Ferrocenium 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. 

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. 

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... 

In the (keto)coumarin/amine/ferrocenium salt system, the ferrocenium salt plays a crucial role that is rather complex. In a three-component photoinitiator system [238,239] consisting of a coumarin, an iron arene complex such as CpFe +Ar and a phenylglycine derivative as an amine, the first step of the photoreaction occurs between the dye and the complex according to an electron process. The amine reacts with the radical (created on the complex) through hydrogen abstraction. Therefore, no detrimental ketyl radicals are formed. 

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. 

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. 

By virtue of their absorption characteristics, many of the compounds listed in Table 10.4 can be employed in conjunction with visible light sources. As the research in organometallic chemistry gained momentum, the potential advantages of organometallic complexes as photoinitiators were also explored, and two such compounds, a ferrocenium salt and a titanocene, were commercialized (see Chart 10.3). 

Fouassier et al.f reported a four component photoinitiating system that consists of a photosensitizer, Rose Bengal, ferrocenium salt, an amine and a hydroperoxide, such as cumene hydroperoxide ... 

Also, it was found that hexaarylbisimidazole will initiate polymerizations as a result of irradiation with visible light. The same is true of bisacylphosphine oxde. Other eompounds are ketocoumarins that are efficient photoinitiators for acrylic and methacrylic monomers in the presences of amines, phenoxy acetic acid, and alkoxy pyridinium salts. It was also shown that free-radical initiation is possible through visible light decomposition of ferrocenium salts in a three component composition, combined with either a hydroperoxide or a epoxide, and a third ingredient, a dicyanobutadiene derivative ... 

Three-component visible light photoinitiator systems exhibit faster polymerization rates than those seen in the dye-electron donor systems mentioned above. The third component is usually a sulfonium or iodonium salt (an example is diphenyliodonium chloride) (12), but may also be a bromocompound, ferrocenium salt, or thiol derivative (3,12,21-24). 

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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