N-alkoxy pyridinium salt

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). 

Synthesis of Block Copolymers by Using N-Alkoxy Pyridinium Salts... 

N-Alkoxy Pyridinium Salts /V-Alkoxy Pyridinium salts are obtained with relatively high yields by a reaction of pyridine /V-oxides with a triethyloxonium salt in methylene chloride or chloroform [13]. Quinolinium salts can also be prepared from the corresponding /V-oxides [40], In both cases, an anion exchange is not necessary since the triethyl oxonium salt is available with nonnucleophilic counter anions. The most frequently used photoinitiators of this type are shown in Chart 11.8. The spectral response of these salts is in 260-310 nm range [13],... 

As mentioned previously, the value determined by polarography for N-alkoxy pyridinium salts is of the order of-16 kcal/mol. This indicates that N-alkoxypyridinium salts would be expected to undergo facile reduction by photoexcited electron-rich compounds. Indeed, electron-transfer photosensitization by typical excited state electron donors such as anthracene, perylene, thioxanthene [YAG 93] and trimethoxybenzene [YAG 93] has been reported. 

YAG 97] Yagci Y., Endo T., N-benzyl and N-alkoxy pyridinium salts as thermal and photochemical initiators for cationic polymerization , Polymer Synthesis/Polymer Catalysis, Springer, Berhn Heidelberg, vol. 127, pp. 59-86, 1997. 

Cationic polymerizations induced by thermally and photochemically latent N-benzyl and IV-alkoxy pyridinium salts, respectively, are reviewed. IV-Benzyl pyridinium salts with a wide range of substituents of phenyl, benzylic carbon and pyridine moiety act as thermally latent catalysts to initiate the cationic polymerization of various monomers. Their initiation activities were evaluated with the emphasis on the structure-activity relationship. The mechanisms of photoinitiation by direct and indirect sensitization of IV-alkoxy pyridinium salts are presented. The indirect action can be based on electron transfer reactions between pyridinium salt and (a) photochemically generated free radicals, (b) photoexcited sensitizer, and (c) electron rich compounds in the photoexcited charge transfer complexes. IV-Alkoxy pyridinium salts also participate in ascorbate assisted redox reactions to generate reactive species capable of initiating cationic polymerization. The application of pyridinium salts to the synthesis of block copolymers of monomers polymerizable with different mechanisms are described. 

Reaction of pyridine-N-oxides with triethyloxonium salt in methylene chloride or chloroform gives directly AT-alkoxy pyridinium salts with high yield. Quinolinium salts can also be prepared from the corresponding N-oxides [46], The synthetic procedure does not require the anion exchange reaction since triethyl oxonium salt possesses a non-nucleophilic counter anion, i.e. PFg ... 

N-Benzyl and iV-alkoxy pyridinium salts are suitable thermal and photochemical initiators for cationic polymerization, respectively. Attractive features of these salts are the concept of latency, easy synthetic procedures, their chemical stability and ease of handling owing to their low hygroscopicity. Besides their use as initiators, the applications of these salts in polymer synthesis are of interest. As shown in this article, a wide range of block and graft copolymer built from monomers with different chemical natures are accessible through their latency. 

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