Dialkylphenacyl sulfonium salts

Dialkyl peroxydicarbonates, 18 477 decomposition of, 18 474 475 as free-radical initiators, 14 289-290 properties of, 18 470-471t Dialkylphenacyl sulfonium salts, 10 414-415... 

Fig. 5. Mechanism of ylid and Bronsted acid formation by photolysis of dialkylphenacyl sulfonium salt and dialkyl- -hydroxyphenylsulfonium salt (X" = PF, BF, SbFg etc.)...

Over the past several years, there have been developed several new classes of onium salt photoinitiators capable of initiating cationic polymerization. The most significant of these are aryldiazonium salts, diaryliodonium salts, triarylsulfonium salts, and dialkylphenacyl-sulfonium salts. The mechanisms involved in the photolysis of these compounds have been elucidated and will be discussed. In general, on irradiation acidic species are generated which interact with the monomer to initiate polymerization. Using photosensitive onium salts, it is possible to carryout the polymerization of virtually all known cationically polymerizable monomers. A discussion of the various structurally related and experimental parameters will be presented and illustrated with several monomer systems. Lastly, some new developments which make possible the combined radical and cationic polymerization to generate interpenetrating networks will be described. 

In contrast, dialkylphenacyl sulfonium salts undergo reversible dissociation upon photolysis with formation of an ylid and a Br0nsted acid. Cessation of UV activation results in termination of epoxy homopol5unerization, since the acid is consumed in a reverse reaction with the ylid. 

This type of behavior provides a means of controlling the degree of cure. Dialkylphenacyl sulfonium salts are thermally stable in epoxy resins at room temperature and up to 150°C for 1-2 h. Significant interest in thermal cationic cure of epoxies, especially cycloaliphatic epoxies, has developed (130). 

J.V. Crivello and J.L. Lee, Photosensitized cationic polymerizations using dialkylphenacyl sulfonium and dialkyl(4 hydroxyphenyl)sulfonium salt photoinitiators, Macromolecules, 14, 1141... 

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). 

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