Triarylsulfonium salts structures

The evolution of nitrogen on photolysis of the aryIdiazonium salts appears to have limited the use of these systems to thin film applications such as container coatings and photoresists (23). Other efficient photoinitiators that do not produce highly volatile products have been disclosed (24-27). These systems are based on the photolysis of diaryliodonium and triarylsulfonium salts. Structures I and II, respectively. These salts are highly thermally stable salts that upon irradiation liberate strong Bronsted acids of the HX type (Reactions 43 and 44) that subsequently initiate cationic polymerization of the oxirane rings ... 

Bowden and L.F. Thompson, Electron irradiation of poly(olefin sulfones) Application to electron beam resists, J. Electrochem. Soc. 120, 1722 (1973) Poly(Styrene sulfone) A sensitive ion millahle positive electron beam resist, J. Electrochem Soc. 121, 1620 (1974) D.R. McKean, U.P. Schaedeli, and S.A. MacDonald, Acid photogeneration from sulfonium salts in solid polymer matrices, J. Polym. Set Polym. Chem. Ed. 27, 3927 (1989) D.R. McKean, U.P. Schaedeli, P.H. Kasai, and S.A. MacDonald, The effect of polymer structure on the efficiency of acid generation from triarylsulfonium salts, J. Polym. Sci. Polym. Chem. Ed. 29, 309 (1991). 

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. 

Based on the above, an initiating composition for cationic photopolymerization, with visible and long-wavelength UV light was described by Crivello et al. The structure of the monomers plays a key role in these photosensitization processes. Useful aromatic ketones are camphoquinone, benzyl, 2-isopropylthioxanthone, or 2-ethylanthraquinone. The monomer-bound radicals reduce diaryliodonium salts or dialkyl phenacylsulfonium salts rapidly to form monomer-centered cations. These cations then initiate the polymerization of epoxides, vinyl ethers, and heterocyclic compounds. Onium salts with high reduction potential, however, such as triarylsulfonium salts, do not undergo this reaction. 

Nevertheless, syntheses of several diaryliodonium and triarylsulfonium salts that incorporate long wavelength absorbing chromophores have been successfully accomplished and their structures 7-9 are presented below. The replacement of one of the phenyl rings in a diphenyliodonium salt with a fluorenone moiety as shown in structure 7 results in the generation of a diaryliodonium salt with two absorption bands at 294 and 378 nm [FOU 94, HAR 01]. Both the absorption and the fluorescence spectra of 7 closely resemble those of fluorenone. Unfortunately, despite the long wavelength absorption of 7, photopolymerization studies showed that it was not more efficient as a photoinitiator than simple diphenyliodonium salts that do not possess the fluorenone chromophore. 

Employing both old and newly developed synthetic methods, the preparation of triarylsulfonium salts with a wide variation in their structures has been achieved. Table 3 lists a sampling of some of these photoinitiators along with the appropriate literature references. Identical considerations with respect to the tailoring of the structures of the cations and anions for specific applications as were discussed for diarylio-donium salts (Table 2) apply to triarylsulfonium salt cationic photoinitiators as well. 

Table 3 Structures of triarylsulfonium salt cationic photoinitiators...

In Table 2 are shown the structures of some representative triarylsulfonium salts. Unsubstituted triphenylsulfonium salts have their major absorption band near 230 nm. 

The above schemes amply demonstrate how the structure of triarylsulfonium salts can be manipulated to produce photoactive absorption bands in the mid region of the ultraviolet spectrum. Using similar synthetic techniques, it should be possible to purposefully design photoinitiators which respond to any given wavelength in the ultraviolet spectrum and into the visible region of the electromagnetic spectrum. 

In addition to the control which can be exercised over photoinitiated cationic polymerizations by manipulation of the structures of the diaryliodonium and triarylsulfonium salts, there are a number of additional factors which also influence these polymerizations. First, the emission spectrum of the irradiation source must be matched as closely as possible to the absorption characteristics of the specific photoinitiator. Fortunately, today there are available commercial light sources which provide intense bands in specific areas of any portion of the ultraviolet spectrum. Since the rate of photolysis of the photoinitiators varies as the first power of the light intensity, a simple doubling of the light intensity doubles the rate of photolysis of the photoinitiator. In practice it has also been observed that the rate of the polymerization of epoxy containing monomers is also doubled by a two fold increase in the light intensity... 

As in the case of the diaryliodonium and triarylsulfonium salts, photoinitiated cationic polymerizations employing dialkylphenacylsulfonium and dialkyl-4-hydroxy-phenylsulfonium salts exhibit a marked dependency on the structures of both the cation and anion portions of the photoinitiator. Here also, the most efficient photoinitiators are those bearing anions having the poorest nucleophilic character possible i.e. BF4, PF, AsFg, and SbF. ... 

Of these, the diaryliodonium, triarylsulfonium, and ferrocenium salts are most practical and most often employed. Shown in (21a) to (21c) are the structures of typical members of these classes of compounds. 

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