Termination, of radical

The termination of radical polymerization cannot be prevented under normal conditions. This would be possible only in a polymerization initiated in rigid media, assuming that no chain transfer occurs, or if the radicals are trapped, for instance, by precipitation of the polymer during the process of its formation. Both methods have been used, and indeed the termination was considerably slowed down or even prevented permanently. However, such systems are of little value for synthesizing polymers according to a preconceived pattern. 

Selenosulphonylation 191, 194, 1107-1109 Selenoxides, as oxidizing agents 253 Self-termination, of radicals 1084, 1085,... 

The first term on the RHS of (III-8) represents mutual termination of radicals in the polymer particles (i.e. second order termination). The second term represents a first order termination of radicals in the polymer particles by monomer soluble impurities (MSI), which are present in the polymer particles due to their transfer in there with monomer during the monomer diffusion phase from monomer droplets. 

Ruhho, H., Parthasarathy, S., Barnes, S., Kirk, M., Kalyanaraman, B., and Freeman, B. A., 1995, Nitric oxide inhibition of hpoxygenase-dependent hposome and low-density hpoprotein oxidation termination of radical chain propagation reactions and formation of nitrogen-containing oxidized hpid derivatives, Arclr. Biochem. Biophys. 324 15-25. 

In presence of triphenyl phosphite or tri(4-methyl 6-f-butylphenyl) phosphite, (6) also acts as an oxidation inhibitor for polyenes. The complexes formed have not been studied, but the effect is attributed to termination of radical chains and the complexes are more active than the phosphites alone.29 Vanadyl acetylacetonate is also employed as a hardener for unsaturated polyesters.30... 

The bimolecular termination of radical active centers involves combination (Eq. (2.89 a)) or disproportionation reactions (Eq. (2.89 b)) ... 

R., (Priest s point 3), reduced by the rate at which the radicals are captured by particles, R, (Priest s point 4). Coagulation will further reduce the number of particles at a rate R (Priest s point 5). If the mutual termination of radicals in solution is neglected, then... 

As long as the distribution of radicals among the particles approximates a steady state situation (a condition that may be violated during periods of rapid acceleration in latices), one may use Stockmayer s result (7) to compute n. When radical desorption is insignificant and water phase termination of radicals is neglected,... 

In this set of equations, n is the number of reaction loci per arbitrary volume of reaction system which contain i propagating radicals, cr is the average rate of entry of radicals into a single reaction locus, xr is the volume of the reaction locus, A. is the rate coefficient for the mutual termination of radicals, and k is a composite constant which quantifies the rate at which radicals are lost from reaction loci by first-order processes. For convenience we put kju s x ... 

Processes such as cross-linking, second-order termination of radical species, and polymerization are all examples of recombination. Here, we consider second-order recombination, where two molecules or radical fragments may join to form a new molecule of higher degree of polymerization. 

It is now suggested that in the absence of Mo 2 as a hydrogen activating phase the thermal coke formation is substantial. A very limited amount of Mo is already sufficient to dissociate hydrogen which leads to termination of radicals, thus preventing their condensation to coke. This explains the strong drop of the coke selectivity going from zero to 0,2% Mo,... 

Two-phase polymerization is modeled here as a Markov process with random arrival of radicals, continuous polymer (radical) growth, and random termination of radicals by pair-wise combination. The basic equations give the joint probability density of the number and size of the growing polymers in a particle (or droplet). From these equations, suitably averaged, one can obtain the mean polymer size distribution. 

The only exceptions are terminations of radical centres by combination, some transfers and isomerizations. 

To date, by far the greatest experience has been accumulated on the termination of radical polymerizations. 

THE DECAY OF RADICAL POLYMERIZATION DUE TO MUTUAL TERMINATION OF RADICALS... 

A general mathematical solution of transfer and the corresponding retardation is not available. Many years ago, a solution was published for stationary radical polymerizations (with mutual termination of radicals). The derived relations remain valid. Other cases of transfer and of degradative transfer are solved individually. 

Noyes has made numerical solutions of the case of special interest at higher pressures, that of the second-order recombination of radicals in the gas phase competing with first-order wall recombination. The interesting result here may be stated in terms of the effective thickness over which the wall exerts an appreciable effect. Noyes finds that r , where D is the diffusion coefficient, /c, the constant rate of initiation of radicals, and fc<2 is the rate constant for homogeneous second-order termination of radicals. Within the shell of thickness Vw near the wall, it may be assumed, if the surface is an efficient radical trap, that the concentration of radicals is essentially zero, while outside this shell the concentration of radicals may be assumed to correspond to that which is unperturbed by the walls. 

If we apply to the above system of eciuations the stationary-state hypothesis, then we can write an equation for the rates of initiation and termination of radicals in the system... 

Here we have assumed that the rate of termination of radicals is independent of their degree of polymerization (i.e., of n) and have represented... 

Now the ratio A p(M)//c<(M-) = fcp(M)(M-)//c (M-) can be seen to be the ratio of the rate at which monomer is converted into polymer to the rate of termination of radical chains. If termination occurs by recombination, then this ratio is just one-half the average number of monomer units per final polymer chain, which we may represent by n, the mean chain length, or mean degree of polymerization. This permits us to write for the stationary radical concentrations... 

The fraction 1 — will therefore represent the probalnlity for termination of radicals stemming from initiator in the aqueous phase before they can be absorbed. 

Numerical solutions have also been obtained by Brooks (1980) using his three-state model (see above). The relevant simultaneous differential equations were solved Euler s method. Brooks has included in his examples the case where o decays exponentially with time. In all the cases investigated. he finds that allowance must be made for bimolecular mutnal termination of radicals and for the re-entry of desorbed radicals into reaction loci he concludes that failure to take account of these possibilities can lead to serious errors. 

Further, when the termination of radicals in the water phase or the desotption of radicals from the particles could be neglected, Eq.(19) is rewritten as... 

Bartlett and Kwnrt" regarded this radical as an ideal inhibitor (terminator) of radical chain reuclions. hut Htimmond" found that (he reaction of this radical with azobisisobulyronltrlle Ih mmstoichiometric and oxygen-sensitive. 

In contrast, the o-hydroxybenzophenone had no noticeable effect on the decomposition chonistry. Though it has been conmonly referred to as an ultraviolet absorber, it actually behaves more like a deactivator since the screen film containing it had a lower oxidation rate in Fig. 11 than the underlying clear film devoid of additives. Absorption can hardly account for the protective action of the stabilizer in thin films which are almost completely transparent to 300nm radiation. Neither can the hydroxybenzo-phenone be an important terminator of radical chain reactions because it is much less effective than other hindered enols in inhibiting oxidation in the dark. 

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