Pseudo-cationic polymerization

Gandini and Plesch concluded that in these systems the chain-carriers are not ionic. Since they are certainly highly polar and in many respects behave as if they were ionic, we called the polymerizations propagated by them pseudo-cationic. Admittedly, in retrospect our original evidence for the non-ionic nature of the chain-carriers looks less convincing, but since that time many other phenomena have been found which support our view very forcibly [18] the case for the reality of pseudo-cationic polymerizations has been presented in detail [7], and therefore the argument need not be repeated here. 

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. 

The discovery of pseudo-cationic polymerizations has made it necessary to re-assess a very large part of the results in this field. The situation is in many ways similar to that created some 20 years ago by the discovery of co-catalysis, but whereas in the late 1940s there was only a handful of papers, there is now a vast body of information dealing with many monomers and catalysts which needs to be scrutinised. 

Before the discovery of the pseudo-cationic reactions, one could say simply that the function of the co-catalyst is to provide cations which can initiate the polymerization [28b]. Although this is still valid for the true cationic polymerizations, it is more difficult to define the function of the co-catalyst in the pseudo-cationic reactions. Very tentatively one can suggest that the co-catalyst is the essential link in the formation of an ester which is the chain-carrier, as in the pseudo-cationic polymerizations catalysed by conventional acids in other words, the co-catalyst and catalyst combine to form an acid, but this, instead of protonating the monomer, forms an ester with it, which is then the propagating species. 

Consider now a pseudo-cationic polymerization in which the non-ionic chain-carrier can form a complex with the compound R. The algebra and resulting equations are the same as above, with the one difference that q now signifies the concentration of uncomplexed chain-carriers and w the concentration of those which are complexed with R. 

In the present context the term cationic polymerization refers to reactions in which compounds with C=C bonds are added to carbenium ions, R R"R" C+, with the reformation of the carbenium ion after each addition, and the eventual formation of polymers in this way. The polymerizations via oxonium ions are excluded, as are pseudo-cationic polymerizations (Plesch 1988). 

Not many initiators belong to this class even though the halogenoacetic, fluorosulphonic, and other acids are included. A detailed analysis of their polymerization mechanism is obscured by complex formation with monomer and with solvent, by the occurrence of aggregates, and by anion reactions in acids with an unstable anion. In spite of its apparent simplicity, initiation by Bronsted acids has not yet been investigated in detail. The pseudo-cationic polymerization of styrene is an instructive example. 

The existence of centres with non-ionic character has already been suspected in studies of polymerizations which are supposed to proceed on carbocat-ions the theory of pseudo-cationic polymerization was proposed [137] (see Chap. 3, Sect. 3.1). The transformation of an ion pair to a covalent compound will evidently be easier for acid centres with heteroatoms, i.e. in heterocycle or vinyl ether polymerizations. Propagation on covalent bonds has actually been observed, first in the studies of oxazoline polymerization [138] and later even with THF [139, 140] and with other monomers (see, for example, refs. 131, 141 and 142). 

Initially, Gandini and Plesch proposed that the perchloric acid-initiated low temperature polymerization of styrene is based on monomer insertion on the nonionic perchlorate chain ends, which was based on the observation that the polymerization mixture was not conductive [68, 69]. These nonionic polymerizations were referred to as pseudo-cationic polymerizations. However, more detailed investigations by stopped-flow UV-vis spectroscopy revealed the presence of short-lived carbocations indicating that these are the propagating species in the cationic polymerization of styrene [70, 71]. This was also confirmed for the polymerization of styrene with trifiic acid for which Matyjaszewski and Sigwalt showed that the covalent triflic ester adduct was unstable even at -78 °C leading to carbocationic propagating species [72]. 

Similar results were reported in polymerizations of styrene with CH3COCIO4, CF3SO3H, CF3COOH, CISO3H, FSO3H [109]. Pseudo-cationic mechanism was also claimed in polymerizations of some styrene derivatives, like /7-methyl styrene, /7-methoxystyrene, andp-chlorostyrene with these protonic acids [109]. 

Although the concept of pseudo-cationic mechanism may be accepted by some, it is not accepted by many. An alternative mechanisms based on ion pairs were offered instead in the past [111-113]. A publication by Scwarc claims that all detailed re-examinations of the evidence for pseudo-cationic polymerizations shows that all the features of styrene polymerization initiated by acids are accounted for satisfactorily and convincingly by the orthodox ionic mechanism. To support his claim, he reported that optical absorption and electric conductance of the styrene solutions undergoing polymerization induced by perchloric acid in stopped-flow studies demonstrated protonations of the monomer by the acid and formation of ionic species [114]. 

Further proof against pseudo-cationic polymerization mechanism came from an earlier work by Matyjaszewski who successfully synthesized the styryl perchlorate and demonstrated that it hydrolyzes rapidly in reaction with water [115] ... 

This contradicts the basic claim of pseudo-cationic polymerization mechanism, as originally proposed by Gandini and Plesch, that the hydrolysis of the ester should be negligible [109]. 

What is pseudo-cationic polymerization Explain and illustrate on polymerization of styrene initiated by perchloric acid. 

Kennedy and co-workers10 studied model cationic polymerization initiation and termination. They determined the effect of halogens in f-BuX and MeX on the rate of reaction between f-BuX and Me3Al. The pseudo second order rate constant decreased (Table 1) as ... 

It is the author s hope that the foregoing detailed discussion has helped to clarify some features of cationic polymerizations. Many other aspects, such as co-polymerization and radiation polymerization, which I have not been able to discuss here, deserve equal attention. But perhaps the most urgent task, and one which is much more widely relevant, is the elucidation of details of reaction mechanism, and in particular the identification of the chain-carriers in many widely differing systems. The next problem then is to measure their concentration, its variation throughout the reaction, and, hence, the absolute rate constants. It is essential that the factors which decide whether a polymerization is ionic or pseudo-ionic be determined as soon as possible. 

The solution properties of complexes formed from 9 (n = 3) and polymethylene-diammonium cations, H3N+(CH2)mNHj have been studied in detail.[16] It was found that when the distance between the two ammonium cations is shorter than that between the crown rings in ( )-9 (n = 3), (e.g., m = 6), they form a polymeric complex (Figure 3). When the two distances are comparable (for example, m= 12), they form a 1 1 pseudo-cyclic complex. Photoisomerized (Z)-9 (n = 3) displayed a different mode of aggregation, because of the change in the distance between the... 

Metallocene complexes of Ti, Zr, and Hf have attracted considerable attention in recent years because of their high activity and since their ligand framework can be tailored to a wide variety of polymerization requirements.102 The active species is the 14-electron cationic alkyl [Cp2M—R]+, with a pseudo-tetrahedral structure and a vacant site suitable for forming a weak complex with ethylene. Calculations show that the alkyl transfer to ethylene is stabilized by an a-agostic interaction with the metal, with a very low (ca. 2 kJ mol-1) activation barrier 103... 

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