Phosphonium cationic photoinitiators

Cationic photoinitiators are frequently found in classes of compounds such as the triaryl sulfonium, tetraaryl phosphonium, and diaryliodonium salts of large protected anions (hexafluorophosphates or antimonates). These compounds are soluble in most epoxy resins, do not activate epoxy cure until exposed to uv light, are insensitive to room lighting, and have long storage life at room temperature. Cationic photoinitiators form an acid catalyst when exposed to uv light and consequently start the cationic chemical reaction. 

It was demonstrated that the P-centered radical of BAPO and its derivatives is oxidized by cationic photoinitiators (onium salts in their ground state). Phosphonium ion, a BAPO fragment, can efficiently initiate cationic polymerization. Thus, under certain conditions, PI used mfree radical polymerization can initiate a cationic polymerization as well. ... 

Other Onium Salts and Organometallic Photoinitiators. The success of the iodonium and sulfonium salts as photoinitiators has led to the investigation of a number of analogous onium salts based on the halides and the Group VIA atoms however, these alternative initiators have not been widely used for various reasons. For example, chloronium and bromonium salts were prepared (57,58) and they were also found to function as cationic photoinitiators, but these salts are difficult to prepare and they have low thermal stability. Similarly, triarylselenon-ium salts have also been investigated and foimd to function as cationic initiators (59) however their preparation has been foimd to be expensive (60). Other onium salts such as phosphonium and arsonium salts, developed by Abu-Abdoun and co-workers for the photopolymerization ofp-methylstyrene and styrene, have also been reported (61-63) as successful cationic photoinitiators. Photopolymerization of carbazolyloxiranes with sulfonium and tropylium salts has been reported (64). Dialkylphenacyl sulfonium photoinitiator (65-67) has been reported with excellent solubility in both polar and nonpolar monomers. Pyridinium and isoquino-linium salts have also been reported and they were found useful for polymerizing both the epoxide and vinyl ether monomers (68). 

T. Takata, K. Takuma, and T. Endo, Photoinitiated cationic polymerization of epoxide with phosphonium salts as novel photolatent initiators. Makromol. Chem. Rapid Commun. 1993, 14(3), 203-206. 

K. Takuma, T. Takata, and T. Endo, Latent cationic initiator Photoinitiated polymerization of epoxides and vinyl monomers with phosphonium salts. J. Photopolym. Sci. Technol. 1993, 6, 67-74. 

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