Pseudocationic

When concentrated sulphuric acid alone was used as the initiator, the polymerization was found to follow a different path. It is well known that Bronsted acids can function as cationic/pseudocationic initiators in the oligomerization of olifins [174]. If the counter ion has a higher nucleophilicity as it forms cation-conjugate pairs, which collapse rapidly, polymerization will not take place. As the counter ion in the case of sulphuric acid is not very strong compared to the cation, oligomerization can take place, but may not be to a very high molecular weight. This, however, depends on the nature of the... 

The discovery of the pseudocationic polymerisations by means of kinetic, conductimetric and spectroscopic studies led eventually to a fairly comprehensive theory of living CP. 

A. Gandini, P. H. Plesch, Cationic and Pseudocationic Polymerisation of Aromatic Olefins. Part I. Kinetic and Mechanism of the Pseudocationic Polymerisation of Styrene by Perchloric Acid, J. Chem. Soc., 1965, 4826. 

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. 

Since the reverse of the reaction Nl is the ionisation of the ester, the equilibrium position for any one system depends critically on the nature, especially the polarity, of the solvent, which determines the AHS terms. The accumulation of the necessary thermochemical data is essential to a rationalisation of the relation between cationic and pseudocationic polymerisations but the prevalence of the former at low temperatures and of the latter at high temperatures is surely related to the fact that the dielectric constant, and with it solvation energies, increases as the temperature of a polar solvent is reduced, so that decreasing temperature favours ionisation. 

The common finding, discussed in the previous section, that when cationic polymerisations are initiated with protonic acids, HA, as in the stopped-flow experiments, the [Pn ] [HA]0 can be explained along the same lines, as will be shown in Section 4.3.2. We have here a useful discriminatory test, because in the pseudocationic polymerisations initiated by HA (with or without a modifier, such as an organic sulphide), the concentration of growing ester is equal to [HA]0 (Plesch, 1992). 

For example, if k k = ca. 10 l-moH-s1 (Kunitake and Takarabe, 1979a), k = 10/8.5 x 10"2 = ca. 120 bmoH S"1. Unfortunately, the situation can be much more complicated, with some, or most, of the acid forming an ester with the monomer which then contributes to the rate by pseudocationic consumption of monomer. 

The kinetics of the formation and disappearance of the ions and the kinetics of the polymerisation are both complicated, and so the interpretation of the kp+A is far from clear. We have at least two complications to contend with. One is the extent to which a pseudocationic polymerisation contributes to the rate, the other is the relative importance of unpaired and paired cations. Neither of these questions is addressed by the authors, but we can attempt to resolve them in the light of what we discovered by analysing Kunitake and Takarabe s results obtained with what was ostensibly the same system. 

Pseudocationic Polymerisation, Renamed ca. 1998 Cationoid Insertion Polymerisation ... 

This Chapter contains most of the papers on pseudocationic polymerisations from the writer s research group and, because the experimental evidence plays such an important part in this story, it includes, exceptionally, the experimental papers which led Professor Gandini and the writer to devise the pseudocationic theory in the 1960s. Also, because of numerous misunderstandings, it is appropriate to include here an account of the circumstances surrounding the origin and fate of the pseudocationic theory of polymerisation. 

The impact which was made by the writer s revival of the old ester mechanism in the context of polymerisations is attested by the number of polymer chemists who set about examining the validity of the theory experimentally. For example, Bywater in Canada confirmed that during the progress of a polymerisation of styrene by perchloric acid the acid could not be distilled out of the reaction mixture, but after exhaustion of the monomer it could be. This regeneration of the initiating acid after the consumption of the monomer is an often attested characteristic of pseudocationic polymerisations with many different protonic acids it is most simply explained by the decomposition of the ester to an alkene and the acid, i.e., a reversal of the initiation, when the monomer has been consumed. Enikolopian in the USSR found that the effect of pressure on the rate of polymerisation in the same system was not compatible with the propagation step involving an ion, and... 

C Several different pieces of kinetic evidence from this writer s laboratory were thought to indicate that for the styrene + perchloric acid system the ester is stabilised by four molecules of styrene (48, 67). This feature seemed so unlikely that it helped to create opposition to the whole idea of the pseudocationic polymerisation by an ester, although that improbable solvation pattern is in no way essential to the theory nor logically connected with it. At this point it is necessary to introduce an important new insight which came to the writer whilst assembling this Prologue (ca. 30 years too late ). 

