Syncatalytic system

The magnitude of c is more difficult to determine. In most studies of cationic polymerizations the nominal catalyst concentration lies in the range 10 2-10 4 mole/l. However, in syncatalytic systems, such as SnCl4-H20, the total concentration of cations, c, at any instant may be equal to the concentration of the scarcer of the two components (usually 10 3 to 10"5 mole/l) or it may be very much less than this. 

The Prague workers generalised this theory in order to explain the action of the syncatalytic systems consisting of halides of two metals [5], but later they apparently abandoned it [6]. What they did not see was that if their idea were supplemented and extended as will be shown below, it could explain most of the important features of the polymerisation of isobutylene by aluminium chloride which had defied explanation until then. They also did not test their theory in any detail. 

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 ... 

In the course of a long and thorough study of the polymerisation of isobutene (IB) by syncatalytic systems based on aluminium-organic compounds (Magagnini et al., 1977 and preceding papers) measurements were made by a Biddulph-Plesch type reaction calorimeter fitted with conductivity (k) electrodes on polymerisations of IB by Et2AlCl + Cl2 in MeCl at -45 °C, from which a kp+A value could be obtained. The reactions and procedures can be summarised as follows ... 

In the second part of the paper 6.10 [143] this writer generalises his new kinetic treatment of living polymerisations [134], By applying it to data from the literature, he shows how this treatment can reveal which one of the components of a binary syncatalytic system (e.g. I2 and organic iodide) determines the concentration of the propagating species and, much more important, how the rate-constant of propagation can be calculated from readily available data. There remains here a rich mine of information to be exploited by others. 

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 polymerisations of olefinic and cyclic compounds catalysed by conventional acids and by syncatalytic systems comprising a metal halide and a co-catalyst have been reviewed from several points of view [1-6]. These reactions are unusual in that the ideas used in their interpretation have changed frequently and the theoretical position is still somewhat confused. I shall examine here some of the reasons for this situation and attempt to clarify the picture by an historical approach. I will also give an account of some recent work leading to a revival of the old ester theory and indicate some of its implications and connections. 

With the syncatalytic system HI + I2 and a wide variety of monomers, living polymerisations can be obtained, especially at low temperatures and in solvents of moderate or low polarity, including toluene [22, 23 and earlier papers]. 

It is the nature of the activator that holds the answer to the inherent question If the polymerisations initiated by protonic acids, e.g., CF3S03H, and by syncatalytic systems, such as that consisting of the same acid + thiolan [25] or Me2S [26] are both pseudo-cationic, why does the one yield a product whose DP is determined by proton transfer to monomer, whilst the other produces a living system A review of the facts shows a simple phenomenological distinction, from which a mechanistic explanation can be deduced. The distinction is this ... 

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