Cationic polymerization covalent active species

It has been found that decarboxylation may be completely eliminated if cationic polymerization of cyclic carbonates is initiated with alkyl iodide or bromide. It is believed that polymerization proceeds with the participation of covalent active species favoring propagation over side reactions leading to C02 elimination [204]. It is interesting to note, that BF3-initiated polymerization of some cyclic carbonates leads to high molecular weight polymers (M > 10s) [205]. 

Controlled/living systems can be usually obtained when the polymerization is sufficiently slow and when either nucleophilic anions or additives are present (Sections IV and V). This means that the proportion of carbenium ions should be low and conversion to dormant species, fast. Nevertheless, under such conditions cationic species can be detected by dynamic NMR, by ligand exchange, salt, and solvent effects, and by other methods discussed in Chapters 2, 3, and in this section. Under typical controlled/living conditions, dormant species such as onium ions and covalent esters predominate. It is possible that the active species are strongly solvated by monomer and by some additives. These interactions may lead to a stabilization of the carbocations. However, in the most general case, this stabilization has a dynamic sense and can be described by the reversible exchange between carbocations and dormant species. 

Correlations of structures and reactivities for anionic and cationic ring-opening polymerization are reviewed. The following topics are discussed chemical structure of active species and their isomerism, determination of active centers concentration, covalent vs ionic growth and correlations between structures of active centers or monomers and their reactivities. 

It should be noted, however, that Scheme 8.2 depicts a highly simplified mechanism for living carhocationic polymerizations and it is in most cases not possible to find a counteranion with intermediate reactivity that spontaneously establishes an equilibrium between cationic and covalent species. Instead, the counteranion is generally a halide that preferably forms a covalent species with the carbenium ion. The addition of a Lewis acid as coinitiator is required to activate the covalently bound halide, resulting in the cationic carbenium ion. Alternatively, a nucleophile or electron donor can be added to the cationic polymerization, to reversibly form a stable cationic addition product with the carbenium ion. Both these deactivation mechanisms are depicted in Scheme 8.3. To achieve a living cationic polymerization it is of critical importance to have fast deactivation equilibria. In addition, the position of the equilibria should be carefully optimized for each monomer by variation of, for example, temperature, solvent, initiator, as well as the addition of halide activators or nucleophilic deactivators. 

High propagation rates and low concentration of active species renders kinetic measurements/calculations difficult and leaves plenty of room for debate concerning the mechanism and kinetics of carbocationic polymerizations. Kinetic evaluation is further hampered by the coexistence of covalent species with ion pairs and free ions as originally suggested by Winstein (119) for reactions involving cationic intermediates ... 

In cationic polymerization of alkenes, there is an equilibrium between active ions or ion pairs and inactive covalent species, where the ionization constant is rather low (10 in the isobutene/BCI3 system). The dynamics of this equilib-... 

Depending on the monomer and catalytic/initiating system and the nature of the resulting active species, a number of mechanisms can operate in the ringopening polymerization (ROP). The mechanisms most often employed include coordination, covalent, ionic (anionic or cationic), metathetic, radical and enzymatic. With regards to location of the active species, active chain-end or activated monomer mechanisms can be distinguished. A more detailed discussion of the ROP mechanisms is presented in Chapter 2. 

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