Heterocyclic monomers, ring-opening polymerization

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

Penczek S, Cypryk M, Duda A, Kubisa P, Slomkowski S (2007) Living ring-opening polymerizations of heterocyclic monomers. Prog Polym Sci 32 247-282... 

Living Ring-Opening Polymerization of Heterocyclic Monomers with Aluminum Porphyrin-Organoaluminum Compound Systems... 

Most cyclic monomers of interest in the field of composites happen to be heterocyclic in nature. Polymerizability of some monomers is summarized in Table 1.1. Ring opening polymerizations invariably follow ionic mechanisms, although a few are known to proceed via the free radical route and some via metathesis involving metallocarbene intermediates. 

Ring-opening polymerization of a number of bicyclic heterocycles provides a series of polymers containing the tetrahydropyran ring (78MI11102). For example, 6,8-dioxabicyclo[3.2.1]octane (125) has been polymerized to stereoregular polymer (126) which has the natural dextran backbone (Scheme 36). Similarly, monomers (127) and (128) can be polymerized to yield polyesters and polyamides, respectively (Scheme 37). Interest in these types of polymer has been spurred by their obvious similarity to polysaccharides and to naturally occurring ionophores. 

Two inorganic water-soluble polymers, both polyclcctrolytcs in their sodium salt forms, have been know ll for some time poly(phosphoric acid) and poly(silicic acid). A more exciting inorganic water-soluble polymer with nonionic characteristics has been reported. This family of phosphazene polymers is prepared by the ring-opening polymerization of a heterocyclic monomer (6) followed by replacement of the chlorine atoms in Ihe resultant polymer. 

Most polymerizations of cyclic monomers are ionic processes. Coordination catalysts are effective only for some heterocycles (oxirane and its derivatives, lactones). Ziegler-Natta catalysts can only be used for cycloalkene polymerization by metathesis heterocycles act as a catalytic poison. Smooth radical polymerization of hydrocarbon monomers with ring strain is unsuccessful [304], The deep-rooted faith that ring strain represents a major contribution to the driving force in ring opening (polymerization) has to be revised [305, 306]. 

Although the pKa or H0 values for several acids are known [10,11], the definition of strong acid is somewhat arbitrary, because the position of equilibrium (13) depends on the basicity of heterocyclic monomer. Because this basicity varies from rather low (e.g., cyclic acetals) to rather high (e.g., cyclic amines) no universal rule describing the behavior of particular protonic acids in ring-opening polymerization exists. 

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