By Stanislaw Penczek, Przemyslaw Kubisa, Krzysztof Matyjaszewski
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Extra resources for Cationic Ring-Opening Polymerization of Heterocyclic Monomers
In this formula, the proton is tacitly assumed to be solvated by (at least) monomer molecules or exists as a secondary oxonium ion (cf. Sect. 1). Initiation of some monomers (e. g. THF) by the Lewis acid - H20 system is often so slow that the addition of so-called promoters is required (oxiranes 111, 117) lactones 117)). First, these promoters rapidly form the secondary oxonium ions replacing the slow initiation with the less reactive monomer (here, less strained THF). Thus, Lewis acids do not appear to be prominent initiators for living polymerizations.
In the second group of systems diarythalonium salts, failing to initiate polymerization generate the carbenium salts by decomposition induced by free radicals formed photochemically. Benzophenone, benzil or maleic anhydride were used as sources of free radicalsll9): hP R2 ~ 2R* (51) Depending on the structure of the carbocation formed and monomer used, initiation can proceed either by hydride abstraction 118) or by direct addition. g. the methoxy groups and an aromatic ring as in Scheme (50). On the other hand, the cationic component should provide, preferably in the radical-induced reaction, a cation which abstracts an electron from the free radical (Scheme 50).
It was possible in this system to use 102 times lower concentration of the triflic anhydride than that of BF3 (n-C4H9)20 initiator and obtain comparable overall rates of polymerization. 4 Propagation Despite the fast progress of our understanding of the cationic polymerization of heterocycles, the number of systems providing truly living polymers are still very limited. However, the proper choice of the necessary conditions for a given monomer (solvent, temperature, anion) gives an access to studies of propagation reactions in optimized systems, i.
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