Cationic polymerization simplified scheme

Both the initiation and propagation processes are, moreover, influenced by equilibria between various degrees of association of the active center and its counterion. As a minimum, it is necessary to consider the existence of solvent-separated ion pairs, and free solvated ions. A simplified scheme [20] is shown in Fig. 8.7. The existence of contact (associated) ion pairs [as in Eq. (8.1)] is neglected in this scheme because the dielectric constants of the solvents usually used for cationic polymerizations are high enough (9-15) to render concentrations of intimate ion pairs negligible compared to those of solvated ion pairs. The observed kp in these simplified reactions will thus be composed of contributions from the ion pairs and free solvated ions ... 

Figure 8.7 A simplified reaction scheme for initiation and propagation in cationic polymerization. (After Ref. 20.)...

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. 

Scheme 1 Reactions constituting ring-chain equilibria in ROP (simplified scheme). In brackets, a variant with hyper-bonded structures, for example, with oxonium cations or catalyst coordinated (activated) compounds, is shown. Z denotes an initiator originated end group (X-) or a linear fragment of polymer (X(M)r). Reversible ring-expansion/contraction reactions were added to this general scheme, although they are usually possible and/or important only in some coordination ROP polymerizations.

Propagation proceeds by nucleophilic attack of an oxygen atom in a monomer molecule on a carbon atom in a-position to an oxygen atom bearing formally the positive charge in a tertiary oxonium ion located at the chain end. Even such a simplified scheme indicates the possibility of a side reaction that is a typical feature of cationic polymerization of cyclic ethers. A nucleophilic center (oxygen atom) is present not only in monomers but also in polymer chains. Therefore, the attack of an oxygen atom from the chain on a carbon atom in a... 

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