Diaryliodonium salts cationic polymerization

The initial development of diaryliodonium salt cationic photoinitiators was quickly followed by the nearly parallel discovery of triarylsulfonium salts as a second general class of highly efficient and thermally stable cationic photoinitiators. Along with triarylsulfonium salts, 77, their S-aryl sulfiir-heterocyclic analogs display good photosensitivity and function well in photoinitiated cationic polymerizations. 

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... 

Crivello (lib) has described a series of diaryliodonium (107) and triarylsulfonium salts (108) which function as initiators for ultraviolet activated cationic polymerization. The... 

Several dyes have been found to sensitize the cationic polymerization of cyclohexene oxide, epichlorohydrin, and 2-chloroethyl vinyl ether initiated by diaryliodonium salts (109,110). Acridinium dyes such as acridine orange and acridine yellow were found to be effective sensitizers. One example of a benzothiazolium dye (setoflavin T) was also reported, but no other class of dye nor any other example of a dye absorbing at longer wavelengths were discovered. Crivello and Lam favored a sensitization mechanism in which direct energy transfer from the dye to the diaryliodonium salt occurred. Pappas (12,106) provided evidence that both energy transfer and electron transfer sensitization were feasible in this system. 

Photo-initiators for cationic polymerization may be classified in four groups the diazonium salts, the diaryliodonium salts, the triarylsulphonium salts, and the mixed ligand arene cyclopentadienyl Fe11 salts. 

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). 

Crivello and Lam [69] have reported that the diaryliodonium salt-ascorbate redox system readily initiates the cationic polymerization of appropriate monomers. N-Alkoxy pyridinium salts were also shown [70] to participate in this redox process. The polymerization mechanism depicted below is quite similar to that described for the iodonium salts (Scheme 17). 

The caged species may escape geminate recombination and produce various species that can initiate cationic polymerization. Solvent (RH) often participates in these reactions producing protonic acids. As shown in Eq. (44), protonic acids are also formed by reaction of radical cations with aryl radicals or by Friedel-Crafts arylation. Up to 70% of the protonic acid is formed upon photolysis of diaryliodonium salts [205]. In addition to initiation by protons, arenium cations and haloarene radical cations can react directly with monomer. The efficiency of these salts as cationic initiators depends strongly on the counterions. Those with complex anions such as hexafluoroantimonate, hexafluorophosphate, and triflate are the most efficient. 

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. 

Figure 4, Photolysis of diaryliodonium salts having nonnucleophilic anions results in the formation of strong acids (HPFe). Nonnucleophilic acids such as HPFe can initiate cationic polymerization of vinyl ethers and cyclic ethers.

Figure 5. Sensitization of diaryliodonium salts via electron transfer results in formation of the sensitizer radical cation (S ). Hydrogen abstraction produces a proton. Both species may initiate cationic polymerization. Note The nonnucleo-philic anion (AsFo ) has been deleted for clarity.

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. 

J.V. Crivello and J.L. Lee, The synthesis and characterization of polymer-hound diaryliodonium salts and their use in photo and thermally initiated cationic polymerization. Polym. Bull. 1986, 16(4), 243-248. 

Photoinitiators for Cationic Polymerization. Recently a class of photoinitiators for cationic polymerization was discovered by Crivello et al. (55). This class includes diaryliodonium (Structure I) (56. 57). triaryIsulfonium (Structure II) (58-62). dialkyIphenacylsulfonium (Structure III) (63). dialkyl-4-hydroxy-phenylsulfonium salts (Structure IV) (64). and triaryIselenonium... 

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 ... 

The origin of our interest in photoinitiated cationic polymerization began with the discovery that certain onium sidts, namely, diaryliodonium (I) and triaiyls onium (II) salts, could rapidly and efficiently photoinitiate the polymerization of virtually all types of cationically polymerizable monomos (2-4). 

Several examples of S-arylation of sulfides and P-arylation of phosphines using diaryliodonium salts were reported in the older literature [877,878]. These reactions generally proceed by a radical chain mechanism. The arylation of phosphines has been used to promote the photo-initiation of cationic polymerization [879]. More recently, the synthesis of diaryl sulfones via S-arylation of sodium arenesulfinates, ArS02Na, by diaryliodonium salts has been reported [880]. 

Diaryliodonium salts are widely used as photoinitiators for cationic photopolymerizations [17-20]. Photoini-tiated cationic polymerization is of great practical interest due to its applicability for the curing of coatings and printing inks and for photoresist technology used in lithography [19,20]. General synthetic methods, properties and photochemistry of diaryliodonium salts as photoinitiators were reviewed by Crivello in 1984 [18]. 

In the same groundbreaking paper [21], CriveUo and coworkers also demonstrated that the anion plays no role in determining the photosensitivity of the iodonium salt and the photolysis rates of diaryliodonium salts having the same cations but different non-nucleophilic counterions (BF4, PFs, AsFs", or SbFe ) are identical. Likewise, the cation structure has little effect on the photodecomposition of diaryliodonium salts. The utility of iodonium salts as photoinitiators has been demonstrated in several cationic polymerizations using olefins, epoxides, cycUe ethers, lactones and cyclic sulfides as the monomers [21],... 

The photoinitiators for cationic polymerizations are mainly of two types. One is based on aryldiazonium salts and the other on diaryliodonium, triarylsulfonium, or triaiylselenonium salts. The diazonium salts photodecompose to yield both Lewis acids and strong protonic acids ... 

Fig. 3. Photoinitiated Cationic Polymerization using Diaryliodonium Salt...

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. 

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. 

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