Pseudocationic.polymerisation

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. 

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). 

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 ). 

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

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. 

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]. 

At the head of Table 2 stands what appears to be the same system as one quoted in Table 1 superficially it is, and the differences are subtle. During the work on the pseudocationic polymerisation of styrene by perchloric acid it was found that under special conditions a quite different type of reaction occurred which had all the... 

When some methylene dichloride containing a suspension of sulphuric acid was added to bulk styrene there was instantaneous development of colour and fast, complete polymerisation [7]. It thus appears that sulphuric acid can, under some conditions, cause a cationic, but not a pseudocationic, polymerisation, but the matter is not yet entirely clear. 

Probably the largest body of systematic evidence concerning the occurrence of cationic and pseudocationic reactions is that of Evans and his co-workers [12-15]. It indicates that 1,1-diphenylethylene and its derivatives are very suitable for an exploration of the conditions under which these reaction modes may occur. It is worth noting that even for those systems, in which the dimerisation takes place in the absence of ions, these are formed at the end of the reaction this same behaviour is found in the pseudocationic polymerisation of styrene by perchloric acid [5]. 

Detailed studies led Gandini and Plesch to formulate the concept of pseudocationic polymerisations. These are reactions which show many of the characteristics of cationic polymerisations, but do not involve ions. Since they could see no other alternative compatible with general chemical knowledge, they formulated the reactive species as an ester, and they were able to support this view by direct experiments (formation of the ester in the styrene solution by metathesis). The evidence indicates that in the system styrene, perchloric acid, methylene dichloride, the poly(styryl perchlorate) ester requires four molecules of styrene for its stabilisation. When these are no longer available, the ester ionises, and the residual styrene is consumed by a very fast, truly cationic polymerisation ionisation of the ester is a complicated reaction which has been only partly elucidated. The initiation and propagation of the pseudocationic polymerisation can be represented thus ... 

Whilst the kinetics, and probably the mechanism of pseudocationic polymerisations are simple, those of the reactions following polymerisation are not. Since generalisations are dangerous in this field, I will confine the discussion here to the system which we have explored and hope that others will find useful analogies in systems well known to them. Our findings on the ionogenic reaction which follows polymerisation in the system styrene, perchloric acid, methylene dichloride and other related evidence can be summarised tentatively by the following scheme [31] ... 

The opposite of the stabilisation of an ester is its activation. If we include in the concept ester the alkyl halides, their Friedel-Crafts reactions provide familiar examples of this phenomenon. An unusual example especially relevant to our present considerations is provided by some results made available to me in advance of publication by Giusti and Andruzzi. Their results [38] on the polymerisation of styrene by iodine and hydrogen iodide can be interpreted in terms of an organic iodide derived from styrene, probably 1-phenylethyl iodide, being activated by the co-ordination of one or two molecules of iodine. This process appears to polarise the C—I bond to such an extent that the normally stable ester becomes activated to a chain-propagating species and induces a pseudocationic polymerisation ... 

Cationic and Pseudocationic Polymerisation of Aromatic Olefins-ll. The Reactions Following Polymerisation of Styrene by Perchloric Acid (1968)... 

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