Triaryl sulfonium salts

Fig. 9. Initiation of epoxy cure. Irradiation of a triaryl sulfonium salt produces a radical cation that reacts with an organic substrate RH to produce a cation capable of releasing a proton. The proton initiates ring-opening polymerization. X = BF , PFg, AsFg, and SgFg. ...

Use of a Difunctional Crosslinker. An alternate approach to chemically amplified imaging through electrophilic aromatic substitution is shown in Figure 6 below. In this approach a polyfunctional low molecular weight latent electrophile is used in a three component system also including a photoactive triaryl sulfonium salt and a phenolic polymer. In this case again crosslinking of the polymer is observed upon... 

Figure 7. Example calculation of AG. Free-energy calculations ( G) show that electron transfer from triplet excited thioxanthane to the diaryliodonium salt is exothermic, and therefore, sensitization is observed. In contrast, electron transfer from triplet excited thioxanthane to the triaryl-sulfonium salt is endothermic thus, sensitization is not observed.

The bulky anion then stabilizes the intermediate adduct from protonation of the epoxy group and then facilitates insertion of epoxide at the cationic propagation site. Rapid polymerization can then occur. Cationic photopolymerization of epoxides often involves the photo-generation of acid from an initiator such as diaryliodonium or triaryl sulfonium salts (Crivello, 1999). The anions are important in controlling the addition at the cationic site and are typically BF4 and PFg. The reactivity of the system depends also on the structure of the epoxide. 

Inhibition of the polymerization by air was controlled by use of 1 1 diacrylate-poly (methyl methacrylate) mixtures or by using the acrylate in a laminate between two glass plates. These cost-effective cures use no external energy and no lamps. Composites of glass fiber and epoxides from vegetable oils have been cured in sunlight with di-aryliodinium and triaryl sulfonium salts in 25 min.311 Further work is needed to speed up this photocationic cure. 

However, Trost et ah treated triaryl sulfonium salts with aryl lithiums to obtain only diaryl sulfides and diaryls, via sulfuranes [38]. These two contradictory results obtained by different groups have not been resolved. 

Sulfonium Salts Triaryl sulfonium salts (TPSs) are generally produced by the method of Pitt [23]—a Friedel-Crafts condensation of aromatic hydrocarbons with... 

It should be pointed out that the photolysis of the diaryliodonium and triaryl-sulfonium salts proceeds differently from the photolysis of the diazonium systems. The reaction mechanism of the photolysis of iodonium salts is described by Reactions [6.46] and [6.47]. 

Diazonaphthoquinones Nitrobenzylsulfonates Triaryl sulfonium salts (O3 S+ SbFg) Diphenyliodonium salts Polymer types... 

Table 1. Structure and Spectral Properties of Triaryl-sulfonium Salts...

The decomposition mechanism of the triaryl sulfonium salt is believed to take place somewhat similarly... 

This modifieation does extend the sensitivity to longer wave length ultraviolet light. The eompounds were reported to display exeellent thermal lateney in the presence of various monomer systems and high effieieney as photoinitiators for cationic polymerization. Furthermore, their initiation efficiency was reported to be on par with current commercial triaryl sulfonium salts. 

Shim et al., reported preparation of several novel mono- and di-propenyl ethers by condensation of perfluoroalkyl alcohol with allyl bromide followed by the ruthenium-catalyzed isomerization of the corresponding allyl ethers. These fluorinated propenyl ethers undergo rapid photoinitiated cationic polymerization with the aid of triaryl-sulfonium salt bearing a long alkoxy... 

Diaryl iodonium and triaryl sulfonium salts are among the most versatile photoinitiators. These compounds have been thoroughly investigated by Crivello... 

Photochemical doping involves treating a CP with a dopant which is initially inert but rendered an active dopant by irradiation. These dopants are usable with common CPs. Examples are diphenyliodonium hexafluoroarsenate (in CH2CI2) or triaryl-sulfonium salts (in aqueous medium), both of which are rendered active by UV radiation [125, 126]. 

In all cases, these photoinitiators were less efficient than diaryliodonium and triaryl-sulfonium salts under comparable conditions. To date, there has been no report of a study of the mechanism of photolysis of these compounds nor have the quantum yields been determined. Since thiopyrylium salts are colored compounds, they may be useful photoinitiators in the visible region of the spectrum. The related thio-xanthylium salts were also examined as potential photoinitiators but were too hydrolytically unstable to be useful... 

---