Bimolecular termination mutual

No support can be regarded as inert with respect to the active centres. By its universally positive effect on the activity of centres, MgCl2 is superior to any other support. In spite of the great technical importance of Mg in active centres, generally not much is known of their structure in third-generation catalysts (or perhaps because of its positive effects all the important producers have published hundreds of patents, but the crucial factors may still be kept secret). It is suspected that the separation (dilution) of transition metal atoms by a barrier of Mg atoms enables the majority of transition metals to become part of the active centres on these centres, the polymer grows more rapidly than on centres without Mg. Mutual contact of the centres is hindered, bimolecular termination of centres (transition metal reduction to a less active oxidation state) is limited, and the centres live longer. 

Termination will occur when the carbocation undergoes reaction with nucleophilic species other than monomer to produce a dead chain and no re-initiation. Since cationic polymerizations are carried out with high-purity reagents and under rigorous conditions this reaction is much less likely than chain transfer to monomer. The mutual repulsion of the charged polymerization sites ensures that bimolecular termination cannot occur (unlike in free-radical polymerization, where this is the most probable termination route). Recombination of the cation with the counter-ion will occur, and these termination reactions are often very specific to the chemistry of the initiator. 

Processes that are kinetically of second order with respect to the crmcentration of radicals within the reaction locus. The most important of these processes is bimolecular mutual termination between pairs of pro-pagating radicals within the same reaction lopus. 

An important further contribution to the analysis of steady-state reacticm systems has been made by Ugclstad et d. (1967), They have shown how account can be taken of the likely possihillty that radicals that exit from the reaction loci contribute to the stationary concentration of free radicals in the external phase which is available for entry into a reaction loci. For this purpose, it is necessary to distinguish blmolecular mutual termination between radicals that occurs in the reaction loci (i.e., within polymer/ monomer particles) from that which occurs in the external phase. The rate at which the former reaction occurs is characterized by the rate coefficient the rate of the latter reaction by. The total rate of entry of radicals into all loci within unit volume of reaction system is then expressed as the sum of three contributions. The first derives from the rate of formation of new "acquirable radicals within the external phase the second derives from the rate at which acquirable radicals become present in the external phase by the process of exit from the loci the third (which is negative) allows for the fact that radicals can be lost from the external phase by bimolecular mutual termination within the external phase. The resultant equation is... 

This problem was first treated in detail by Haward (1949). He considered the case of a bulk polymerization that has been compartmentalized by subdividing the reaction system into a large number of separate droplets, each of volume v. Radicals are generated exclusively within the droplets and always in pairs. An example would be the polymerizatiim of styrene in emulsified droplets dispersed in water initiated the thermal decomposition of an oil-soluble initiator which partitions almost exclusively within the monomer droplets. In the model considered by Haward, radicals are unable to exit from the droplets into the external phase. The only radical-loss process is in fact bimolecular mutual termination. It therefore follows that all the droplets must always contain an even number (including zero) of propagating radicals, and that the state of radical occupancy will change in increments of 2. The conclusion reached by Haward is that in this case the effect of compartmentalization is to reduce the overall rate of polymerization per unit volume of disperse phase. The f ysical reason for this is that, as the volume of the droplets is reduced, so are the opportunities for a radical to escape from the others—and hence to avoid mutual... 

The termination of polymer radicals occurs by various bimolecular recombination reactions. When the oxygen supply is sufficient the termination is almost exclusively via reaction (8) of Scheme 1.55. At low oxygen pressure other termination reactions may take place (Zweifel, 1998). The recombination is influenced by cage effects, steric control, mutual diffusion and the molecular dynamics of the polymer matrix. In melts the recombination of polymer peroxy radicals (POO ) is efficient due to the high mte of their encounter. 

Termination. Just as peroxy radicals are key to the propagation sequence, so the bimolecular recombination of these radicals is the major termination process in the unstabilized polymer. The existence of an intermediate tetroxide has been established in solution (25). Several factors influence the competitive pathways of subsequent decomposition to form alcohols, ketone and singlet oxygen or to form alkoxy radicals which can couple before separation from the reaction center to form a peroxide. This latter process is a route to crosslinking in the case of polymeric peroxy radicals. The effect of steric control, viscosity and temperature have been studied in solution. However, in the solid phase the rates of bimolecular processes which require the mutual diffusion of the reactant groups will be limited by the diffusion process. As a standard, we have assumed a value close to that determined from oxygen absorption (26) and by ESR spectra (27) for oxidized polypropylene films. 

Reactions that are bimolecular can be affected by the viscosity of the medium [9]. The translational motions of flexible polymeric chains are accompanied by concomitant segmental rearrangements. Whether this applies to a particular reaction, however, is hard to tell. For instance, two dynamic processes affect reactions, like termination rates, in chain-growth polymerizations. If the termination processes are controlled by translational motion, the rates of the reactions might be expected to vary with the translational diffusion coefficients of the polymers. Termination reactions, however, are not controlled by diffusions of entire molecules, but only by segmental diffusions within the coiled chains [10]. The reactive ends assume positions where they are exposed to mutual interaction and are not affected by the viscosity of the medium. 

Interval III Particle Growth in the Absence of Monomer Droplets.—James and Sundberg have published the results of an experimental study of ideal and non-ideal behaviour in the seeded emulsion polymerization of styrene. Unlike the experiments on seeded emulsion polymerization reported in papers referred to above, the amounts of monomer added to the seed latices were less than those required to saturate the particles and form a separate monomer droplet phase. The reaction systems were therefore the seed analogues of Interval III of a conventional emulsion polymerization reaction. The results are found to be in good agreement with the predictions of the Stockmayer-O Toole theory, provided that allowance is made for the effect of monomer/polymer ratio at the reaction locus upon the rate coefficient for bimolecular mutual termination. A paper by Hamielec and Marten is concerned with the effects of chain entanglements and the rubber-glass transition... 

Recent interest in this aspect of the subject commenced with the appearance in 1974 of a paper by Gilbert and Napper, in which solutions were reported for a special case of the Smith-Ewart differential difference equations, namely, the case where the loss of radicals from reaction loci occurs exclusively by processes which are kinetically of first order in radical concentration within the loci. Loss through bimolecular mutual termination of radicals is assumed either not to occur at all or, at most, to account for the loss of a negligible proportion of the propagating radicals. Radical loss is assumed to occur almost exclusively by processes such as diffusion from the reaction loci back into the external... 

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