Thermally Initiated cationic Initiation

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

Turning from the intramolecular process to the intermolecular ones, we now extend our comparison of the thermal and cation-radical cyclizations. It is also interesting to take sonication into account as a route to initiate cyclizations. The reaction between 2-butenal A,A-dimethylhydrazone (a diene) and 5-hydroxy-l,4-naphthoquinone (a dienophile) gives such an opportunity. In toluene, at 20°C, the reaction follows as depicted in Scheme 7.28 (Nebois et al. 1996). 

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. 

Intramolecular cyclization of o-diethynylbenzene gives us an opportunity to compare the results of the thermal and cation radical variants of the reaction. There are three possible modes of cyclization (Scheme 6-21). While 1,6 cyclization takes place in the thermal process, cation radical initiation leads to 1,5 cylization (Ramkumar et al. 1996). Chemical oxidation of o-diethynylbenzene bearing two terminal phenyl groups by tris(/>bro-mophenyl)aminiumyl hexachloroatimonate as the catalytic oxidizing agent in the presence of oxygen yields 3-benzoyl-2-phenylindenone in 70% yield (Scheme 6-22). 

Although cyclobutanes with varying substitution patterns are known, cyclopropanes present a much wider variety and much greater ease of synthesis. Ethyl 2-(p-methoxyphenyl)-l-cyanocyclopropanecarboxylate has been shown to thermally initiate the diradical polymerization of acrylonitrile [138]. In the presence of zinc chloride as activator, it also initiates the diradical polymerization of styrene [139]. On the other hand, this same initiator also initiates the thermal cationic polymerization of AT-vinylcarbazole [140]. This direction of tetra- and trimethylene chemistry is currently under active investigation. 

Free Radical Initiators as Reducing Agents for Dlaryllodonlua Salts. A final method by which dlaryllodonlum salts can be used as thermal Initiators of cationic polymerization has recently been reported by Ledwlth and his coworkers(14,15) and Is shown In Equations 17-19. 

Cationic vesicles typically used for DNA delivery often self-aggregate or bind to plasma proteins in vivo. Wu et al. [104] attempted to improve vesicle stability using a cationic lipid with a cross-linkable acrylamide attached to the headgroup (Fig. 16). Vesicles were polymerized using thermal initiation with AAPD. Compared to monomeric vesicles, polymerized vesicles were less cytotoxic, more resistant to aggregation in serum, and comparable in transfection activity using a vector encoding firefly luciferase. 

K. Takuma, T. Takata, and T. Endo, Cationic polymerization of epoxide with benzyl phosphonium salts as the latent thermal initiator. Macromolecules 1993,26(4), 862-863. 

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. 

T. Wang, et al., Several ferrocenium salts as efficient photoinitiators and thermal initiators for cationic epoxy polymerization. J. Photochem. Photobiol. A Chem. 2007, 187(2-3), 389-394. 

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... 

Crivello and Lee have described the synthesis and characterization of a series of (4-alkoxyphenyl)phenyliodonium salts 7, which are excellent photo- and thermal-initiators for the cationic polymerization of vinyl and heterocyclic monomers [17]. Iodonium salts 7 are conveniently prepared by the reaction of alkoxyphenols 6 with [hydroxy(tosyloxy)iodo]benzene followed by anion exchange with sodium hexafluoroantimonate (Scheme 7.2). Products 7 have very good solubility and photoresponse characteristics, which make them especially attractive for use in UV curing applications. Compounds 7 with alkoxy chains of eight carbons and longer are essentially nontoxic, compared to diphenyliodonium hexafluoroantimonate, which has an oral LD50 of 40 mg kg (rats) [17]. 

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). 

Photo-initiation for A -vinylcarbazole with azoisobutyronitrile or benzoyl peroxide leads to simultaneous radical and cationic polymerizations, as when the peroxide is used as a thermal initiator with this monomer. Triarylsulphonium salts can initiate both types of polymerization in some systems. The use of sources of radicals for indirect initiation of cationic polymerization has been reviewed. ... 

Although this question is off the subject of inhibitor design, what prospect do we see for a chemical synthesis of 8, the intermediate itself We are optimistic that chemistry recently demonstrated by Uday Maitra in a model system can be applied to the appropriate shikimate substrate. He showed that the pyruvate ketal phosphate of 2-phenylethanol (14) is formed on tin hydride reduction of bromide 13, prepared by the sequence in Figure 8.3 This reduction is carried out with UV irradiation at 0 °C (in place of the usual radical source and thermal initiation) and is facilitated by a cationic micellar catalyst (Figure 10). 

CARs have been obtained on a laboratory or an industrial scale [92-94]. In this respect, thermal oUgomerization is employed [95], the cationic technique [96] or other procedures that combine thermally initiated reactions or Diels—Alder additions with cationic oligomerization [97,98],... 

The initial development of diaryliodonium salt cationic photoinitiators was quickly followed by the nearly parallel discovery of triarylsulfonium salts as a second general class of highly efficient and thermally stable cationic photoinitiators. Along with triarylsulfonium salts, 77, their S-aryl sulfiir-heterocyclic analogs display good photosensitivity and function well in photoinitiated cationic polymerizations. 

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... 

---