Cationic polymerization, of alkenes

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. 

Cationic polymerization of alkenes and alkene derivatives has been carried out frequently in aqueous media.107 On the other hand, the reaction of simple olefins with aldehydes in the presence of an acid catalyst is referred to as the Prins reaction.108 The reaction can be carried out by using an aqueous solution of the aldehyde, often resulting in a mixture of carbon-carbon bond formation products.109 Recently, Li and co-workers reported a direct formation of tetrahydropyranol derivatives in water using a cerium-salt catalyzed cyclization in aqueous ionic liquids (Eq. 3.24).110... 

Developments in the Theory of Cationic Polymerization VII. Theoretical Attempts at Improving Initiators for Cationic Polymerization of Alkenes, S.D. Pask and P.H. Plesch. European Polymer Journal, 1982, 18, 839-846. 

Developments in the Cationic Polymerization of Alkenes - A Personal View, P.H. Plesch, Macromolecular Symposia, 1994, 85,1-31. 

Matyjaszewski, K. and C. Pugh, Introduction, Chap. 1 and Mechanistic Aspects of Cationic Polymerization of Alkene, Chap. 3 in Cationic Polymerizations Mechanisms, Synthesis, and Applications, K. Matyjaszewski, ed., Marcel Dekker, New York, 1996. 

Plesch, P H., Propagation Rate Constants in the Cationic Polymerization of Alkenes, pp. 1-16 in Cationic Polymerization and Related Processes, E. J. Goethals, ed., Academic Press, New York, 1984. Pregaglia, G. F., M. Minaghi, and M. Cambini, Makromol. Chem., 67, 10 (1963). 

A variety of initiator systems, of the types used in the cationic polymerization of alkenes (Sec. 5-2a) can be used to generate the tertiary oxonium ion-propagating species [Dreyfuss and Dreyfuss, 1969, 1976 Inoue and Aida, 1984 Penczek and Kubisa, 1989a,b]. 

Historically acid-catalyzed (cationic) polymerization of alkenes is the oldest polymerization. At the end of the eighteenth century Bishop Watson described resin... 

Cationic polymerization of alkenes involves the formation of a reactive carbo-cationic species capable of inducing chain growth (propagation). The idea of the involvement of carbocations as intermediates in cationic polymerization was developed by Whitmore.5 Mechanistically, acid-catalyzed polymerization of alkenes can be considered in the context of electrophilic addition to the carbon-carbon double bond. Sufficient nucleophilicity and polarity of the alkene is necessary in its interaction with the initiating cationic species. The reactivity of alkenes in acid-catalyzed polymerization corresponds to the relative stability of the intermediate carbocations (tertiary > secondary > primary). Ethylene and propylene, consequently, are difficult to polymerize under acidic conditions. 

It has been shown by Olah et al.928,929 that cationic polymerization of alkenes can be initiated by stable alkyl or acyl cations as well as nitronium ion salts. 

If concentrations of carbenium ions are too low to be observed directly, they must be detected indirectly in kinetic studies of the racemiza-tion of optically active dormant species, ligand exchange and/or detailed studies of the effect of substituents, solvent and salts. Some of the most convincing and elegant work in this area was presented in Chapter 2 using primarily benzhydryl derivatives. As discussed in the next section, correlations between ionization rates and equilibrium constants, rates of solvolysis and rate constants of electrophilic addition can be interpolated and in some cases extrapolated to cationic polymerizations of alkenes to evaluate the reactivities of various active species and the dynamics of their isomerization. 

Several classes of compounds initiate cationic polymerizations of alkenes, including protonic acids, Lewis acids (usually in combination with a cation or proton source), stable carbenium ions, oxidizing reagents, and other strong electrophiles. This section attempts to explain the mechanism of initiation with quantitative information when available physical means of initiation (electric current, y-rays, field ionization and emission, nuclear chemical initiation) will not be discussed. 

Photoinitiators which produce species-inducing cationic polymerization of alkenes and heterocycles are usually based on diarylhalonium salts and triarylsulfonium salts [203]. The mechanism of the direct photolysis of these salts is quite complex and involves formation of excited singlet species which can reversibly convert by intersystem crossing (ISC) to the... 

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