Diaryliodonium and Triarylsulfonium Salt Photoinitiators

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. ... 

Diaryliodonium and triarylsulfonium salts act as photoinitiators of cationic polymerization. Photolytic celeavage of an Ar—I or Ar—S bond yields a radical-cation (Eq. 5-8) that reacts with HY to yield an initiator-coinitiator complex that acts as a proton donor to initiate... 

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... 

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. 

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 ... 

When photoinitiated cationic polymerizations employing diaryliodonium and triarylsulfonium salts are carried out under the usual laboratory "dry conditions, the chief initiators of polymerization are protonic acids. The general mechanism by which these salts initiate polymerization is shown as follows using triarylsulfonium salts as an example ... 

Many other electron-rich polynuclear aromatic compounds also act as efficient electron-transfer photosensitizers for both diaryliodonium and triarylsulfonium salt cationic photoinitiators. Examples are pol5muclear aromatic compounds like pyrene, and perylene. The drawback, however, to using... 

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. 

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. 

A final piece of evidence for the formation of radical species, i.e., aryl radicals and cation-radicals, in the photolysis of diaryliodonium and triarylsulfonium salts is the ability of these onium salts to photoinitiate typical radical polymerizations Indeed, using such photoinitiators, it is uniquely possible to simultaneously initiate both cationic and free radical polymerizations. 

Photoinitiated Cationic Polymerization Using Diaryliodonium and Triarylsulfonium Salts... 

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... 

Two other patents have also appeared describing the use of alternate free radical sources as reducing agents for diaryliodonium and triarylsulfonium salts Crivello and his coworkers have discovered several other types of reactions which can be used to thermally induce cationic polymerization using onium salt photoinitiators. Diaryliodonium salts are thermally decomposed in the presence of catalytic amounts of copper compounds An organocopper compound is proposed as an intermediate which undergoes electrophilic arylation of cationally polymerizable monomers as shown in Scheme 14 to initiate polymerization. 

Commercially important photoinitiators for crosslinking by cationic polymerization are onium salts, particularly the thermally and hydrolytically stable diaryliodonium and triarylsulfonium salts,and mixed ligand arene cyclopentadienyl metal salts with complex metal halide anions. Direct and sensitized photolysis of these systems have been reviewed " and are discussed in detail in Volume 4, Chapter 20. 

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. 

The cationic photoinitiator triarylsulfonium, diaryliodonium, and related aryl diazonium salts containing nonnucleophilic counterions, were used by Konarski et al. (2) to prepare adhesives. 

Such compounds as polynuclear aromatics, heteroaromatics, ketones, quinones and dyes can serve as donors. Both excited singlet and triplet-states of these products can be involved in the PET. Diaryliodonium salts, triarylsulfonium salts, phosphonium salts, ammonium salts, pyrylium and thiapyrylium salts possess enough thermal stability and corresponding reduction potential to function as electron acceptors (R X+). In order to select suitable photoinitiator systems based on compounds discussed, the Weller-Eq. (5) can be employed. 

Many different photoinitiators based on onium -type compounds with anions of low nucleophilicity also have been described in the literature as effective catalysts for the polymerization of epoxides Thus, diaryliodonium salts diaryliodosyl salts triarylsulfonium salts and related compoundstri-phenylsulfoxonium saltsdialkylphenacylsulfonium salts and dialkyl-4-hydroxyphenylsulfonium salts seem to be most suitable as photoinitiators for epoxy curing. Some of the principles of the reaction mechanism involving these initiators are discussed in detail in the following Sections. Various other onium photoinitiators such as diarylchloronium and diarylbromonium salts , thiopyrylium salts 3), triarylselenonium salts and onium salts of group Va elements >... 

During the past twenty years, development of compounds that efficiently initiate polymerization on irradiation have made possible the development of several new commercially important technologies based on these photoinitiators [1]. Their use in UV curable coatings is particularly notable. The most useful photoinitiators that have been explored to date are radical photoinitiators. Many applications today use this technology, in spite of important drawbacks [2]. The recent development of diaryliodonium, triarylsulfonium and ferrocenium salts as highly efficient photoinitiators for cationic polymerization has generated a new class of fast polymerizations. 

---