Styrene cationic polymerisation

Considerable advances have taken place in the 1990s with regard to cationic polymerisation of styrene. Its uses to make block copolymers and even living cationic polymerisation have been reported (171). 

G. E. Holdcroft, P. H. Plesch. The Propagation Rate-Constants for the Cationic Polymerisation of Acenaphthylene and Styrene in Nitrobenzene, Makromol Chem., 1984, 185, 27. 

In the light of our new knowledge about the pseudo-cationic polymerisation of styrene it appears that many, perhaps all, the main differences between aliphatic and aromatic monomers may be due to the fact that one is not comparing like with like that is, the differences arise because under many of the most commonly used experimental conditions the two groups of monomers polymerise by different mechanisms. In order to make valid comparisons between two monomers it is necessary to ascertain first that they do both polymerise by the same mechanism under the same conditions. 

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. 

Reaction with solvent - The solvent influences the course of cationic reactions not only through its dielectric constant, but also because many substances used as solvents are far from inert in these reactions [22, 23]. Although much more experimental material is required before a full treatment of the subject becomes possible, at least one example, the cationic polymerisation of styrene in toluene, is amenable to quantitative discussion. Experiment shows that polymerisation is rapid and complete, the molecular weight is low and the polymer contains para-substituted rings which are almost certainly tolyl endgroups [22]. Theoretically, a polystyryl carbonium ion can react with toluene in six different ways, only two of which (a.l and b. 1 below) can lead to tolyl endgroups in the first case the tolyl group is at the end of the terminated chain, in the second it is the start of a new chain. The alternative reactions can be represented as follows... 

Probably the earliest quantitative experiments on what are now known as cationic polymerisations were made by Gwyn Williams (1938) with styrene and stannic chloride, and by the early 1940s the general belief had become established that in reactions initiated by metal halides the active species is a cation. It appears that Evans and Meadows [3] were the first to state specifically that in hydrocarbon solvents the propagating cations must be paired with the anions and Plesch [4] made the first attempt at calculating the dissociation constant, KD, for an ion-pair comprising a growing cation in a hydrocarbon solvent ... 

This is the third report on attempts to measure the propagation rate constant, kp+, for the cationic polymerisation of various monomers in nitrobenzene by reaction calorimetry. The first two were concerned with acenaphthylene (ACN) [1, 2] and styrene [2]. The present work is concerned with attempts to extend the method to more rapidly polymerising monomers. With these we were working at the limits of the calorimetric technique [3] and therefore consistent kinetic results could be obtained only for indene and for phenyl vinyl ether (PhViE), the slowest of the vinyl ethers 2-chloroethyl vinyl ether (CEViE) proved to be so reactive that only a rough estimate of kp+ could be obtained. Most of our results were obtained with 4-chlorobenzoyl hexafluoroantimonate (1), and some with tris-(4-chlorophenyl)methyl hexafluorophosphate (2). A general discussion of the significance of all the kp values obtained in this work is presented. 

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... 

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. 

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 ... 

Another, necessarily much less precise, method for determining n is available from the kinetic experiments [1]. At the end of these reactions, at the precise instant at which the reaction mixtures turned yellow, a very fast reaction took place (Figure 5). This represents the polymerisation of the residual styrene by a true cationic reaction caused by the ions formed from the ester at the point where the styrene concentration was reduced to a level no longer sufficient to stabilise the ester. From the very small temperature rise during this final fast reaction, the number of styrene molecules polymerised could be calculated it was always about four times the initial concentration of perchloric acid. This phenomenon was particularly evident in the reactions carried out at -19 °C in which relatively high acid concentrations were used so as to obtain reasonably fast polymerisations [1]. 

In some pseudo-cationic systems, e.g., styrene + H2S04, polymerisation ceases when the yield, Y, is less than 100%, but it can be restarted by the addition of a very small quantity of trichloroacetic acid [31]. 

Pseudocationic and True Cationic Polymerisation of Styrene by Various Catalysts, A. Gandini and P.H. Plesch, Journal of Polymer Science, Part B,... 

