Equilibrium polymerisation

Details are given of the successful construction of a novel reversible system of network polymers between bifunctional monomers by utilising the equilibrium polymerisation system of a spiro orthoester. Molecular structures were determined by NMR and IR spectroscopy. 9 refs. 

This publication is the record of the papers given and of the discussions at a meeting convened in May 1950 at Trinity College, Dublin by D.C. Pepper which is usually referred to as the First International Cationic (occasionally just Ionic) Symposium (A). It is important in the history of polymer science because many important new ideas were discussed there, some for the first time. These included Dainton and Ivin s theory of equilibrium polymerisations, co-catalysis (Plesch, Polanyi and Skinner), and the energetics of polymerisations. The present author made several contributions to that discussion, the most substantial of which was a joint theoretical paper which is reproduced here ... 

The question whether this reaction is monoeidic or enieidic was not discussed by that author, but in view of the low polarity of bulk THF it seems likely that the principal propagating species is ion-pairs but since there is no definite evidence we will denote the rate constant by kp, but write the equations in terms of x rather than Exr. For this equilibrium polymerisation therefore... 

The big advantage of THF copolymerisation with alkyleneoxides is the fact that the equilibrium polymerisation characteristic of THF homopolymerisation is practically suppressed, at relatively high concentrations of alkylene oxides (30-50%). This behaviour leads to high yields of the resulting copolyether, THF-alkylene oxides, of around 85-90% (Figure 7.3). 

A bacterial isolate APN has been shown to convert a-aminopropionitril enantioselectively to L-alanine (94% yield, 75% e e). However, the major disadvantage of this approach, is the low stability of most aminonitriles in water (for example a-aminophenylacetonitrile in water of pH 7, degrades completely within 48 hours). The aminonitriles are always in equilibrium with the aldehyde or ketone and ammonia/HCN. Polymerisation of hydrogen cyanide gives an equilibrium shift resulting in the loss of the aminonitrile. Therefore, a low yield in amino adds is to be expected, which makes this method less attractive for the industrial synthesis of optically active amino adds. 

Acetaldehyde, b.p. 21°, undergoes rapid polymerisation under the influence of a little sulphuric acid as catalyst to give the trimeride paraldehyde, a liquid b.p. 124°, which is sparingly soluble in water. The reaction is reversible, but attains equilibrium when the conversion is about 95 per cent, complete the unreacted acetaldehyde and the acid catalyst may be removed by washing with water ... 

A common characteristic to all the chemical reactions involved in step polymerisation that should be emphasised is that they are most often equilibrated reactions. For instance, the polyesterification reaction is based on the esterification/hydrolysis equilibrium... 

Performing what is known as post-condensation. Most step polymerisations are exothermic and, consequently, the equilibrium constant K decreases with increasing temperature. Hence, one way to increase the molecular mass would be to decrease the polymerisation temperature, but kinetics prohibits using a too low temperature as it will lead to an excessively long residence time in the reactor and/or too high viscosities. Thus, in order to reach very high molecular... 

One of the basic assumptions of this theory is that the polymerisation rate can be computed from the transition rate from an initial electronic state E to a final one Ef of the crystal at a given polymerisation state. The energies of these states depend on the nuclear configuration and their changes around the equilibrium positions for the initial and final electronic states can be expressed (43) in terms of vibrational oscillators which at a given temperature are either classical 1ui)c[Pg.181]

Yet, during the last decade, considerable advances have been made towards a quantitative understanding of the structural and energetic factors controlling chain cyclisation. Thanks to the application of modern technology there has been a substantial accumulation of reliable data in the form of accurate kinetic or equilibrium measurements of cyclisation reactions of bifunctional chains, as well as of careful analyses of ring-chain polymerisation equilibria. These will be dealt with in the remaining part of this section. 

The important feature was the recognition that because the relation between k and [TiClJ was rectilinear, the number of concentration terms on each side of the equilibrium had to be the same, which ruled out the involvement of ion-pairs. Our work opened up the new field of binary ionogenic equilibria (BIE) which was to prove an essential preliminary to unravelling the mechanism of the polymerisation of isobutene by A1C13. We also showed... 

There remains, of course, the question why apparently isobutene (and perhaps other aliphatic olefins) do not polymerise by the pseudo-cationic mechanism - or do so much less readily than, say, styrene. Probably the short answer lies in the relative stabilities of the esters in the polymerisation conditions, (e.g., perchlorate stabilised by co-ordination of styrene). The long answer will have to be based on a detailed understanding of all the factors which determine this stability and thus govern the equilibrium between ester and ions. 

