ar involves the complexation mechanism [31] further discussed below, but illustrated for the first part of this reaction in Eqs. (4-6). 

The utility of the arylsulfonium salts as arylating agents in the above reactions depends upon the availabihty of these salts. Triphenykulfonium bromide is commercially available. Condensation of organometallics with diarylsulfoxides 34 or diarylethoxysulfonium fluoroborates 35 suffers from low yields. Use of diaryladamantoxysulfonium fluoroborate circumvents... 

Ultraviolet curing of epoxy resins has become important. Cycloaliphatic epoxy resins are combined with substances that induce polymerization under the influence of UV radiation or an electron beam. Industrially important products include tri-arylsulfonium salts (UVE 1014, 1016, General Electric) and arene-ferrocenium compounds (CG 24-061, Ciba-Geigy). To ensure good film flexibility and chemical resistance, polyols should be added. The films should be postcured for 30-120 s at ca. 100"C [2.126]. 

The papers presented in the following chapters represent advances in pressure sensitive adhesives (ultraviolet light activated acrylate monomer - low Tg polyether formulations) photoinitiated cationic polymerization (light activated aryliodonium and arylsulfonium salts of lewis acids in epoxy resin formulations) polymer and formulation design criteria for radiation curable adhesives radiation curable composites (dynamic thermal analysis characterization of electron beam cured... 

While the photolysis of diaryliodonium and tri-arylsulfonium salts has been observed to be independent of temperature, there is a marked dependence of the rate of photolysis on the intensity and wavelength of UV radiation. As the light intensity is increased, the photolysis rate also increases proportionally, indicating a first power relationship between these two factors. 

Synthesis of TriaryIsulfonium Salts. The synthesis of a range of arylsulfonium salts (4) from arylsulfides 3 via the thermolysis of la in the presence of copper salts was reported (eq 2). In all cases, microwave-assisted heating proved superior in terms of reaction times and/or yields compared to conventional heating (Table 1). 

Hydroxyalkylaryl sulfides cyclize with the reagent to yield cyclic arylsulfonium salts, which are useful for increasing the water resistance of carboxylated polymers. ... 

The arylsulfonium metal halide complex salts are not readily available. Triphenylsulfoniurn hexafluorophosphate can be prepared from diphenyliodonium hexafluorophosphate by heating with phenyl sulfide, as described by Knapczyk and McEwen ( ), or by reaction of phenyl magnesium bromide with diphenyl sulfoxide followed by reaction with hexafluorophosphoric acid according to the procedure of Wildi, Taylor and Potratz (60). 

Photochemistry of Triarylsulfonium Salts. A similar mechanism was proposed for the photolysis of triarylsulfonium salts having nonnucleophilic anions. The arylsulfonium and to a lesser extent, the aryliodonium salts have improved thermal stability as compared to the diazonium salts, however, their absorption maximum occurs at a much shorter wavelength since the aromatic rings are isolated by the heteroatom, and therefore not conjugated. 

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