Photoinitiated cationic polymerization, application

Applications of Photoinitiated Cationic Polymerization to the Development of New Photoresists... 

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. 

Over the years, a number of compounds have been identified to photoinitiate cationic polymerization of monomers designed for radiation curing applications generally, and negative-resist applications specifically. The most prominent of these initiators are onium salts. 

Photoinitiated cationic polymerizations have numerous applications involving photoimaging. Epoxy-based photoresists with high resolution derived from commercially available resins have been developed (22) and the use of these same materials in plastic flexographic printing plates has been shown in our laboratory. 

Over the past ten years the development of onium salt and other cationic photoinitiators has moved from the realm of speculative investigation to the point today at which they are being employed in numerous commercial applications. Much work still needs to be done in this fields particularly to improve our understanding of the relationship between the structure and the photosensitivity of these photoinitiators. As the field advances, one can expect still other new classes of onium salt photoinitiators to be developed as well as continued improvements to be made in the efficiency of the present systems. An understanding of the mechanism of photosensitization should lead to discovery of more efficient photosensitizers and a further broadening of their spectral response in photoinitiated cationic polymerization. 

Lastly, the process and polymers produced by onium salt photoinitiated cationic polymerizations are eminently useful. Like the corresponding photoinitiated radical polymerizations which they complement, these cationic systems will find a wide range of applications where polymerization speed and economy of energy utilization are of prime concern. 

Included among the many types of vinyl monomers that have been subjected to photoinitiated cationic polymerization are styrene," substituted styrenes, a-methylstyrenes, N-vinylcarbazole, alkyl vinyl ethers, prop-l-en-l-yl ethers, ketene acetals, and alkoxyallenes. Most useful in the crosslinking photopolymerizations employed for UV curing applications are multifunctional vinyl ethers and multifunctional prop-l-en-l-yl ethers. A number of multifunctional vinyl ether monomers are available from commercial sources, while multifunctional prop-l-en-l-yl ethers can be readily prepared by catalytic isomerization from their corresponding allyl ether precursors. The photoinitiated cationic... 

Onium salt cationic photoinitiators present many unique and interesting opportunities for basic studies of cationic ring-opening polymerizations. Since they are latent photochemical sources of strong Bronsted adds, they can be dissolved in the subject monomers and then precisely tri ered on demand by the application of light. Mixing problems and the use of complex stopped-flow devices and other apparatuses required to overcome them are thus avoided. Only the rate of initiation is different in a photoinitiated cationic polymerization as compared to a conventional thermally initiated polymerization. The rate of initiation for an onium salt-photoinitiated cationic potymerization (eqn [68]) is... 

Chien CC, Liu JH (2014) Optical behaviors of flexible photonic films via the developed multiple UV-exposed fabrications. Macromol Rapid Commun 35 1185-1190 Crivello JV (1983) Photoinitiated cationic polymerization. Annu Rev Mater Sci 13 173-190 de Gennes PG (1975) The physics of liquid crystals. Clarendon, Oxford Decker C (1996) Photoinitiated crosslinking polymerization. Prog Polym Sci 21 593-650 Decker C (2002) Kinetic study and new applications of UV radiation curing. Macromol Rapid Commun 23 1067-1093... 

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. 

Cationic polymerizations induced by thermally and photochemically latent N-benzyl and IV-alkoxy pyridinium salts, respectively, are reviewed. IV-Benzyl pyridinium salts with a wide range of substituents of phenyl, benzylic carbon and pyridine moiety act as thermally latent catalysts to initiate the cationic polymerization of various monomers. Their initiation activities were evaluated with the emphasis on the structure-activity relationship. The mechanisms of photoinitiation by direct and indirect sensitization of IV-alkoxy pyridinium salts are presented. The indirect action can be based on electron transfer reactions between pyridinium salt and (a) photochemically generated free radicals, (b) photoexcited sensitizer, and (c) electron rich compounds in the photoexcited charge transfer complexes. IV-Alkoxy pyridinium salts also participate in ascorbate assisted redox reactions to generate reactive species capable of initiating cationic polymerization. The application of pyridinium salts to the synthesis of block copolymers of monomers polymerizable with different mechanisms are described. 

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. 

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. 

The larger portion of the current literature on photochemistry of onium salts deals either with fundamental mechanisms of onium salt photodecomposition (see above) or application of this chemistry to generation of catalysts for cationic polymerizations (see below). Application to the photoinitiation of radically polymerizable monomer systems developed in parallel with applications to cationic polymerizations [17,96, 111]. 

The evolution of nitrogen on photolysis of the aryIdiazonium salts appears to have limited the use of these systems to thin film applications such as container coatings and photoresists (23). Other efficient photoinitiators that do not produce highly volatile products have been disclosed (24-27). These systems are based on the photolysis of diaryliodonium and triarylsulfonium salts. Structures I and II, respectively. These salts are highly thermally stable salts that upon irradiation liberate strong Bronsted acids of the HX type (Reactions 43 and 44) that subsequently initiate cationic polymerization of the oxirane rings ... 

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