Photoinitiators for cationic polymerization

These initiators rely on the photochemical generation of strong Brbnsted acids which are capable of initiating the chain polymerization of epoxy compounds, vinyl ethers, cyclic ethers, lactones and many other compounds. 

Diaryl iodonium and triaryl sulfonium salts are among the most versatile photoinitiators. These compounds have been thoroughly investigated by Crivello 

A diaryl iodonium salt can be depicted as Ar2l X in which X is a counterion such as PFg , BF, AsF , etc. Its photolysis proceeds as follows  

Together with the strong acid HX, radicals are also formed. A major advantage of these systems is that they can be polymerized in the presence of oxygen a potential draw-back is that many of them absorb at wavelengths far below 300 nm. This makes the polymerization of, e.g., aromatic di-epoxides a rather slow process. Sensitization has proved possible but structural modification is a more useful method, especially with the sulfonium salts 

Iron-arene salts may also exhibit an appreciable absorption in the visible or near-UV part of the spectrum. The photochemistry of these compounds as well as their use as photoinitiators has recently been described by Lohse and Zweifel In certain applications the presence of strong acid m the photopolymer is desirable since the reaction continues after exposure. 

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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)... 

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. 

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). 

Table 1. Iodonium and sulfonium salts as photoinitiators for cationic polymerization 2)...

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. 

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... 

Photoinitiators for Cationic Polymerization. Recently a class of photoinitiators for cationic polymerization was discovered by Crivello et al. (55). This class includes diaryliodonium (Structure I) (56. 57). triaryIsulfonium (Structure II) (58-62). dialkyIphenacylsulfonium (Structure III) (63). dialkyl-4-hydroxy-phenylsulfonium salts (Structure IV) (64). and triaryIselenonium... 

Recent review articles on the following topics were published the controversy concerning the cationic ring-opening polymerization of cyclic acetals (213), photoinitiators for cationic polymerization (21A), living polymerization and selective dimerization (215). raacroraonomers (216), and functional polymers and sequential copolymers by carbocationic polymerization (217). 

The metal arene bond in (arene)FeCp complexes is photolabile [93], and this forms the basis of the use of these complexes as photoinitiators for cationic polymerizations of epoxides [94]. Arene exchange via this route has not been used extensively. On photolysis of (p-xylene)FeCp in CH2CI2 or acetone by a medium pressure Hg-arc or by bright sunlight , the complex undergoes arene exchange with hexamethylbenzene, paracyclophane and thiophene under mild conditions... 

The photoinitiators for cationic polymerizations are mainly of two types. One is based on aryldiazonium salts and the other on diaryliodonium, triarylsulfonium, or triaiylselenonium salts. The diazonium salts photodecompose to yield both Lewis acids and strong protonic acids ... 

Our research group at General Electric, along with similar groups at 3M Company and ICI, have been successful in developing several new types of very active photoinitiators for cationic polymerization, i.e., diaryliodonium (1-5), triarylsulfonium (6-9), triaryl-selenonium (10), dialkylphenacylsulfonium (11), and dialkylhydroxyphenylsulfonium salts (12). The most practical of these photoinitiators are diaryliodonium salts, I, and triarylsulfonium salts, II. 

ESR evidence supports the presence of an Fe(lll) radical intermediate. Irradiation of the iron(ll) arene compound shown in Figure I9.40 leads to slippage of the linkage to the >74 bonding mode. The resulting compound is coordinatively unsaturated (16 e ). Continued irradiation leads to the loss of the arene li d and replacement by three solvent molecules. This species has been used as a photoinitiator for cationic polymerization reactions, as shown in Figure 19.40. 

This modifieation does extend the sensitivity to longer wave length ultraviolet light. The eompounds were reported to display exeellent thermal lateney in the presence of various monomer systems and high effieieney as photoinitiators for cationic polymerization. Furthermore, their initiation efficiency was reported to be on par with current commercial triaryl sulfonium salts. 

Crivello et al., described the synflieses of 5-arylthianthrenium salts that form efficient triarylsulfonium salt photoinitiators for cationic polymerization. Through the use of electron-transfer photosensitizers, the response of these photoinitiators can be spectrally broadened to the long wavelength ultraviolet-visible light regions of the spectrum. The results obtained suggest that. 5-arylthianthrenium salts are potential replacements for now available triarysulfonium salt photoinitiators in many applications. 

Wang et al., prepared a ferrocene tetrafluoroborate photoinitiator for cationic polymerizations of epoxides ... 

Crivello and Lam introduced the diaryliodonium salts (I) as photoinitiators for cationic polymerization in 1976 (2). The efficiency of photopolymerization was shown (not surprisingly) to be... 

The development of photoinitiators for cationic polymerization is an important one, with both practical and fundamental applications. Practical uses are primarily in the area of light-induced curing of coatings, a process which is expected to be energy-conservative in comparison with conventional thermal curing (4). 

PAP 03] Pappas S.P., Tilley M.G., Pappas B.C., Anthracene-bound sulfonium salts highly efficient photoinitiators for cationic polymerization anew synthesis of sulfonium salts which avoids the use of silver salts . Journal of... 

In recent years, there has been a considerable effort to expand the range of iodonium salts that can be employed as photoinitiators for cationic polymerizations. Stable iodonium salts can be obtained when one of the aryl groups in 51 is replaced by a moiety that can provide resonance stabilization to the positively charged iodine atom. For example, Koser et prepared stable aryl (phenyl ethynyl)iodonium salts, 55, that were shown by Kitamura et al. and by Hofer and Liska to exhibit good activity as cationic photoinitiators. Similarly, iodonium salts, 56, in which the positively charged iodine atom is attached to the central carbon of a resonance-stabilized 1,3-diketone moiety such as dimedone are also isolable compounds that display the ability to serve as photoinitiators for cationic polymerization. Finally, it has been reported that diaryl(oxo)iodonium salts, 57, can be readily prepared by the base-catalyzed self-condensation of diaryliodyl compounds. Replacement of the initial... 

Li., H., Ren, K., and Neckers, D.C. (2001) Substituted cyclopropenium salts as photoinitiators for cationic polymerization of glycidyl phenyl ether. Macromolecules,... 

Of these, only the triarylsulfonium and triarylselenonium salts have been investigated as photoinitiators for cationic polymerization. Triarylsulfonium salts, in particular, are attractive photoinitiators since they can be readily prepared in high yields by straightforward synthetic techniques. 

As in the case of the analogous diaryliodonium salts, only those triarylsulfonium salts bearing anions of low nucleophilic character are broadly applicable as photoinitiators for cationic polymerization. Triarylsulfonium halides and bisulfates undergo facile... 

Dialkylphenacylsulfonium salts (111) and dialkyl-4-hydroxyphenylsulfonium (IV) salts have recently been described as useful photoinitiators for cationic polymerization 1 2-123)... 

During the past few years there has been considerable activity by various research groups directed toward the development of new types of onium salt photoinitiators for cationic polymerization. Although much of this work has appeared in fragmentary fashion in the literature, an effort will be made here to briefly report on the most significant developments in this area. 

The emergence of photosensitive onium salts as novel classes of stable, highly efficient photoinitiators for cationic polymerization has stimulated much recent interest on the part of several academic and industrial institutions. As a result of this activity, a number of interesting applications have been developed which will be briefly described here. 

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