Dialkylphenacylsulfonium salts photoinitiators

J.V. Crivello and S.Q. Kong, Synthesis and characterization of second-generation dialkylphenacylsulfonium salt photoinitiators. Macromolecules 2000, 33(3), 825-832. 

Dialkylphenacylsulfonium salts are also known to be highly photosensitive [3b, c] and have been adapted to photoinitiated cationic polymerization [9]. These salts have a higher intrinsic quantum yield for photoinitiation of cationic polymerization on direct photolysis than the simple aryl or aryl-alkyl onium salts [10], but do not appear to undergo the same photodecomposition as the other onium salts [1-4]. 

J.V. Crivello and J.L. Lee, Structural and mechanistic studies on the photolysis of dialkylphenacylsulfonium salt cationic photoinitiators. Macromolecules 1983, 16(6), 864-870. 

However, unlike the triaryIsulfonium salts, these compounds undergo reversible photoinduced ylid formation rather than homolytic carbon-sulfur bond cleavage. Because the rate of the thermal back reaction is appreciable at room temperature, only those monomers that are more nucleophilic than the ylid will polymerize. Epoxides, vinyl ethers, and cyclic acetals undergo facile cationic polymerization when irradiated in the presence of dialkylphenacylsulfonium salts as photoinitiators. 

Crivello and J.FI.W. Lam,. Polym. Set Polym. Lett. Ed. 17, 759 (1979) J.V. Crivello and J.L. Lee, Photosensitized cationic polymerizations using dialkylphenacylsulfonium and dialkyl(4 hydro xyphenyl)sulfonium salt photoinitiators, Macromolecules, 14, 1141 (1981) S.P. Pappas, Photo generation of acid Part 6 A review of basic principles for resist imaging applications, J. Imaging Technol. 11, 146 (1985) J.L. Dektar and N.P. Hacker, Triphenylsulfonium salt photochemistry. New evidence for triplet excited state reactions, J. Org. Chem., 53, (1988) J.L. Dektar and N.P. Hacker, Photochemistry of triarylsulfonium salts, J. Am. Chem. Soc. 112, 6004 (1990) G. Pohlers, J.C. Sciano, R.F. Sinta, R. Brainard, and D. Pai, Mechanistic studies of photoacid gen eration from substituted 4,6 bis(trichloromethyl) 1,3,5 triazines, Chem. Mater. 9, 1353 (1997). 

The major portion of the article will be devoted to a discussion of the synthesis and mechanistic aspects of photoinitiation by individual photoinitiator systems. Cationic polymerizations induced by these photoinitiators are dark, i.e., non-photo-chemical, processes which are governed by the same parameters which must be taken into account in polymerizations which occur in the presence of conventional initiators such as Lewis and Bronsted acids. Accordingly, cationic polymerizations induced by halogen and sulfur based onium salt photoinitiators will be discussed only from the context in which they are influenced by factors which have their origin in the photoinitiator. Since the photochemistry of diaryliodonium and triarylsulfonium salts is similar, these two types of photoinitiators will be discussed together. The photolysis of dialkylphenacylsulfonium and dialkyl-4-hydroxyphenylsulfonium salts proceeds by a different mechanism, and they will be discussed separately. 

Dialkylphenacylsulfonium salts (111) and dialkyl-4-hydroxyphenylsulfonium (IV) salts have recently been described as useful photoinitiators for cationic polymerization 1 2-123)... 

In contrast to the reversibility of the direct photolysis of dialkylphenacylsulfonium salts, photosensitized photolysis results in fragmentation of the photoinitiator (Scheme 11), Thus, in this latter case, the initiating species, the photosensitizer cation-radical is formed by an irreversible process. Photosensitization of dialkylphenacylsulfonium salts would, therefore, be expected to result in more efficient initiation of cationic polymerization. Indeed, the polymerizations of THF, E-caprolactone, and N-vinyl-2-pyrrolidone proceed smoothly under photosensitized conditions. 

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. ... 

Photoinitiation of radical polymerization has long been known. Recently, a group of photoinitiators for cationic polymerization hase been discovered and developed by Crivello et al.1J. They include diaryliodonium (7),2) triarylsulfonium (2), 3 5) dialkylphenacylsulfonium (J), 6) and dialkyl-4-hydroxyphenylsulfonium salts (4) 7). 

Our research group at General Electric, along with similar groups at 3M Company and ICI, have been successful in developing several new types of very active photoinitiators for cationic polymerization, i.e., diaryliodonium (1-5), triarylsulfonium (6-9), triaryl-selenonium (10), dialkylphenacylsulfonium (11), and dialkylhydroxyphenylsulfonium salts (12). The most practical of these photoinitiators are diaryliodonium salts, I, and triarylsulfonium salts, II. 

As in the case of other sulfonium salts, the above simple halide-containing salts are not directly suitable for use as photoinitiators because of the tendency of these anions to act as terminating agents n in cationic polymerizations. Direct metathesis of the sulfonium halides with the appropriate acid or alkali metal salt of such weakly nucleophilic anions as PF, and SbFg, and AsFg" suffices to convert them to the active photoinitiators. Tables 6 and 7 give the structures and UV absorption characteristics of some representative dialkylphenacylsulfonium and dialkyl-4-hydroxyphenyl-... 

Irradiation of dialkylphenacylsulfonium and dialkyl-4-hydroxyphenylsulfonium salts at wavelengths from 190 to 360 nm in the presence of suitable monomers results in facile cationic polymerization. Shown in the following scheme is the mechanism which has been proposed for photoinitiation using these onium salts The first step involves the reversible photogeneration of the respective ylides and the solvated acid In the following nonphotochemical or dark step, initiation takes place by protonation of a monomer molecule by HX,s, to generate a carbenium or onium species. In the case where the solvent is the monomer, photolysis and protona-... 

The spectral response of both dialkylphenacylsulfonium and dialkyl-4-hydroxy-phenylsulfonium salts can be broadened through the use of photosensitizers A separate mechanism is involved in the photosensitization of each of these two photoinitiators. 

The appearance of sulfonium salts as end groups in polymers produced using these photoinitiators supports this conclusion Among those monomers which undergo facile polymerization with dialkylphenacylsulfonium and dialkyl-4-hydroxyphenyl-sulfonium salts include oxiranes, oxetanes, thiiranes, vinyl ethers and s-trioxane. Polymerizations of tetrahydrofuran, e-caprolactone, a-methylstyrene, and N-vinyl-2-pyrrolidone are very sluggish and often fail using these photoinitiators. 

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