Cationic polymerization of monomers with

Crivello, J.V. Malik, R. Synthesis and photoini-tiated cationic polymerization of monomers with the silsesquioxane core. J. Polym. Sci. Polym. Chem. 1997, 35, 407. 

Various initiators can be used to bring about the polymerization of monomers with electronreleasing substituents [Faust and Shaffer, 1997 Gandini and Cheradame, 1980, 1985 Kennedy and Marechal, 1982 Matyjaszewski and Pugh, 1996 Sauvet and Sigwalt, 1989]. Additional information on cationic initiators is described in Chap. 7. 

Ring-opening polymerizations are generally initiated by the same types of ionic initiators previously described for the cationic and anionic polymerizations of monomers with carbon-carbon and carbon-oxygen double bonds (Chap. 5). Most cationic ring-opening polymerizations involve the formation and propagation of oxonium ion centers. Reaction... 

Rakova and Korotkov compared the rates of homopolymerization and copolymerization of styrene and butadiene [226], Styrene polymerizes very rapidly and butadiene slowly. Their copolymerization is slow at first, with preferential consumption of butadiene. When most of the butadiene is consumed, the reaction gradually accelerates yielding a product with a high styrene content. In the authors opinion, this is caused by selective solvation of the active centres by butadiene only after butadiene has polymerized, does styrene gain access to the centres [227], A similar behaviour was observed by Medvedev and his co-workes [228] and by many others. In our laboratory we observed this kind of behaviour in the cationic polymerization of trioxane with dioxolane. Although trioxane is polymerized much more rapidly than dioxolane, their copolymerization starts slowly, and is accelerated with progressing depletion of dioxolane from the monomer mixture [229],... 

Some radical sources will, in the presence of oxidizing agents, or light or heat energy, initiate cationic polymerizations of monomers, like n-butyl vinyl ether. Those that are most readily oxidized are carbon atom centered radicals that have substituents like benzyl, allyl, alkoxy, or structures with nitrogen or sulfur. Also, radicals that are formed by addition of other radicals to alkyl vinyl ethers are particularly reactive. 

Styrene is one of those monomers that lends itself to polymerization by free-radical, cationic, anionic, and coordination mechanisms. This is due to several reasons. One is resonance stabilization of the reactive polystyryl species in the transition state that lowers the activation energy of the propagation reaction. Another is the low polarity of the monomer. This facilitates attack by free radicals, differently charged ions, and metal complexes. In addition, no side reactions that occur in ionic polymerizations of monomers with functional groups,are possible. Styrene pol erizes in the dark by a free-radical mechanism more slowly than it does in the presence of light. Also, styrene formed in the dark is reported to have a greater amount of syndiotactic placement. The amount of branching in the polymer prepared by a free-radical mechanism increases with temperature. This... 

Despite difficulties in direct polymerization of monomers with a lateral heterogroup (amine, acids) the most intensively investigated polymers of this type are certainly vinyl polymers which are usually obtained without appreciable racemization during the polymerization itself both stereoregular and atactic polymers are obtained and can be fractionated. With cationic initiators, some racemization can take place because of hydride shift in the propagation step. 

Strong Lewis acids, that is, electron acceptors, are often capable of initiating addition polymerization of monomers with electron-rich substituents adjacent to the double bond. Cationic catalysts are most commonly metal trihalides such as AICI3 or BF3. These compounds, although electrically neutral, are short of two electrons for having a complete valence shell of eight electrons. Traces of a cocatalyst, usually water, are usually required to initiate polymerization, first by grabbing a pair of electrons from the cocatalyst ... 

It was reported that the rate of living cationic polymerization of IBVE with Al-based initiating systems was enhanced if the basicity of an added base was reduced. Thus, a weaker base was examined in the polymerization using SnCU and FeCU. An alternative weaker base, ethyl chloroacetate, realized very fast polymerization with SnCU in toluene at -78 °C, being completed within 2 s (determined using a high-resolution digital video camera).Moreover, FeCU induced faster polymerization with 1,3-dioxolane, a weaker base than 1,4-dioxane, which was completed in 2-3 s in toluene at 0 °C. ° In both cases, product polymers had very narrow MWD (Mw/Mn< 1.1), irrespective of the monomer conversion. 

The cationic polymerization of NVK with iodine satisfies the requirements of a living polymerization [107,108]. Living polymerization was carried out in dichloromethane solution containing tetrabutylammo-nium iodide at low temperature. The molecular weight of the polymer increased linearly with monomer conversion during the reaction and upon the addition of successive aliquots of the monomer. Sawamoto et al. [109] used hydrogen iodide as initiator for the living cationic polymerization of NVK in toluene and dichloromethane solution. 

