Pseudocationic Polymerizations

Similar results were reported in polymerizations of styrene with CH3COCIO4, CF3SO3H, CF3COOH, CISO3H, and FS03H. Pseudocationic mechanism also takes place in polymerizations of some styrene derivatives, like p-methyl styrene, p-methoxystyrene, and p-chlorostyrene with these protonic acids.  

Although the pseudocationic mechanism is now fairly well accepted, it was argued against in the past and alternative mechanisms based on ion pairs were offered instead.  

In some cationic polymerizations, the monomers may rearrange in the process of placements into the chains. The isomerizations are to eneigetically preferred configurations. The result is that the units in the final polymers are structurally different from the original monomers. Such rearrange- 

The isomerization polymerizations were classified by Kennedy according to the type of rearrangement that accompanies the propagation and by the particular processes.  

Propagation reactions accompanied by bond or election rearrangement  

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It needs to be said at the outset that my attempts at clarification have not been made easier by the discovery [4] of the pseudocationic polymerizations early in 1964. Since exploration and revaluation of these reactions are still only in their early stages, there are inevitably many loose ends and open questions and probably also some inconsistencies in the present work. Some aspects of pseudocationic polymerization have been reviewed [5-7]. It should be noted that this discovery makes many of the theoretical discussions in Reference 1 of purely historical interest. Since the publication of Reference 1 several reviews on, and relevant to, cationic polymerization [8] and on carbonium ions [9] have appeared. 

It appeared to us that the only reasonable non-ionic reaction product of an acid and an olefin would be an ester, and for this reason we put forward the idea that this is the active species in the pseudo-cationic polymerizations. Of course, the idea of an ester in this role has a respectable ancestry which has been discussed in this new context [6]. The ester mechanism of polymerization will be discussed in sub-section 3.3. It must be understood that our conclusion concerning the non-ionic nature of the chain-carriers in the pseudocationic polymerizations is quite independent of our view that the chain-carriers are esters this is at present merely an hypothesis to explain our factual conclusion. 

Nevertheless, some research groups challenge the concept of a dynamic equilibrium between dormant covalent species and carbenium ions, and instead insist that covalent species can react directly with monomer. They propose that this occurs by a pseudocationic mechanism involving a multicenter rearrangements [259], such as those shown in Eq. (78), for the pseudocationic polymerization of styrene initiated by perchloric acid [260]. 

What is pseudocationic polymerization Explain and illustrate on polymerization of styrene initiated by perchloric acid. 

Pseudocationic polymerizations can be distinguished from true ones by the temperature dependence of the rate of polymerization and the effect of added water. That is, pseudocationic reactions proceed slowly at low temperatures —90 C), whereas cationic polymerizations are still vary rapid. Furthermore, the rate of a pseudocationic polymerization is practically unaffected by the addition of water (up to [H2O]/[initiator] = 10 1). By contrast, in true cationic polymerizations, even at very low concentrations, water strongly affects the polymerization. Carbocations from olefins, in fact, are instantly destroyed by added water (see Chapter 18). Metal halides form hydrates with water. The concentration of these hydrates, and therefore the water concentration, then affects the polymerization rate and the degree of polymerization. 

This interpretation indicates why not every cocatalyst is equally effective with every Lewis acid. The behavior of water as cocatalyst is especially notable. In many cases, a small amount of water increases the polymerization rate, while in other cases, it has practically no influence. This characteristic of water appears to be especially useful as a diagnostic tool for distinguishing between genuine cationic and pseudocationic polymerizations. Pseudocationic polymerization (see Section 19.2.2), of course, is only very slightly affected by addition of water. 

An interesting feature of certain so-called pseudocationic polymerizations is that they involve propagation of polarized covalently-bonded species (e.g. a/vvCH2CRiR2—OCIO3 species when initiation is by perchloric acid). 

The kinetics scheme analysed in Section 2.6.4 naturally leads to formation of polymers with a most probable distribution of molar mass (cf. Section 2.4.9). However, polymers formed by cationic polymerization sometimes have complex (e.g. bimodal) molar mass distributions and in certain cases (e.g. pseudocationic polymerizations) this is thought to be due to simultaneous growth of different types of active species with very different but characteristic values of the rate constants for propagation and ion-pair rearrangement. 

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