Sulfonium cationic photoinitiators

While electron beams can produce cations, they are not effective at producing cationic cure in the absence of suitable photoinitiators. The same cationic photoinitiators used for UV cure are often also e-beam sensitive. Examples are triaryl sulfonium or diaryl iodonium salts [41]. 

Crivello, J. V. Cationic Polymerization — Iodonium and Sulfonium Salt Photoinitiators, Vol. 62, pp. 1-48. 

The development of new classes of cationic photoinitiators has played a critical role in the production of highly sensitive, acid-catalyzed deep-uv photoresists. Sulfonium salts have been widely used in this respect (4). These materials are relatively easy to prepare and structural modifications can be used to produce desired wavelength sensitivity. Triphenylsulfonium salts are particularly well suited for deep-uv application and in addition can be photosensitized for longer wavelength. These salts are quite stable thermally and certain ones such as the hexafluoroantimonate salt are soluble in casting solvents and thus easily incorporated within resist materials. 

Cationic photoinitiators are compounds that, under the influence of UV or visible radiation, release an acid, which in turn catalyzes the desired polymerization process. Initially, diazonium salts were used, but they were replaced by more thermally stable iodonium and sulfonium salts. Examples of cationic initiators are in Table 4.3. 

Cationic photoinitiation is based on the ring opening of the oxirane group. The photoinitiators of practical importance belong to three main classes of compounds diazonium salts, onium (e.g. iodonium and sulfonium) salts, and organometallic complexes,55 which upon irradiation by UV light decompose and yield an acid catalyst. 

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. 

J.V. Crivello and J.L. Lee, Photosensitized cationic polymerizations using dialkyl-phenacylsulfonium and dialkyl(4-hydroxyphenyl)sulfonium salt photoinitiators. Macromolecules 1981, 14(5), 1141-1147. 

Crivello and J.FI.W. Lam,. Polym. Set Polym. Lett. Ed. 17, 759 (1979) J.V. Crivello and J.L. Lee, Photosensitized cationic polymerizations using dialkylphenacylsulfonium and dialkyl(4 hydro xyphenyl)sulfonium salt photoinitiators, Macromolecules, 14, 1141 (1981) S.P. Pappas, Photo generation of acid Part 6 A review of basic principles for resist imaging applications, J. Imaging Technol. 11, 146 (1985) J.L. Dektar and N.P. Hacker, Triphenylsulfonium salt photochemistry. New evidence for triplet excited state reactions, J. Org. Chem., 53, (1988) J.L. Dektar and N.P. Hacker, Photochemistry of triarylsulfonium salts, J. Am. Chem. Soc. 112, 6004 (1990) G. Pohlers, J.C. Sciano, R.F. Sinta, R. Brainard, and D. Pai, Mechanistic studies of photoacid gen eration from substituted 4,6 bis(trichloromethyl) 1,3,5 triazines, Chem. Mater. 9, 1353 (1997). 

Recently [Sangermano et al., 2008] reported for the first time the paeparation of antistatic epoxy coatings via cationic UV curing of an epwxy resin in the presence of a very low content of carbon nanotubes (CNT). After dispersing the CNT into the epexy resin, in the range between 0.025-0.1 wt.-%, the formulations were cured by means of UV light in the presence of a sulfonium salt as cationic photoinitiator. 

There are mainly three types of cationic photoinitiators. The first one consists of aryldiazonium salts. The second one consists primarily of onium salts. Prominent among them are iodonium, sulfonium and selenonium salts. The third type is based on organometallic complexes. 

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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