Needless to say, vinyl polymerization is one of the most important methods for polymer synthesis. A variety of carbon-carbon (C-C) main chain polymers have been prepared by the vinyl polymerization of monomers with diverse substituents, via radical, cationic, anionic, or coordination mechanism. Furthermore, with the technological achievement such as living and stereoselective (or stereospecific) polymerizations, fine-tuning of the polymer structure with respect to molecular weight and tacticity has been realized in a number of examples. In particular, polymers obtained with vinyl polymerization (vinyl polymer) as represented by polyethylene, polypropylene, polystyrene, and poly(methyl methacrylate) have contributed to the progress of modern society in various aspects as useful synthetic materials. 

ATRP is successfully employed in the polymerization of a large variety of vinyl monomers such as styrenes, methacrylates, acrylates, acrylonitrile, and some others [2,9-15]. However, at present, available catalytic systems seem to be unsuitable for the less reactive monomers such as ethylene, olefines, vinyl chloride, and vinyl acetate. In the polymerization of monomers with strong electron-donating groups such as /7-methoxy styrene, some side reactions arising from the involvement of cationic intermediate are observed. Acrylic and methacrylic acids are also not prone to ATRP because they form Cu(II) carboxylates, which are inefficient deactivators. However, hydroxy derivatives such as hydroxyethyl acrylate and hydroxyethyl methacrylate can be polymerized by ATRP. 

The key initiation step in cationic polymerization of alkenes is the formation of a carbocationic intermediate, which can then interact with excess monomer to start propagation. We studied in some detail the initiation of cationic polymerization under superacidic, stable ion conditions. Carbocations also play a key role, as I found not only in the acid-catalyzed polymerization of alkenes but also in the polycondensation of arenes as well as in the ring opening polymerization of cyclic ethers, sulfides, and nitrogen compounds. Superacidic oxidative condensation of alkanes can even be achieved, including that of methane, as can the co-condensation of alkanes and alkenes. 

The observation in 1949 (4) that isobutyl vinyl ether (IBVE) can be polymerized with stereoregularity ushered in the stereochemical study of polymers, eventually leading to the development of stereoregular polypropylene. In fact, vinyl ethers were key monomers in the early polymer Hterature. Eor example, ethyl vinyl ether (EVE) was first polymerized in the presence of iodine in 1878 and the overall polymerization was systematically studied during the 1920s (5). There has been much academic interest in living cationic polymerization of vinyl ethers and in the unusual compatibiUty of poly(MVE) with polystyrene. 

Complexation of the initiator and/or modification with cocatalysts or activators affords greater polymerization activity (11). Many of the patented processes for commercially available polymers such as poly(MVE) employ BE etherate (12), although vinyl ethers can be polymerized with a variety of acidic compounds, even those unable to initiate other cationic polymerizations of less reactive monomers such as isobutene. Examples are protonic acids (13), Ziegler-Natta catalysts (14), and actinic radiation (15,16). 

Polymerization Reactions. The polymerization of butadiene with itself and with other monomers represents its largest commercial use. The commercially most important polymers are styrene—butadiene mbber (SBR), polybutadiene (BR), styrene—butadiene latex (SBL), acrylonittile—butadiene—styrene polymer (ABS), and nittile mbber (NR). The reaction mechanisms are free-radical, anionic, cationic, or coordinate, depending on the nature of the initiators or catalysts (194—196). 

A long-standing goal in polyolefins is the synthesis of polymers bearing polar functional groups such as acrylate, esters, or vinyl ethers, etc [24,40]. These copolymers might endow polyolefins with useful properties such as adhesiveness, dyeability, paintability, and print-ibility. Advances have recently been made in polymerizing polar monomers with cationic metallocene catalysts... 

Block copolymers have been synthesized on an industrial scale mainly by anionic or cationic polymerization, although monomers for block components are limited to ones capable of the process. Intensive academic and technological interest in radical block copolymerization using macroinitiators is growing. This process can be implemented in plants with easier handling of materials, milder conditions of operation, and a variety of materials to give various kinds of block copolymers to develop a wide application area [1-3]. 

A comparison of the cationic polymerization of 2,3-dihydrofurans with that of furan and 2-alkylfurans shows that the complications of the latters two, arising from the dienic character of the monomers, obviously vanish when the monomer is a simple cyclic vinyl ether with just one reactive site, viz. the carbon-carbon double bond. However, it also points out that ring opening in the polymerization of furans by acidic catalysts in the absence of water is unlikely, because otherwise it would also occur to some degree in the polymerization of dihydrofurans. 

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