Triarylsulfonium salts, photolysis

Triarylbismuth dilahides, 4 31-33 Triarylbismuthines, 4 26-28 Triaryl carbonium dyes, 9 259-261 Triaryl phosphates, 11 493-494 Triarylpyrazolines, 19 113 Triarylstibines, 3 69 Triarylsulfonium salts, photolysis of,... 

Lewis acids such as BF3 and SbCl5, almost always in conjuction with water or some other protogen, initiate polymerization of cyclic ethers. The initiator and coinitiator form an initiator-coinitiator complex [e.g., BF3 H20, H+(SbCl6) ], which acts as a proton donor (Sec. 5-2a-2). Cationic photopolymerizations are achieved when similar proton donors are formed by the photolysis of diaryliodonium and triarylsulfonium salts (Sec. 5-2a-4). 

Photoinitiators which produce species-inducing cationic polymerization of alkenes and heterocycles are usually based on diarylhalonium salts and triarylsulfonium salts [203]. The mechanism of the direct photolysis of these salts is quite complex and involves formation of excited singlet species which can reversibly convert by intersystem crossing (ISC) to the... 

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. 

Although triarylsulfonium salts are highly thermally stable, they undergo rapid photolysis when irradiated at wavelengths from 200-300nm. Under these conditions, efficient ( 313,360 " 0.17-0.19) homolytic rupture of one of the carbon sulfur bonds results. The following mechanism has been proposed for this photolysis (12). 

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. 

Typical photosensitizers for diaryliodonium salts are condensed ring aromatic hydrocarbons, diaryl ketones, and acridinium dyes. Condensed ring aromatic hydrocarbons are particularly effective photosensitizers for triarylsulfonium salts. The use of photosensitizers in onium salt photolysis permits the photoimaging processes induced by these compounds to be optimally fitted to the specific irradiation source used for their exposure. 

Cationic Polymerization. Protonic acids such as HBF, HPFg, HSbFg, etc., derived from the photolysis of onium salts I-III are well known initiators of cationic polymerization (9). In equations 13-15 is shown the proposed mechanism of cationic polymerization using a triarylsulfonium salt and a typical monomer, M. 

Simultaneous Radical and Acid Catalyzed Condensation Polymerization. As shown in Equations 1-7, the photolysis of diaryliodonium and triarylsulfonium salts produces in addition to strong protonic acids, a variety of radical fragments. These photoinitiators are, therefore, capable of initiating free radical polymerizations. A number of hybrid imaging systems which take advantage of both radical and acidic species formed from the photolysis of these salts have been designed. For example, Equation 26 illustrates one such system based on simultaneous radical and acid catalyzed condensation polymerizations which has been explored in our laboratory. 

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... 

Wu et al. in 1988 [43], discussed the synthesis of triarylsulfonium salts by the photolysis of a diaryliodonium salt in the presence of diphenyl sulfide. They explained the reaction using an electron transfer mechanism (see below). 

Photolysis of 4-phenylthio-substituted triarylsulfonium salts, on the other hand, proceeds through a triplet state, unlike most other triarylsulfonium cations, hence only a homolytic pathway is available for this reaction [82b]. 

Under conditions of nanosecond laser-flash photolysis, a long-lived transient absorption assigned to VI, A 465 nm, analogous to V, the intermediate proposed in formation of iodobiphenyls from iodonium salts, is observed [83], The diphenylsulfinyl radical cation, /Lmai340, 750 nm, is similarly observed in acetone sensitized laser-flash photolysis. Photo-CIDNP observations, namely emissive polarization for benzene, suggest that the homolytic cleavage pathway also operates under conditions of direct photolysis, accounting, in part, for the diphenylsulfide product [83], as well as the photochemistry of polymeric triarylsulfonium salts [81] (see above). 

Dektar and Hacker have studied the sensitized photolysis of sulfonium salts extensively [70,83]. Like diphenyliodonium cation, triphenylsulfonium is reduced by anthracene singlet to triphenylsulfur radical which cleaves incage to yield phenyl radical and diphenylsulfide [91]. Naphthalene sensitized photolysis of triarylsulfonium salts yielded some of the same photoproducts observed in direct photolysis, namely arylated diarylsulfides [94a] ... 

Abstraction of labile hydrogen atoms, for example, from tetrahydrofuran (THF), by photoexcited ketones also yields easily oxidized radicals [60a]. Quantum yields for benzophenone sensitized photolysis of diphenyliodo-nium hexafluoroarsenate in acetonitrile with hydrogen atom donors are shown in Table 9. Diaryliodonium salts are capable of oxidizing electron rich radicals from isopropanol and THF but triarylsulfonium salts are not [96]. The oxidation potential of tetrahydrofuranyl radical is —0.35 V versus SCE [109], apparently sufficient for irreversible reduction of diphenyliodonium cation ( red = approx. —0.7 V versus SCE, see above) but not of triphenyl-sulfonium cation ( red = —1.2V versus SCE). 

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. 

Photolysis of appropriate combinations of the mono- and bifunctional monomers, DOP and BDOP in the presence of diaryliodonium or triarylsulfonium salts yields expanded polymer networks [126]. The resulting cross-linked polymers have suitable Tg values for desired applications. 

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 ... 

CIDNP spectroscopy has been applied to both direct and sensitized photoreactions of onium salts (diaryliodonium and triarylsulfonium salts) [134] see Chart XVIII. In a recent investigation [134f] ambiguities of earlier studies on sensitized photolysis of iodonium salts [134a, 134b], where S-T -type polarizations had to be postulated to rationalize the polariz-... 

Knapzyck and W.E. McEwen, Photolysis of triarylsulfonium salts in alcohol, J. Org. Chem. 

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. 

Attempts to quench the photolysis of diaryliodonium and triarylsulfonium salts using various triplet quenchers have failed suggesting that cleavage of these compounds occurs from the excited singlet state. The photolysis rate of diaryliodonium salts has been shown to be solvent dependent (17rl8). In alcohols and ethersr higher quantum... 

The photolysis of diaryliodonium and triarylsulfonium salts may proceed via formation of a radical cation, which abstracts a hydrogen atom from a suitable donor. 

The protonic acid needed to initiate the chain reaction is usually produced by UV-irradiation of diaryhodonium or triarylsulfonium salts. The photolysis reaction, which produces both Bronsted acid and free radicals in the presence of a hydrogen donor molecule, can be formally written as follows for the iodonium salt [55] ... 

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. 

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