Solution polymerisation

Similar equations may of course be deduced for a polymerised solute in this case i < 1 in (18). 

When the reaction is then killed with tritiated water, the polymer contains tritium which is not exchangeable, and the tritium content is correlated with the conductivity of the polymerising solution. These findings support the suggested reaction sequence ... 

The assumption that tertiary alkyl cations are not stable in solvents other than super-acids is widespread and was apparently well founded on many experiments by different workers over many years [20, 24]. For this reason the stability of our polymerised solutions was astonishing and it seemed at first unlikely that the cation of the electrolyte could be a simple tertiary ion the tert-butyl cation in the experiment with tert-butyl bromide and the ions 2-4 in the polymerised solutions. This was because we did not know then that Cesca,... 

We concluded, therefore, that in sufficiently pure alkyl halide solvents the tert-alkyl tetrahaloaluminates are stable electrolytes and that previous failures to produce them, and the consequent legend of the instability of tcrt-alkyl carbenium ions, arose from the use of inappropriate and insufficiently rigorous experimental techniques. On this basis it seems highly probable that in the polymerised solutions the cations R+ partaking in reaction (viii) were also original ions, i.e., 2 at the end of a live chain and 3 and 4 formed by alumination of a terminal double bond, and not derived ions formed by degradative reactions of monomer or polymer. 

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... 

The principle of the short-stop method of determining Y.xr, in cationic polymerisations is that to the polymerising solution a basic reagent Q is added which combines with the growing centre and thus forms an end-group in the dead polymer ... 

Because our main concern is the composition of the ionic population of the polymerising solutions, we need to consider the principal factors which affect it, namely the polarity of the solvent and the ionic concentration with particular reference to the formation of ionic aggregates. The simplest of these are the ion-pairs, and we will not consider any higher aggregates because the ionic concentrations are usually far too low for their formation to be significant. This means that we are concerned with the equilibrium (11) ... 

The role of ion-pairs is discussed at some length. Conductivity measurements on polymerised solutions of EVE with successive dilutions gave results from which ion-pair dissociation constants KD were calculated conventionally by means of Shedlovsky plots. However, since the conductivity of solutions of the model system EtOCHMe+ SbCl6" can be interpreted much more plausibly in terms of a BIE (Plesch and Stannett, 1982),... 

The results obtained by Jaacks s ethoxide method, shown in Tables 1 and 2, prove that for all systems the concentrations of tert-oxonium ions are very considerably smaller than those of the perchloric acid. In view of the correlations shown above, they must also be much smaller than the concentrations of ions in the polymerising solutions. We conclude, therefore, that the principal growing ions are not tertiary, and that they must, therefore, be secondary. 

The method of Jaacks and his co-workers was used by Firat and Plesch to study the nature of the cations present when DXT, 4-methyl-DXL, and 1,3-dioxepan (DXP) were polymerised by anhydrous perchloric acid in methylene dichloride. They showed that when these polymerisations are done under the most rigorously dry conditions, the concentration of tert.-oxonium ions in the polymerising mixtures is at most close to the limit of detection and much less than the original concentration of perchloric acid and the concentration of ions in the polymerising solution. 

Expertise in Emulsion Polymerisation, Solution polymerisation, Process, Product development, Molecular Metal Catalyst and Elastomer,... 

H NMR spectrum of the solution was identical to that of polymerising solutions initiated by methyliodide and proceeding via the oxazinium ion iodide. 

It was found (43) that dilution of the polymerising solution increases the yield of the hydantoin-3-acetic acid and this result, predicted by Bamford s mechanism, provides additional evidence in its favour. 

The self-acceleration of y-benzyl-L-glutamate NCA polymerisation initiated by sodium methoxide was reported by Idelson and Blout (50). This reaction produces a high-molecular weight polymer and acceleration, although less pronounced, is still observed when fresh monomer is added to a polymerised solution (see Fig. 17). It is significant that the straight portions of curves A and B, shown in Fig. 17, are parallel. Apparently, the same stationary state eventually is attained in both reactions. 

It is well known that even modern Fourier-transform instruments require concentrations of at least 10 M to give a reasonably resolved NMR spectrum. Also, the minimum time necessary for a spectrum to be taken or for one given signal to be scanned is of the order of a minute. These constraints rule out the possibility of identifying short-lived transients or any compound present in low concentrations formed during a cationic polymerisation. Thus, while carbenium ions have been characterised by this technique under suitable conditions, their concentration in a polymerising solution is always too low to permit detection by NMR spectroscopy. On the other hand, one can easily follow the disappearance of the monomer and/or the formation of the polymer and of any other product (such as an ester) formed in appreciable quantities, an operation which is extremely convenient, not only from the point of view of the kinetics of pol)onerisation, Init also because it can lead to mechanistic information concerning the nature and the structure of various products. 

SimOar work was carried out by Giusti s group on acenaphthylene and the same mechanistic conclusions were reached. The lack of effect of hi electric fields applied to the polymerising solutions was an additional element in favour of pseudocationic propagation. 

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