Styrene undergoes polymerisation with the cationic organonickel(II) complex / 3-methallyl ( /4-cycloocta-l,5-diene)nickel hexafluorophosphate [(MeAll) (Co<7)/W]+[PF]6 combined in situ with tricyclohexylphosphine. The product of such polymerisation is a styrene oligomer with Mn = 1900, characterised by a... 

The first study of the cationic polymerisation of styrene by triflic acid was undertaken before 1970 by Mathias and Plesch They fixind that very tow catalyst con-centratimis (10 — 10 M) were needed to induce a fairly rapid polymerisation in dichloromethane. These reactions were followed calorimetric y, conductimetrically and spectrophotometrically. The mqor obstacle precluding a stematic study was the poor reproducibility of the results. This problem was finally traced to the ng of the acid solutions contained in sealed phials. It was concluded that a slow reaction between the catalyst and the solvent was the cause of this aging phenomenon ... 

Also in 1960, Asami and Tokura showed that 60% perchloric acid was a very effective catalyst for the polymerisation of styrene in sulphur dioxide But it was the now classic study of Pepper and Reilly on the pdiymerisation of styrene by the anhydrous acid in various solvents that initiated a Icrng series of investigations on the rde of this promoter in the cationic polymerisation of arcmatic defins and more recently of cyclopentadiene. 

The system styrene-perchloric acid is however more complicated than originally thought. Gandini and Plesch reported in 1%5 that under special conditicms the tme cationic polymerisation of styrene had been observed. The presence of ionic cies resulted in a very fast polymerisaticm even at temperatures as low as —90 °C. These observations were complemented in a later paper, but never studied stematically. It was in Pepper s laboratory that this investigation was ccmtinued and led to the use of stop-flow techniques to attempt a ccmplete raticmalisaticai of the very complex features displayed by this system at low temperature 18,340,341)... 

Two recent studies cast more light on this problem and they are particularly interesting because they relate directly to the context of cationic polymerisation. In the first Bogomolova et al. examined the electronic spectra of mixtures of stannic chloride and styrene or a-methylstyrene in ethyl chloride and cyclohexane. Complexation with the first moncaner gave a band at 295 nm with an extinctimi coefficient of about 1,000 cm in ethyl chloride and about 2,000 in cyclohexane. The in-... 

The use of ethers as cocatalysts for the cationic polymerisation of alkenyl monomers induced by Lewis acids has received little systematic attention and the mechanism through which these compounds operate is not well understood. The complex diethyl-ether-boron fluoride has been extensively used as a very convenient cationic initiator, but mostly for preparative purposes. As in the case of alcohols and water, ethers are known to act as inhibitors or retarders in the cationic polymerisation of olefins, if used obove cocatalytic levels, because they are more nucleophilic than most rr-donor monomers. Imoto and Aoki showed that diethyl ether, tetrahydrofuran, -chloro-diethyl ether and diethyl thioether are inhibitors for the polymerisation of styrene-by the complex BF3 EtjO in benzene at 30 °C, at a concentration lower than that of the catalyst, but high enough (0.5 x 10 M) to quench the active species formation for a time. Their action was temporary in that the quenching reaction consumed them, and therefore induction periods were observed, but the DP s of the polystyrenes were independent of the presence of such compounds, as expected from a classical temporary inhibition. 

In 1962 Tokura and Kawahara reported that styrene could be polymerised by various alkyl and aryl chlorides in sul ur dioxide. The solvent did not participate in the polymerisation as drown by the absence of polysul one copolymers among the products. Benzyl, 1-phenylethyl, propyl and Ixxtyl chlorides were all modestly active initiators in these experiments. Typically about 10% polystyrene, DP = 100—300, was obtained in 150 minutes with 0.37 M of initiator at 25°C. Under these same conditions, both trityl chloride and hydrogen diloride failed to give any polymer. We do not know if these results have ever been repeated and confirmed by other workers. If they are genuine and do not arise from some unknown artifact, they could be interpreted in terms of specific solvation of the chlorides by the solvent to give a C—Cl polarisation sufficient to induce the pseudo cationic polymerisation of styrene. However, more work would be necessary to confirm these proposals. 

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