Cationic heterocyclic polymerization

Heterocyclic polymers containing the 1,4-dioxane ring have been mentioned only briefly in the literature. Cationic addition polymerization of 1,4-dioxene (151) produces polymer (152 Scheme 48) which, along with other poly(l,2-dialkoxyethylenes), is reported to form polymeric complexes with poly(methacrylic acid) (79MI11109). 

Heterocycles form a specific class of monomers. They do not usually undergo radical polymerization, and the kind of ionic polymerization mechanism is determined by the kind of heteroatom, substituent and ring size. Oxiranes and, aziridines are polymerized by both ionic mechanisms. With the exception of lactone, four-membered and larger heterocycles with oxygen and with substituted nitrogen can only be polymerized cationically heterocycles with unsubstituted nitrogen can also be polymerized anionically. 

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Certain heterocycles polymerize by very peculiar reaction pathways. For example, the cationic polymerization of lactams proceeds by activation of the monomer, as for their anionic polymerization ... 

Except for some heterocycles (2), living cationic polymerization has been considered almost impossible, particularly for vinyl monomers, which generate unstable carbocationic intermediates. Despite this pessimistic view, we have recently found that living cationic vinyl polymerization is indeed possible by stabilizing the growing carbocations with nucleophilic counteranions or with externally added bases (3). 

Pencek, S., Kubisa, P. and Matyjaszewski, K. Cationic Ring-Opening Polymerization of Heterocyclic Monomers. Vol. 37, pp. 1 —149. 

Af-Acyliminium ions are known to serve as electron-deficient 4n components and undergo [4+2] cycloaddition with alkenes and alkynes.15 The reaction has been utilized as a useftil method for the construction of heterocycles and acyclic amino alcohols. The reaction can be explained in terms of an inverse electron demand Diels-Alder type process that involves an electron-deficient hetero-diene with an electron-rich dienophile. Af-Acyliminium ions generated by the cation pool method were also found to undergo [4+2] cycloaddition reaction to give adduct 7 as shown in Scheme 7.16 The reaction with an aliphatic olefin seems to proceed by a concerted mechanism, whereas the reaction with styrene derivatives seems to proceed by a stepwise mechanism. In the latter case, significant amounts of polymeric products were obtained as byproducts. The formation of polymeric byproducts can be suppressed by micromixing. 

Studies on the cationic polymerization of cyclic ethers, cyclic formals, lactones and other heterocyclic compounds have proliferated so greatly in the last few years that a detailed review of the evidence concerning participation of oxonium and analogous ions in these reactions cannot be given here. Suffice it to say that there is firm evidence for a few, and circumstantial evidence for many such systems, that the reactive species are indeed ions and there appears to be no evidence to the contrary. A few systems will be discussed in sub-sections 3.2 and 4.4. 

Goethals, E. J., Cationic Ring-Opening Polymerization Amines and V-Containing Heterocycles, Chap. 52 in Comprehensive Polymer Science, Vol. 3, G. C. Eastmond, A. Ledwith, S. Russo, and... 

Kubisa, P., Cationic Polymerization of Heterocyclics, Chap. 6 in Cationic Polymerizations Mechanisms, Synthesis, and Applications, K. Matyjaszewski, ed., Marcel Dekker, New York, 1996. 

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