Cationic photopolymerization of epoxide

Cationic photopolymerizations of epoxides and vinyl ethers offer tremendous potential in the area of high-speed, solvent-free curing of films and coatings. The polymers formed exhibit excellent clarity, adhesion, abrasion, and chemical... 

Presumably, a PET is involved also in the cationic photopolymerization of epoxides with metal arene complexes [181, 182]. 

The bulky anion then stabilizes the intermediate adduct from protonation of the epoxy group and then facilitates insertion of epoxide at the cationic propagation site. Rapid polymerization can then occur. Cationic photopolymerization of epoxides often involves the photo-generation of acid from an initiator such as diaryliodonium or triaryl sulfonium salts (Crivello, 1999). The anions are important in controlling the addition at the cationic site and are typically BF4 and PFg. The reactivity of the system depends also on the structure of the epoxide. 

TOB 99] Toba Y., Saito M., Usui Y., Cationic photopolymerization of epoxides by direct and sensitized photolysis of onium tetiakis(pentafluoropheityl)borate irniiaXors , Macromolecules, vol. 32, no. 10, pp. 3209-3215, 1999. 

The Lewis acid is important for polymerization. Surprisingly, the present author did not find evidence for or against the aryl cation (Ar+) as a polymerization initiator. Green and Stark (1981) briefly reviewed the photopolymerization of epoxides. 

Various bifunctional resins are based on acrylic epoxide monomers. Such systems can photopolymerize by the radical and/or cationic mechanism. With iron arene photoinitiators in the presence of an oxidant, radical as well as cationic photopolymerization of these monomers is possible . Onium -type photoinitiators form radical species upon photolysis, as shown in Figs. 3 and 4. The local radical concentration is, however, too low to permit the polymerization of such systems... 

Derivatives of anthracene and carbazole are typical electron-transfer photosensitizers. Of practical interest are derivatives containing cationically polymerizable epoxide groups (see Chart 10.5) [54]. During the ring-opening photopolymerization of epoxides, these sensitizers are covalently incorporated into the polymeric network and cannot be removed by extraction. Therefore, the potential risk of toxic effects of the sensitizers is strongly diminished. 

Cationic photoinitiation is generally based on a ring-opening reaction of an oxirane group (see Scheme 6). There are three main classes of cationic photoinitiator molecules, namely diazonium salts, onium salts, and organometallic complexes. When photopolymerization of epoxides is considered, the use of diazonium salts is limited due to stability issues. In addition, they release nitrogen gas that forms bubbles and pinholes in the coatings. Onium salts... 

Figure 7 (150) contains profiles of the polymerization rate and conversion as function of time for cationic photopolymerization of an epoxide monomer obtained using a photo-DSC. As shown in the figure, the reaction rate profile has a general shape that is characteristic of cationic photopoljunerizations. Immediately... 

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. 

Because of releasing very stable cations upon irradiation, phosphonium salts containing pyrenylmethyl groups are excellent initiators for photopolymerization of convenient monomers such as epoxides and vinyl monomers [35,39]. 

Monomers possessing 1 -propenyl ether groups are reported to be the most reactive class of monomers toward cationic species. Therefore, a series of hybrid epoxide monomers with 1-propenyl ether group was designed and their behavior in cationic photopolymerization initiated by a diaryliodonium salt was investigated [120]. The... 

Different initiation techniques have been investigated in polymerizations induced by iodonium salts, such as visible laser irradiation [32], dual photo- and thermally initiated cationic polymerization [23, 26] and a two-photon photopolymerization initiation system [33,34]. For example, dual photo- and thermal-initiation systems based on selective inhibition of the photoinifiated cationic ring-opening polymerization of epoxides by dialkyl sulfides have been developed [26]. Such a dual system, iodonium salt/dialkyl sulfide, in the... 

Although, in contrast to free radical polymerizations, cationic polymerizations are unaffected by O2, their importance is somewhat limited by the scarcity of appropriate macromolecules and suitable photoinitiators [3]. However, this does not apply to the photopolymerization of low molar mass epoxides (see Section 10.3). In this context, applications of photo-cross-linked epoxides in various fields such as stereolithography, volume holography, and surface coating are notable [16]. 

In fact, the importance of the cationic photopolymerization lies beyond epoxides (39). Many oxygen-containing compounds, e.g., vinyl ethers, tetra-hydrofuran, oxetane, lactones, trioxane, and some unsaturated compounds (Fig. 3) can be polymerized by the same mechanism to form adhesives or coating materials. Crivello (43) reviews the scope of cationic photopolymerization, providing us with a perspective on this promising process. 

Based on the above, an initiating composition for cationic photopolymerization, with visible and long-wavelength UV light was described by Crivello et al. The structure of the monomers plays a key role in these photosensitization processes. Useful aromatic ketones are camphoquinone, benzyl, 2-isopropylthioxanthone, or 2-ethylanthraquinone. The monomer-bound radicals reduce diaryliodonium salts or dialkyl phenacylsulfonium salts rapidly to form monomer-centered cations. These cations then initiate the polymerization of epoxides, vinyl ethers, and heterocyclic compounds. Onium salts with high reduction potential, however, such as triarylsulfonium salts, do not undergo this reaction. 

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