Photoinitiation of cationic polymerization

Diaryliodonium and triarylsulfonium salts act as photoinitiators of cationic polymerization. Photolytic celeavage of an Ar—I or Ar—S bond yields a radical-cation (Eq. 5-8) that reacts with HY to yield an initiator-coinitiator complex that acts as a proton donor to initiate... 

Dialkylphenacylsulfonium salts are also known to be highly photosensitive [3b, c] and have been adapted to photoinitiated cationic polymerization [9]. These salts have a higher intrinsic quantum yield for photoinitiation of cationic polymerization on direct photolysis than the simple aryl or aryl-alkyl onium salts [10], but do not appear to undergo the same photodecomposition as the other onium salts [1-4]. 

Dialkyphenacylsulfonium salts [9] undergo on photolysis reversible ylid formation with simultaneous generation of Bronsted acid. They are thus specifically useful for photoinitiation of cationic polymerization. Insofar as they exhibit moderately strong absorption at approximately 300 nm (unassigned), they may be useful photoinitiators in certain cases without additional sensitization. 

Photoinitiated cationic polymerization has been the subject of numerous reviews. Cationic polymerization initiated by photolysis of diaryliodonium and triarylsulfonium salts was reviewed by Crivello [25] in 1984. The same author also reviewed cationic photopolymerization, including mechanisms, in 1984 [115]. Lohse et al. [116], reviewed the use of aryldiazonium, diphenyliodonium, and triarylsufonium salts as well as iron arene complexes as photoinitiators for cationic ring opening polymerization of epoxides. Yagci and Schnabel [117] reviewed mechanistic studies of the photoinitiation of cationic polymerization by diaryliodonium and triarylsulfonium salts in 1988. Use of diaryliodonium and sulfonium salts as the photoinitiators of cationic polymerization and depolymerization was again reviewed by Crivello [118] in 1989 and by Timpe [10b] in 1990. 

The general mechanisms of photoinitiation of cationic polymerizations by iodonium, sulfonium, and selenonium salts are similar. These salts can be illustrated as follows. ... 

Under some circumstances, radical initiators can give rise to non-radical polymerizations this effect arises for benzoyl peroxide with Mvinyl carbazole. Generation of radicals in the presence of a fragmentable salt, e.g. (p-CHjC,114)2 + PF,-, can lead to the polymerization of monomers susceptible only to cationic polymerization. Salts of this type can also act as photoinitiators of cationic polymerization. ... 

MAN 92] Manivannan G., Fouassier J.P., Crivello J.V., Chlorothioxanthone-onium salts efficient photoinitiators of cationic polymerization . Journal of... 

PAP 84a] Pappas S.P., Gatechair L.R., Jilek J.H., Photoinitiation of cationic polymerization III photosensitization of diaryliodonium and triaiylsulfonium... 

Pappas, S.P., Pappas, B.C., Gatechair, L.R., and Schnabel, W., Photoinitiation of cationic polymerization. II. Laser flash photolysis of diphenyhodonium salts, /. Polym. Sci., Polym. Chem. Ed., 22, 69, 1984. 

Muneer, R. and Nalli, T.W., Use of diaryhodonium/phosphine radical-chain chemistry for visible photoinitiation of cationic polymerizations, trimethyl phosphite as a co-initiator of the ringopening polymerization of cyclohexene oxide. Macromolecules, 31, 7976, 1998. 

The recent development of several new classes of practical photoinitiators for cationic polymerization has now made it possible to utilize this chemistry in a number of ways to produce highly sensitive photoresists (142-144). The facile synthesis of onium salts (I-III)... 

Cyclic ethers, cyclic acetals, and some vinyl compounds can be polymerized by cationic processes. Photoinitiation of these polymerizations by ultraviolet light are known (12,106). Some extension of direct photolytic sensitization of cationic processes to visible wavelengths is obtained by the use of colored diazonium salts as initiators. For example, Schlesinger (11a) used diazonium salts substituted in the para position with electron withdrawing groups, but sensitivity was limited to the blue to green regions of the spectrum. 

