Diaryliodonium salts photoinitiators

Diarylide yellow pigments, 74 317 79 433 Diaryliodonium salt photoinitiators, 74 270 Diaryliodonium salts, 79 108 photolysis of, 70 414... 

Cho and Hong (2005) used photodifferential scanning calorimetry to investigate the photocuring kinetics of UV-initiated cationic photopolymerization of 1,4-cyclohexane dimethanol divinyl ether (CHVE) monomer with and without a photosensitizer, 2,4-diethylthioxanthone (DETX) in the presence of a diaryliodonium-salt photoinitiator. Two kinetic parameters, the rate constant (k) and the order of the initiation reaction (m), were determined for the CHVE system using an auto-catalytic kinetics model as shown in the following equation ... 

On irradiation in the presence of an epoxy monomer and a diaryliodonium salt photoinitiator, it has been observed that the initial bright yellow color of curcumin shifts to deep orange. On continued irradiation or simply standing in ambient light, the photopol mer is bleached resulting ultimately in a pale-yellow-colored cross-linked epoxy polymer. 

Table 2 Functions of the cation and anion of a diaryliodonium salt photoinitiator...

Many attempts have been made to find alternatives to the most reactive onium salt photo initiators bearing the AsFe" and SbFg" anions. These efforts have mainly been in response to the perception of toxicity and/or the possibility heavy metal contamination due to arsenic- and antimony-containing residues that are of concern in certain applications. In recent years, there has been considerable progress in the development of novel anions with weakly coordinating (i.e., low nucleophilic) character. For example, Castellanos et al7 have introduced diaryliodonium salt photoinitiators with the tetrakis(pentafluor-ophenyl)borate ((C6F5)4B ) anion. Following closely on this... 

Cationic photoinitiators are frequently found in classes of compounds such as the triaryl sulfonium, tetraaryl phosphonium, and diaryliodonium salts of large protected anions (hexafluorophosphates or antimonates). These compounds are soluble in most epoxy resins, do not activate epoxy cure until exposed to uv light, are insensitive to room lighting, and have long storage life at room temperature. Cationic photoinitiators form an acid catalyst when exposed to uv light and consequently start the cationic chemical reaction. 

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 >... 

For applications, such hybrid systems are limited to either diaryliodonium salts or aryldiazonium photoinitiators and suitable radical initiators. Triphenylsulfonium salts, however, are not active as cocatalysts in the presence of free-radical initiators. 

Diaryliodonium Salts. The use of diaryliodonium salts as photoinitiators for epoxide polymerization was first described in Belgian Patents issued in 1975 and 1976 (, ). Diaryl-... 

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 cure rates by the triarylsulfonium salts are also influenced by the size of the anions. Larger anions give faster cures. The rates are also influenced by the temperatures. However, they reach an optimum. The mechanisms of photoinitiations by triaiylselenonium salts are believed to be generally similar to those by triarylsulfonium and diaryliodonium salts. 

Crivello reported that only diaryliodonium salts were used as catiOTiic photoinitiators in this study. Photo activation was carried out by UV irradiation prior to thermal initiation. Initiation was carried out using an electrically heated wire immersed in the monomer. The velocity of the resulting propagating front is quite high with the temperature of the front reaching 110°C. 

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. 

Another photoinitiating system that is effective in visible light that is based on a combination of a squarylium dye " and diaryliodonium salt was reported ... 

Photoinitiators used for epoxy curing include aryldiazonium salts (ArN2+X ), diaryliodonium salts (Ar2l+X ), and onium salts of Group Via elements, especially salts of positively charged sulfur (ArsS+X ). The anions must be of low nucleophilicity, such as tetrafluoroborates or hexafluorophosphates, to promote pol5uner chain growth rather than chain termination. Upon UV irradiation. 

Figure 6 Photoinitiated cationic polymerisation of an aliphatic dicycloepoxide and a dicycloepoxy silicone. Diaryliodonium salt (1 wt%) photoinitiator (/= 85 mW cm ).

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