Photochemical reaction cationic polymerization

This article reports on the synthesis of photosensitive polymers with pendant cinnamic ester moieties and suitable photosensitizer groups by cationic copolymerizations of 2-(cinnamoyloxy)ethyl vinyl ether (CEVE) (12) with other vinyl ethers containing photosensitizer groups, and by cationic polymerization of 2-chloroethyl vinyl ether (CVE) followed by substitution reactions of the resulting poly (2-chloroethyl vinyl ether) (PCVE) with salts of photosensitizer compounds and potassium cinnamate using a phase transfer catalyst in an aprotic polar solvent. The photochemical reactivity of the obtained polymers was also investigated. 

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. 

In general, an alternating eopolymer is formed over a wide range of monomer compositions. It has been reported that little chain transfer occurs, and in some cases, conventional free radical retarders are ineffective. Reaction occurs with some combinations, like styrene-acrylonitrile, when the monomers are mixed with a Lewis acid, but addition of a free-radical source will increase the rate of polymerization without changing the alternating nature of the copolymer. Alternating copolymerizations can also be initialed photochemically and electrochemically. The copolymerization is often accompanied by a cationic polymerization of the donor monomer. 

One of the focal points of this work was the influence of traces of moisture on the cationic polymerization of epoxides. Using the previously described iodonium salt as an example, it was shown that water not only played an important role as co-catalyst in the photochemical formation of the superacid, but it also caused the hydrolysis of the iodonium salt via a nucleophihc attack at the T center. This reaction was mainly investigated by X-ray photoelectron spectroscopy (XPS). Interestingly, this hydrolysis only occurs with the hexafluoroantimonate salt and not with the corresponding chloride. Here it should be mentioned that the chloride salt resembles an interhalogen compound and the hexafluoroantimonate salt forms a real ion pair. Hydrolysis of the hexafluoroantimonate anion of the iodonium salt was not observed, but slow hydrolysis was observed for sodium hexafluoroantimonate. The hydrolysis is so slow overall that the influence of this effect can be excluded from the discussions into the influences of water and alcohol on the cationic polymerization of epoxides. 

Cationic ring-opening polymerization of oxiranes can also be carried out photochemically (photochemical reactions are discussed in Chap. 10). Yagci and coworkers reported polymerizations of cyclohexene oxide with the aid of highly conjugated thiophene derivatives [12]. The reaction is illustrated as follows ... 

Photochemical, Electrochemical, and Radiation Initiation.— In contrast to conventional chemically-induced cationic polymerizations, photo-polymerizations, electro-polymerizations and radiation polymerizations require the application of energy from a source external to the reaction medium. 

XIA 14] Xiao P., Fouassier J.P., Lalevee J., Photochemical production of interpenetrating polymer networks simultaneous initiation of radical and cationic polymerization reactions , in Sperling L.H. (ed.), Photopolymerization of Acrylate/Epoxide Blends, Polymers, Special Issue Interpenetrating Polymer Networks , available at http //www.mdpi.com, 2014. 

Cationic polymerizations can also be initiated by a photochemical process either in the presence of a photo-initiator or under the effect of ionizing radiations. These reactions are described in Section 8.3. 

Nishikubo, T., Kameyama, A., Kishi, K., Kawashima, T., Fujiwara, T., and Hijikata, C., Synthesis of new photoresponsive polymers bearing norbomadiene moieties by selective cationic polymerization of 2- [ [ (3-phenyl-2,5-norbomadienyl)-2-carbonyl] oxy] ethyl vinyl ether and photochemical reaction of the resulting polymers. Macromolecules, 25,4469-4475,1992. 

Electron transfer to or from a conjugated tr-system can also induce pericyclic reactions leading to skeletal rearrangements. A typical example is the Diels-Alder cycloaddition occurring after radical-cation formation from either the diene or the dienophile [295-297], The radical cation formation is in most cases achieved via photochemically induced electron transfer to an acceptor. The main structural aspect is that the cycloaddition product (s Scheme 9) contains a smaller n-system which is less efficient in charge stabilization than the starting material. Also, the original radical cations can enter uncontrollable polymerization reactions next to the desired cycloaddition, which feature limits the preparative scope of radical-type cycloaddition. 

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