All of these species (XIV, XV) have been for the most part applied towards function in the olefin polymerization arena use of these novel anions for the stabilization of other electrophilic species remains to be explored. Recently, the imidazolide anion XVI, as well as the perfluorinated tetraaryl borate derived from the diborane IX of Chart 2, have been used to stabilize iodonium cations.222 These cations are used as photoinitiators for cationic polymerization of epoxy resins in photolithography applications. While use of the [B(C6F5)4] led to a breakthrough in this area of research,223 higher activities are observed for more WCAs. 

Compounds with a nucleophilic sulphur are readily arylated by iodonium salts in their anionic form, and less readily in other cases, notably in sulphides triaryl sulphonium salts are useful photoinitiators in cationic polymerizations. A list of chalcogen compounds arylated by iodonium salts appears in Table 8.7. 

Photoinitiation of radical polymerization has long been known. Recently, a group of photoinitiators for cationic polymerization hase been discovered and developed by Crivello et al.1J. They include diaryliodonium (7),2) triarylsulfonium (2), 3 5) dialkylphenacylsulfonium (J), 6) and dialkyl-4-hydroxyphenylsulfonium salts (4) 7). 

The generation of a protonic acid (HX) is responsible for the initiation of cationic polymerization. Various monomers are polymerized by sulfonium photoinitiators. Especially this system has been shown to be potentially useful for UV curable coatings of metal and plastics with epoxy resins. 

As outlined in Scheme 2, for cationic photopolymerization a photoinduced formation of species X+ or Lewis acids is required [162]. Such species are formed both by PET between neutral donors and acceptors (see Scheme 3), between neutral donors and positive charged acceptors (see Schemes 9 and 11), respectively, and by an indirect PET between nucleophilic radicals and onium salts or halogen compounds (see Eq. (16) and Scheme 12). Therefore, combinations of compounds, whose light-induced reactions are based on the pathways given above, are usable as photoinitiators for cationic polymerizations, too. Prerequisites for the use of cations... 

Crivello JV, Dietliker K. (1991) Photoinitiators for cationic polymerization. in Oldring PKT (ed.), Chemistry and Technology of UV and EB Formulation for Coatings, Inks and Paints. Vol. 3, pp. 329-373. Selective Industrial Training Associates Technologies Ltd., London. 

Initiators based on halonium and sulfonium salts are used commercially in various microlithographic processes and in the coating industry. Onium salts were developed commercially as photoinitiators due to the lower sensitivity of cationic polymerizations to oxygen compared to radical polymerizations. Aromatic halonium and sulfonium salts with complex anions such as SbF6, AsF6 and BF4- do not initiate cationic polymerizations spontaneously, but must be activated by UV irradiation. 

Recently, Ledwith described combined systemscomposed of a radical initiator and a cationic photoinitiator, which are very suitable for hybrid systems. It is particularly convenient to employ common photochemical sources of free radicals for this purpose. Since many of them possess aromatic carbonyl groups (e.g. benzoin and acetophenone derivatives), these groups provide an extension of the absorption to longer wavelengths and promote the initiation of cationic polymerization. Figure 19 shows the proposed formation mechanism of free-radical and cationic active species by electron transfer ... 

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. 

In this paper, we would like to report some recent work which has led to the development of triarylsulfonium salts (III) as a third class of useful photoinitiators for cationic polymerization and in particular, describe their application to the polymerization of epoxides. 

Both of the methods shown in equations 2 and 3 give rise to triarylsulfonium halides which are inactive as photoinitiators for cationic polymerization. These salts must, therefore, be converted to the corresponding salts in which the anion is of the type X" = BF4 , AsF5, PFc , etc. This conversion may be accomplished using either of the two methods shown in equation 5. 

The recent development of several new classes of highly efficient photoinitiators for cationic polymerization makes possible their application in the design of novel photoresists. The concepts on which these imaging processes are based are set forth in this article. 

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

As discussed in a previous section, a number of studies have been conducted to increase the rate of cationic polymerization of epoxides. In curing applications, polymerization should be rapid enough for high output of production. In a recent work, the effect of addition of tetraethylene glycol (TEG) or polyEPB on the rate of photoinitiated cationic polymerization of CY179, limonene dioxide (LDO), and 1,2,7,8-diepoxyoctane (DEO) has been investigated [150]. These hydroxyl containing additives were shown to obviously accelerate the polymerization, increase the total epoxide conversion and decrease the induction period. 

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