Dialkylphenacylsulfonium

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). 

The photolysis of dialkylphenacylsulfonium salts 3 generates an ylid 5 and a strong acid HX (Eq. (13)), the latter being responsible for the initiation of cationic polymerization 6). 

The photolysis of dialkylphenacylsulfonium salts and dialkyl-4-hydroxyphenyl-sulfonium salts is different from that of triphenylsulfonium salts. The latter compounds undergo irreversible photoinduced carbon-sulfur bond cleavage the former compounds, however, react by reversible photodissociation and form resonance-stabilized ylids as shown in Fig. 5. Because of the slow thermally induced reverse reaction, only small equilibrium concentrations of the ylid and acid arc present during irradiation and the concentration will rapidly decrease when photolysis has been terminated. Therefore, in contrast to triarylsulfonium salt initiation, no dark reaction will continue after the irradiation step. 

Dialkylphenacylsulfonium salts (III), in contrast, undergo reversible dissociation on photolysis with generation of ylids and protonic acids (6). 

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

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. 

Irradiation of dialkylphenacylsulfonium salts also produces strong protonic acids (63). 

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. 

Bohme and Krause312 describe the preparation of various dialkylphenacylsulfonium salts, e.g., dimethylphenacylsulfonium bromide co-Bromoacetophenone (9 g) is dissolved in a mixture of water (1 volume) and acetone (20 volumes), and dimethyl sulfide (3 g) is added. After about 0.5 h the sulfonium salt separates as long, colorless needles, m.p. 145° (10.2 g, 85 %). 

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). 

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. 

Two additional classes of sulfonium salt initiators, also introduced by Crivello and Lam, are the dialkylphenacylsulfonium salts (II) (12,13) and the dialkyl-4-hydroxyphenylsulfonium salts (III) (13,14). The initiating species generated upon... 

Pyrenemethanol, 19, is readily soluble in various epoxide monomers and has the advantage that it undergoes acid catalyzed and radical reactions that covalently bind it into the pol5mier matrix as photopolymerization proceeds. Similar considerations apply to compound 20 which, in addition to the reactive benzylic hydroxyl group, has a long alkyl chain that markedly improves solubility in a variety of vinyl and heterocyclic monomers. Like the above anthracene compounds, pyrene and its derivatives 19-21 are active as PSs for diaryliodonium, triarylsulfonium and also dialkylphenacylsulfonium salts. 

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)... 

Dialkylphenacylsulfonium and dialkyl-4-hydroxyphenylsulfonium salts are in general colorless crystalline compounds of well defined character which are insoluble in aqueous media but soluble in many common organic solvents from which they may be purified by crystallization,... 

Dialkylphenacylsulfonium salts can readily be prepared in yields generally above 80% by the method of Bohme and Krause which involves the condensation of phenacyl halides with dialkylsulfides (Eq. (64)). 

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-... 

Several reports of the photolysis of dialkylphenacylsulfonium salts appear in the literature Qjj basis of the results of product studies after long irradiation... 

More recent investigations of the mechanism by Crivello and Lam showed that in addition to the reaction shown above, dialkylphenacylsulfonium salts undergo the even more facile process shown in Scheme 9 involving the formation of a ylide and a protonic acid. 

Table 8. Rate Constants for the Photolysis of Para Substituted Dialkylphenacylsulfonium...

Modifications at other sites in the dialkylphenacylsulfonium salt molecule have less pronounced effects on the photolysis rates. In general, substituents other than hydrogen on the carbon adjacent to the carbonyl tend to slightly accelerate or decelerate the rate due to a balance in steric and electronic effects. Similarly, the highest photolysis rates were obtained when the dialkyl groups were both methyls which conformationally and sterically favor the six member transition state of the Norrish Type II reaction. 

The photochemistry of dialkyl-4-hydroxyphenylsulfonium salts has received little attention in the literature. Recently, on the basis primarily of NMR data and analysis of the photolysis products, Crivello and Lam have proposed the following mechanism for the photolysis of these compounds which is related to that of the dialkylphenacylsulfonium salts. 

The key step in this mechanism involves the photoexcitation of the sulfonium salt which rapidly dissociates to form the resonance-stabilized ylide (V) and the solvated protonic acid. Recombination of these two products to form starting material via a thermal back reaction results in the establishment of a small steady-state concentration of ylide and protonic acid. As in the case of the dialkylphenacylsulfonium salts, solvents or monomers may play an important role in this mechanism by assisting in the removal of a proton from the excited sulfonium salt. 

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