The polymerisation of styrene by anhydrous perchloric acid in methylene dichloride was shown not to be an ionic reaction (1) and we have now found further examples of what we propose to call pseudocationic reactions and have formulated a tentative mechanism for these. This involves as chain-carrier an ester which, if it is a perchlorate, is stabilised in solution by coordination of at least three molecules of monomer (or possibly unsaturated oligomer) ... 

However, we believe that we have also been able to diagnose true cationic polymerisations of styrene, but only under special conditions in calorimeter experiments with relatively large quantities of perchloric acid, especially at low temperature, down to -90 °C, it was found that when the phial of acid was broken in such a way that the mixing of acid with the solution was relatively slow, the solution turned yellow for a fraction of a second, near the broken phial, and there was an abnormally fast polymerisation which settled down after a few seconds to the rate appropriate to the pseudocationic reaction. The rate of these very fast reactions was much greater than could be accounted for by the high local concentration of acid if the reactions had been of the normal pseudocationic type, and we believe them to betray the transient presence of, and polymerisation by, true ions. 

Pseudocationic and True Cationic Polymerisation of Styrene with Various Catalysts... 

The kinetics of the polymerising system styrene-perchloric acid-methylene dichloride have been studied in the temperature range +19 °C to -19 °C, by a calorimetric technique. The propagation is pseudocationic, its rate constant at 19 °C is kp = 10.6 1 mole 1 s 1, and Ep = 11.6 kcal mole1. The elementary reactions are interpreted in detail by a mechanism involving an ester as chain carrier. 

Cationic and Pseudocationic Polymerisation of Aromatic Olefins, Part I... 

The above evidence strongly suggests that the pseudocationic reactions involve the ester 1-phenylethyl perchlorate and its oligomeric homologues as catalyst. It also shows that the ester is only stable when an excess of styrene is present in the reaction mixture. Spectroscopic and conductimetric studies on the present system confirmed this interpretation and indicated that at least four molecules of styrene are required for the stabilisation of one molecule of ester. Details of the experiments carried out to investigate the stoicheiometry of ester stabilisation will be given in a later paper. The mode of this stabilisation is not clear at present and we do not known the location of the four styrene molecules with respect to the ester. 

Since, however, we have disproved the chemical interpretation in terms of ions proposed by these authors we can conclude that the values for the rate constants and for the corresponding activation energies do not refer to cationic but to pseudocationic polymerisations. This distinction is most fundamental not only because of its chemical significance, but also because the two chain carriers, i.e., the carbonium ions and the ester molecules, exhibit a vast difference in activity, the former giving reaction rate constants several powers of ten higher than the latter [2], The true cationic polymerisation of styrene will receive our attention in a later paper of this series. 

The subject has been very thoroughly reviewed [1-3], but certain fundamental aspects can profitably be reconsidered in the light of some recent developments [4-6]. Certainly the most startling of these is the discovery that under conventional conditions the polymerisation of styrene by perchloric and other acids, and by the syncatalytic system stannic chloride-water, is not an ionic process. These polymerisations have been named pseudocationic . This finding is in direct contradiction to the beliefs held previously about this and related reactions. Hence a new survey of the whole field has become necessary which must start with an enumeration of those systems for which the nature of the polymerisation reaction has been established with reasonable certainty. From these boreholes one can then try to assess the nature of the intervening territory and to decide where further detailed exploration would be most profitable. 

The systems for which the polymerisation has been shown definitely to be pseudocationic are listed in Table 1. From this evidence and the similarities in kinetic behaviour, it follows that probably almost all the polymerisations of styrene and of some related monomers by conventional acids and by stannic chloride in hydrocarbons and chlorinated hydrocarbons are of this type. 

An interesting feature of pseudocationic polymerisations is that they are relatively insensitive to water which may be without effect on the kinetics even in concentrations up to ten times greater than that of the catalyst. This is of great diagnostic value since the carbonium ions derived from polymerisable olefins are instantly destroyed by water if the ionogenic catalyst is a conventional acid. If it is a metal halide, water may form a catalytic hydrate and its effect on rate and degree of polymerisation (DP) will depend on its concentration relative to the catalyst, and other factors. 

The pseudocationic polymerisations are tentatively explained in terms of an ester as the active species. For the styrene-perchloric acid system it is known that this ester is only stable in the presence of an at least four-fold excess of styrene. When, at the end of a polymerisation, the styrene concentration has fallen to this level, the ester ionises. When styrene is mixed with an excess of acid in dilute solution, protonation is the only reaction [5-7]. 

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