As Skinner has pointed out [7], there is no evidence for the existence of BFyH20 in the gas phase at ordinary temperatures, and the solid monohydrate of BF3 owes its stability to the lattice energy thus D(BF3 - OH2) must be very small. The calculation of AH2 shows that even if BFyH20 could exist in solution as isolated molecules at low temperatures, reaction (3) would not take place. We conclude therefore that proton transfer to the complex anion cannot occur in this system and that there is probably no true termination except by impurities. The only termination reactions which have been definitely established in cationic polymerisations have been described before [2, 8], and cannot at present be discussed profitably in terms of their energetics. It should be noted, however, that in systems such as styrene-S C/4 the smaller proton affinity of the dead (unsaturated or cyclised) polymer, coupled, with the greater size of the anion and smaller size of the cation may make AHX much less positive so that reaction (2) may then be possible because AG° 0. This would mean that the equilibrium between initiation and termination is in an intermediate position. 

If one wants to use only a metal halide as the initiator for an alkene polymerisation, an analysis similar to ours can be made for the metal halide alone. In addition, there is a 1 2 equilibrium for the organic halide [35, 36] and a molecular aggregate <-> single molecule equilibrium associated with the metal halide. Thus, a solution of a carbocation salt is more exactly described by the series of linked equilibria summarised in Scheme 5. 

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 ether-catalyst complex (II) splits into a complex anion (III) and a carbonium ion (IV), which rearranges to the configuration of maximum stability (V). This carbonium ion (V) could itself initiate polymerisation, but it is more likely that it attacks the double bond of the closely associated anion (III), giving the double ion (VI) in equilibrium with the aldehyde (VII). Rearrangements of the type (I)-(VII) have been observed for vinyl ethers [7], and a closely parallel isomerisation is that of isobutyl phenyl ether into para-tertiary butyl phenol under the influence of A1C13 [8]. It is unlikely that the steps from (II) to (VI) take place in a well defined succession. The process probably proceeds by a single intramolecular transformation. 

The fact that the results of some earlier studies can be explained better in terms of equilibrium (9) than equilibrium (11) is undoubtedly related to the relatively high level of impurities prevalent in the earlier experiments, but it has not yet been explained in detail. This matter is, however, not as irrelevant as it may seem, because (as mentioned above) in the actual solutions of aluminium halides which have been, or are likely to be, used in most polymerisation experiments, an important fraction of the ions present probably arises from reactions of the initiator with impurities, solvent or both, and the case discussed here in detail - ions arising only from reaction of the initiator molecules with their own kind - is surely an idealisation. 

We can now begin to see some of the implications of the theory and in which directions its applicability can be tested. In the course of these considerations one must always keep in mind that the rate of the ionogenic reaction (iii) ( left to right rate-constant k() is very small in comparison with the rates of polymerisation and of complex formation (reaction (ii)) and that the equilibrium concentration of ions is very small (see Table 1). 

Figure 3 The final, equilibrium conductivity of polymerised mixtures of isobutylene and AlBr3 in methyl bromide as a function of the isobutylene concentration [IB]. Each point represents the conductivity resulting from an addition of isobutylene conditions...

In the present context it will be useful to establish the conditions under which free cations or paired cations might be expected to determine the behaviour of a cationic polymerisation some aspects of this problem have been discussed previously [5]. Consider a system in which Pn+ are the growing polymer molecules and A is the anion derived from the catalyst or the syncatalytic system. Let [Pn+] + [Pn+ A"] = c, let [Pn+] = [A ] = i, [Pn+ A"] = q, and let K be the equilibrium constant for the dissociation of ion-pairs ... 

As far as the polymerisation of heterocyclic monomers is concerned, the situation is qualitatively similar, but quantitatively different. As a model for the active species in oxonium polymerisations, Jones and Plesch [10] took Et30+PF6 and found its K in methylene dichloride at 0 °C to be 8.3 x 10"6 M however, in the presence of an excess of diethyl ether it was approximately doubled, to about 1.7 x 10 5 M. This effect was shown to be due to solvation of the cation by the ether. Therefore, in a polymerising solution of a cyclic ether or formal in methylene dichloride or similar solvents, in which the oxonium ion is solvated by monomer, the ion-pair dissociation equilibrium takes the form... 

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