Photoinitiated cationic polymerization nucleophilic anions

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. ... 

Compounds with a nucleophilic sulphur are readily arylated by iodonium salts in their anionic form, and less readily in other cases, notably in sulphides triaryl sulphonium salts are useful photoinitiators in cationic polymerizations. A list of chalcogen compounds arylated by iodonium salts appears in Table 8.7. 

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],... 

Cationic polymerizations proceed as chain reactions involving initiation and propagation. In most cases, there is no termination by neutralization, and the growing chains are terminated by nucleophilic impurities. Cationic chain reactions are photoinitiated by special initiators (Table 3.10) which date back to the pioneering studies of Crivello on sulfonium and iodonium salts containing non-nucleophilic anions such as and AsFs ... 

For reasons which will be detailed later, the simple diaryliodonium bisulfate and halide salts produced in the above synthetic procedures have only very limited use as photoinitiators in cationic polymerization. More generally applicable photoinitiators are obtained by converting the bisulfate and halide salts to those possessing anions of low nucleophilic character such as CF3SO3, BF, PF, AsF, or CIO4. This may be achieved by using one of the three metathetical methods shown in Eqs. 

As in the case of the analogous diaryliodonium salts, only those triarylsulfonium salts bearing anions of low nucleophilic character are broadly applicable as photoinitiators for cationic polymerization. Triarylsulfonium halides and bisulfates undergo facile... 

As in the case of other sulfonium salts, the above simple halide-containing salts are not directly suitable for use as photoinitiators because of the tendency of these anions to act as terminating agents n in cationic polymerizations. Direct metathesis of the sulfonium halides with the appropriate acid or alkali metal salt of such weakly nucleophilic anions as PF, and SbFg, and AsFg" suffices to convert them to the active photoinitiators. Tables 6 and 7 give the structures and UV absorption characteristics of some representative dialkylphenacylsulfonium and dialkyl-4-hydroxyphenyl-... 

A large number of cationic photoinitiators are known. The most significant from the commercial point of view are aryldiazonium, diaryliodon-ium, triarylsulfonium, and ferrocenium salts. These salts possess anions of very low nucleophilicity which do not terminate the polymerization process. Nonionic cationic photoinitiators such as organosilanes, latent sulfonic acids, and some other miscellaneous compounds are also used. 

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