Ferrocenium cationic photoinitiators

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. 

Attention was given to the synthesis of bulky borate anions that seemed to display the required properties. In particular, the tetrakis(pentafluorophenyl)borate anion has focused our interest. This compound is a very stable, crystalline compound and is insensitive to air or moisture. We have found that certain of these salts, as will be described further in this paper, give excellent cationic photoinitiators when associated with a diaryliodonium cation, a triarylsulfonium cation or a ferrocenium cation... 

T. Wang, B.S. Li, and L.X. Zhang, Carbazole-bound ferrocenium salt as an efficient cationic photoinitiator for epoxy polymerization. Polym. Int. 2005, 54(9), 1251-1255. 

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. 

Iron Arene Complexes-Based Photoinitiators Iron arene complexes or ferrocenium salts are attractive photoinitiators for cationic polymerization of... 

T. Wang, et al., Several ferrocenium salts as efficient photoinitiators and thermal initiators for cationic epoxy polymerization. J. Photochem. Photobiol. A Chem. 2007, 187(2-3), 389-394. 

This is also probably the case with the other classes of photoinitiators, with the exception of the diazonium and ferrocenium salts. The photolysis of diazonium salts is well known to generate Lewis acids which initiate polymerization either by themselves or in combination with a protogen. Similarly, the photolysis of ferrocenium salts proportedly proceeds by the mechanism shown in equation (24), which involves the formation of the Lewis acid shown. Apparently, this iron-containing Lewis acid is strong enough to initiate many types of cationic polymerization, including those of vinyl ethers. 

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