Chain breaking reactions

Fig. 16.5 Synergistic regeneration of a-tocopherol by quercetin at a lipid-water interphase. a-tocopherol is reacting with a lipid peroxyl radical in a chain-breaking reaction. According to the standard reduction potential, the phenoxyl radical of quercetin can further be regenerated by ascorbate.

The rate of a chain reaction is usually sensitive to the ratio of surface to volume in the reactor, since the surface serves to allow chain-breaking reactions (recombination of chain carriers) to occur. Thus, if powdered glass were added to a glass vessel in which a chain reaction occurred, the rate of reaction would decrease. 

The proportions of the different end-groups depend upon the relative rates of the various chain-breaking reactions by which they are formed. These are determined by the temperature, the solvent, the nature of the catalyst and co-catalyst, and the concentration of the chain-breaking agents. These features will be discussed below. 

Kennedy and Thomas attempted to construct a formal kinetic theory to account for these phenomena. They started from the assumption, for which there is no evidence, that the propagation reaction takes place with free ions only, but that the chain breaking reactions involve ion-pairs. 

If the DP is indeed independent of monomer concentration, the chain-breaking reactions which remain important at -180° must be of the same order with respect to monomer concentration as the propagation reaction. The most obvious conclusion is that monomer transfer is the dominant chain breaking reaction, so that DP = kp/km it follows that the activation energy, EDP, characterising the low temperature branch of the Arrhenius plot is Ep -Em = -0.2 kcal/mole. 

The higher EDP (-3.6 kcal/mole) at the higher temperatures cannot be interpreted without detailed kinetic information, but is probably associated with chain breaking reactions other than monomer transfer which gain in importance as the temperature is raised. 

It is not possible to give here a detailed account of, nor to take issue with, every aspect of the interpretation which the authors give to their results. Their main conclusion is that the inverse correlation between DP and conductivity proves that the principal chain breaking reaction must be a bimolecular termination between free cations at the growing end of the chain and free anions in the solution. However, the arguments which lead to... 

It does not follow, as the authors affirm, that this chain-breaking reaction with free ions is a termination, nor that the entity at the growing end of the polymer chain, which reacts with the free ions, is itself a free ion. 

It is evident that by far the most effective chain breaking reaction is that involving the chain breaking agent X, the concentration of which is proportional to that of the monohydrate. 

It is an obvious next step to identify this termination reaction with the second order chain breaking reaction involving the agent X, which was shown up by the Mayo plots. The rate of this is given by the third term of equation ii ... 

Most of the other important chain-breaking reactions are transfer reactions and much confusion has been created by authors failing to distinguish clearly the introduction of the term molecular termination [99] to designate the transfer by aromatic compounds [100] was not helpful. 

The adventitious chain-breaking reactions are those which involve the adventitious components of the polymerization system in other words, the impurities. The inherent chain-breaking reactions are those which are characteristic of the system, such as reactions between cation and anion, monomer transfer, solvent transfer. Each system has its own inherent chain-breaking reactions and for any one monomer the relative importance of these can be changed by changing the solvent, catalyst or co-catalyst [27b, 101]. 

One interesting difference between vinyl compounds and cyclic oxygen compounds is that with some of the latter, e.g., tetrahydrofuran, all kinds of chain-breaking reactions... 

In this way from appropriate plots other chain-breaking coefficients can be found - with corresponding cross-checks. It is evident that there is no way in which kn and / can be separated, and one can only hope to identify all chain-breaking reactions by exhaustive study and then take any chain-breaking effect which is independent of all concentrations as ktl. 

The termination reactions are probably more obscure than the other chain-breaking reactions in cationic polymerisation because in most systems they are unimportant compared to the transfer reactions. The only unambiguous evidence for the existence of a termination reaction can come from rate studies of the whole course of the reaction. Several types of behaviour can be distinguished ... 

The number of chain-breaking reactions which have been identified in various systems is... 

The DPs obtained in cationic polymerizations are affected not only by the direct effect of the polarity of the solvent on the rate constants, but also by its effect on the degree of dissociation of the ion-pairs and, hence, on the relative abundance of free ions and ion-pairs, and thus the relative importance of unimolecular and bimolecular chain-breaking reactions between ions of opposite charge (see Section 6). Furthermore, in addition to polarity effects the chain-transfer activity of alkyl halide and aromatic solvents has a quite distinct effect on the DP. The smaller the propagation rate constant, the more important will these effects be. 

Various reversible chain-breaking reactions between the growing cations and the various kinds of complex anions are discussed for the first time in some detail. It is also emphasized that terminations, i.e., irreversible chain-breaking, may be rather rare. 

Finally, the dependence of the DP and DPD on conversion, on mQ and on c0 must always be obtained, so that chain-breaking reactions can be identified by means of Mayo plots. 

The formation of the ester is very fast and the propagation is rate-determining. The chain-breaking reactions (which do not include a termination) are essentially those characterised by Pepper and Reilly [32]. 

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