Photopolymerization epoxides

Although a number of unsaturated monomers theoretically may undergo cationic photopolymerizations, there have been few studies of these photopoiymerization systems reported in the literature. In the last decade some systematic studies of the cationic photopoiymerization of vinyl ethers have been reported therefore these systems are the focus of our discussion. Vinyl ether and propenyl ether monomers are among the most reactive monomers (which polymerize by a cationic mechanism) and they exhibit low vapor pressure, low toxicity, and relatively low viscosity. Hence, these monomers may be used as reactive diluents to enhance the effective cure rates of epoxide photopolymerizations. 

Olefin isomerization can be catalyzed by a number of catalysts such as molybdenum hexacarbonyl [13939-06-5] Mo(CO)g. This compound has also been found to catalyze the photopolymerization of vinyl monomers, the cyclization of olefins, the epoxidation of alkenes and peroxo species, the conversion of isocyanates to carbodiimides, etc. Rhodium carbonylhydrotris(triphenylphosphine) [17185-29-4] RhH(CO)(P(CgH )2)3, is a multifunctional catalyst which accelerates the isomerization and hydroformylation of alkenes. 

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. 

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 first efficient catalysts for the photopolymerization of epoxides to be found were aromatic diazonium salts with anions of low nucleophilicity Upon... 

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... 

Besides the most important area of surface coatings, the use of photopolymers as photoresists in the manufacture of printed circuits is well established. Photoimaging with aryldiazonium salt photoinitiators and multifunctional cresol-novolac epoxides was first described by Schlesinger Crivello has mentioned several new photoresists based on the photopolymerization of epoxides with onium initiators Meier and Zweifel have shown that iron arene salts in combination with multifunctional cresol-novolac epoxides yield photoresists with high resolution and contrast. Dual functional epoxides (cf. Sect. 5) containing chalcone groups as light-sensitive units have been described as suitable photoresists especially... 

In studies directed towaids the synthesis of the fungal isonitrile antibiotic isonitrin B (103), a cy-clopentanoid metabolite related to trichoviridine (104), various trialkylsilyl enol ethers were irradiated with propionaldehyde to produce equimolar amounts of stereoisomeric bicyclic oxetanes (105). The corresponding silyl dienol ethers, however, gave little oxetane product owing to preferential diene photopolymerization. The use of vinyl epoxide derivatives such as (106), however, offers a possible access to the epoxycyclopentanoid system with simultaneous control of the face selectivity of the photoaddition by the epoxide moiety. 

Due to the limitations of non crossiinkable cyclic liquid crystalline siloxanes resulting from the fact that the Tg is dominated by the mesogenic groups connected to the cyclic siloxane backbone and, therefore, other aspects for the selection of the mesogens are limited, a crossiinkable system had to be established. This strategy requires the presence of polymerizable moieties within the side chain groups, such as epoxide, einnamate or (meth)acrylate groups. Methacrylates are preferred due to their reasonably fast photopolymerization and thermal stability. 

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. 

The polymerization is said to be fast with a low energy input, is not inhibited by air, uses no solvents, is nonpolluting, and uses inexpensive materials. The photopolymerization of various epoxidized oils and terpenes also does this. The propenyl ethers were made by isomerization of the corresponding allyl ethers with a ruthenium catalyst. The allyl ethers were made from the hydroxy compounds with allyl bromide and base, a process that produces waste salts. It is possible that the monomers could be made by reaction of the hydroxy compounds with propylene oxide followed by dehydration so that no waste salts would be formed. Isosorbide is made by the acid-catalyzed dehydration of sorbitol, in turn, obtained by the reduction of glucose. 

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]. 

In the recent decay, there is a considerable interest in making green photopolymerizations using photosensitizers. One way to obtain nontoxic polymers through sensitization is to copolymerize compounds that can behave either as a photosensitizer or monomer with different monomers [113]. Another way is to polymerize these monomeric photosensitizers and afterward subject them to sensitize the polymerization of convenient monomers. In both ways, nontoxic and odorless polymers can be obtained after polymerization. Compounds introducing phenothiazine moiety and their polymeric analogs were found the display high efficiency in PCP of vinyl ethers and epoxides. 

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... 

In another work, improvement of polymerization reactivity of epoxides was attempted [148]. Styrene monomers were modified with epoxide functions that are inert toward radicallic species, and then polymerized in the presence of azobisiso-butyronitrile (AIBN). The polymers, ultimately, were cross-linked by sulfonium salt induced photopolymerization of the epoxide groups (Scheme 11.45) [148]. Kinetic results indicate that polymers containing silicon in their pendant group has the highest cross-linking reactivity. 

F. Hamazu, et al., Photopolymerization of epoxide by benzyl (para-hydroxy phenyl) methylsulfonium salts as novel photolatent initiators. Makromol. Chem. Rapid Commun. 1992, 13(4), 203-206. 

S. Hayase, et al., Photopolymerization of epoxides a new type of photopolymerization with photodecomposahle silyl ether as coinitiator. Macromolecules 1985, 7<5(10), 1799-1804. 

A luminescence study of the solid-state photopolymerization of p-diethynyl-benzene,58 the photopolymerization of epoxides,57 other photochemical poly-... 

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... 

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