Thermally Initiated cationic polymerization

J.V. Crivello and J.L. Lee, The synthesis and characterization of polymer-hound diaryliodonium salts and their use in photo and thermally initiated cationic polymerization. Polym. Bull. 1986, 16(4), 243-248. 

More recently, iodonium salts have been widely used as photoinitiators in the polymerization studies of various monomeric precursors, such as copolymerization of butyl vinyl ether and methyl methacrylate by combination of radical and radical promoted cationic mechanisms [22], thermal and photopolymerization of divinyl ethers [23], photopolymerization of vinyl ether networks using an iodonium initiator [24,25], dual photo- and thermally-initiated cationic polymerization of epoxy monomers [26], preparation and properties of elastomers based on a cycloaliphatic diepoxide and poly(tetrahydrofuran) [27], photoinduced crosslinking of divinyl ethers [28], cationic photopolymerization of l,2-epoxy-6-(9-carbazolyl)-4-oxahexane [29], preparation of interpenetrating polymer network hydrogels based on 2-hydroxyethyl methacrylate and N-vinyl-2-pyrrolidone [30], photopolymerization of unsaturated cyclic ethers [31] and many other works. 

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... 

The usefulness for this purpose of triarylsulphonium hexafluoroantimonate comes from its considerable thermal stability, stability in the presence of highly reactive monomer, and highly efficient photolysis to yield reactive cations capable of initiating cationic polymerization. These properties arise from the unique chemical composition of the photo-initiator, the effectiveness of which can be shown to be a result of the presence of a very weak cation and a similarly weak anion. 

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. 

TGA analyses were performed for polymer samples having different degrees of cross-linking. The decomposition of the linear oligomer starts at about 200 °C. Once cured and baked, the formed siloxane network is more thermally stable, and the decomposition begins at temperatures higher by 100-150 °C. The results are similar to those reported for analogous Tsi-modified siloxanes cross-linked by means of photo-initiated cationic polymerization of epoxides [8]. 

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 ... 

In the presence of triarylsulfonium and diaryliodonium salts, polymerization continues even if UV irradiation is terminated. This phenomenon is called dark cure and is due to the living nature of the superacid generated cation. The cure regime can be thought of as UV-initiated but thermally cured. Thermally initiated cationic catalysts are also available (129). 

Two other patents have also appeared describing the use of alternate free radical sources as reducing agents for diaryliodonium and triarylsulfonium salts Crivello and his coworkers have discovered several other types of reactions which can be used to thermally induce cationic polymerization using onium salt photoinitiators. Diaryliodonium salts are thermally decomposed in the presence of catalytic amounts of copper compounds An organocopper compound is proposed as an intermediate which undergoes electrophilic arylation of cationally polymerizable monomers as shown in Scheme 14 to initiate polymerization. 

The features and detail of the IPN kinetics were also studied in other works [274-276]. The kinetics of thermally initiated cationic epoxy polymerization and free radical acrylate photopolymerization were investigated in [277]. It was found that the preexistence of one polymer has a significant effect on the polymerization of the second monomer. The reaction kinetics and phase separations were studied for sequential IPNs in [278]. The kinetics of IPN formation was studied for IPNs based on PDMS-cellulose acetate butyrate [279]. All these and other works [280-282] confirm the general regularities of the reaction kinetics and its connection with phase separation in forming systems. 

The cationic polymerization of cardanol under acidic conditions has been referred to earlier [170,171], NMR studies [16] indicated a carbonium ion initiated mechanism for oligomerization. PCP was found to be highly reactive with aldehydes, amines, and isocyates. Highly insoluble and infusible thermoset products could be obtained. Hexamine-cured PCP showed much superior thermal stability (Fig. 12) at temperatures above 500°C to that of the unmodified cardanol-formaldehyde resins. However, it was definitely inferior to phenolic resins at all temperatures. The difference in thermal stability between phenolic and PCP resins could be understood from the presence of the libile hydrocarbon segment in PCP. 

Crivello, J. V., Lockhart, T. P., and Lee, J. L., Diaryl-iodonium salts as thermal initiators of cationic polymerization,... 

Yagci, Y. and Endo, X N-Benzyl and N-Alkoxy Py ridium Salts as Thermal and Photochemical Initiators for Cationic Polymerization. Vol. 127, pp. 59-86. 

The radiolysis of olefinic monomers results in the formation of cations, anions, and free radicals as described above. It is then possible for these species to initiate chain polymerizations. Whether a polymerization is initiated by the radicals, cations, or anions depends on the monomer and reaction conditions. Most radiation polymerizations are radical polymerizations, especially at higher temperatures where ionic species are not stable and dissociate to yield radicals. Radiolytic initiation can also be achieved using initiators, like those used in thermally initiated and photoinitiated polymerizations, which undergo decomposition on irradiation. 

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