Recombination of free radicals

Rapid bimolecular reactions are limited by diffusion of reactants in the liquid and solid phases. Diffusion occurs in polymers much more slowly than in liquids. Hence, such rapid reactions as recombination of free radicals occurs in polymers with rate constants of a few order of magnitude more slowly than in solution. For example, the reaction of sterically hindered phenoxyl with the peroxyl radical... 

The chain fragments formed by the recombination of free radicals can be reconverted into radicals by a variety of reinitiation processes, some of which are listed in Table 1. Such reactions can occur in the gas phase via electron collision and on the polymer surface by impact of charged particles or photon absorption. Reinitiation may also be induced in both the gas phase and on the polymer surface by hydrogen transfer reactions. These last processes are similar to the chain transfer processes which occur during homogeneous polymerization. Expressions for the rates of reinitiation are given by Eqns. 20 through 23. 

Dr. Halpem But all that is necessary to explain your observation is that this intermediate be fairly long lived and then that two of these ultimately combine. There is some evidence for a similar phenomenon in the case of free radicals. J. K. Kochi and F. F. Rust (9) have shown that transition metal ions, among them Fe(II), catalyze the recombination of free radicals by what appears to be substantially this kind of mechanism—i.e., the transition metal stabilizes the radical against abstraction presumably by forming a complex with it, which then lives long enough to combine with another one. 

Some Rate Constants for the Bimolecular Recombination of Free Radicals... 

The recombination of free radicals can be highly exergonic, since in general a new bond is formed by the formerly non-bonding electrons. In the example of eqn. (4.59) the closed-shell molecule, N02, is then formed in an excited state which can decay through luminescence emission. Some organic free radicals show similar reactions but it must be stressed that in most cases the emission yields are low. 

The emission spectra are similar but often not identical to those excited by UV (18). The energy source is the recombination of free radicals that occurs in the flame and thus flame-excited luminescence is the same as the radical recombination luminescence observed when free neutral radicals from plasmas are used as an excitation source. A simple hydrogen diffusion flame is the simplest source for demonstrating the phenomenon. 

Experimental evidence of the part played by free radicals in a chemical reaction was soon forthcoming. In 1934 Frey24 found that butane decomposed very slowly at 525° but that if one per cent of dimethyl mercury was introduced the decomposition proceeded rapidly. In the same year Sickman and Allen25 found that acetaldehyde was stable at 300° but that it was decomposed completely when a few per cent of azomethane was added. The introductions of dimethyl mercury or azomethane at these temperatures apparently liberated free radicals which initiated chains. Moreover when mixed gases decomposed simultaneously they did not do so independently. The products contained groups from each in a way that could be easily explained on the assumption of the liberation and recombination of free radicals. Again the appearance of butane from the decomposition of propane is difficult to explain on any hypothesis except on the assumption that some free radicals of CH3 are split out and that they become attached to propane molecules. More direct examples will be given later in the discussion of photochemistry. 

Triplet RPs in nonviscous solutions exit the cage with/ 1. An increase in viscosity leads to an increase in a RP lifetime and slows down molecular diffusivity these features aUow S-T transitions to occur in the RP, and geminate recombination of free radicals is expected to occur, increasing the cage effect Experimental measurements demonstrate that the cage effect O increases with an increase in solvent viscosity. An increase of media viscosity, which usually takes place upon... 

In our subsequent considerations of reactions in solution we shall be interested in the rate at which encounters take place. Such a rate will be of interest when the probability of chemical reaction between two species during an encounter is very close to unity. Under these conditions, the rate of the reaction will be limited by the rate of encounters and we describe the reaction as dijJusion-controlletL Among the reactions in this category are the quenching of fluorescence and the recombination of free radicals. 

This resemblance is highly significant if one considers that 10,359 structural isomers exist for saturated hydrocarbons with 16 C atoms (Lederberg, 1972). Apparently the meteoritic hydrocarbons were made by FTT reactions, or some other process of the same extraordinary selectivity. The Miller-Urey reaction, incidentally, shows no such selectivity. Gas chromatograms of hydrocarbons made by electric discharges in methane show no structure whatsoever in the region around Cjg (Ponnamperuma et al., 1969). Apparently all 10 possible isomers are made in comparable yield, as expected for random recombination of free radicals. 

Parylene Thermal dissociation of cyclic dimer Difunctional free radicals Recombination of free radicals... 

In the case of vitamin B12, eq 19 is well investigated and its rate constant k = 4 x 109 L/mohs for primary radicals.203 Comparison of this value with the rate constant for methyl radical dimerization of 1010 L/mohs191 indicates that eq 19 proceeds at diffusion-controlled rates. Newer approaches may provide greater accuracy in the measurement of rates of recombination of free radicals, providing a better understanding of hydrogen atom abstraction (eq 18).205... 

Heterogeneous processes involving natural liquid or solid aerosols play an important role in the chemistry of the Earth s atmosphere. For example, they can provide the downstream flux of many atmospheric compounds via sedimentation in the form of the absorbed or adsorbed species, facilitate recombination of free radicals, as well as provide topochemical, thermal catalytic and photocatalytic chemical reactions in the atmosphere. 

It is one of the causes of inefficiency among initiators. The average time for recombination of free radicals inside a cage and the time for their diffusion out of the cage is about 10 s. In addition, the efficiency of the initiator is affected by the monomer and the solvent. Viscosity of the medium is inversely proportional to the initiator efficiency, because the more viscous the solution, the greater the cage effect. " ... 

In flame treatment the oxidizing portion of a gas flame (6-10 mm from the tip of the blue inner cone) is brought into contact with the surface of the plastic for a brief period (0.02-0.1 s). The flame temperature is high (1100-2800 °C) and the flame is a plasma. Thus the plastic is modified by oxidation by the plasma and by recombination of free radicals created in the polymer with free radicals in the plasma. Changes are observed to a depth of 4-9 nm and wetting and adhesion are improved. This is a popular technique for improving the adhesion of printing inks to the surfaces of moulded polyolefin containers. 

The same experiment was done in parallel with a 10 p. 100 solution of 1-lysine in water. In both cases, the activated frozen material gave, at — 196°C, a strong paramagnetic signal after irradiation. However, if the temperature rose over — 100°C, this signal almost completely disappeared at the same time, evidence of the recombination of free radicals in the solid state could be witnessed by thermoluminescence, essentially below -100°C. 

Plasma treatment is subdivided into the case where reactive gases such as O2, N2 and NH3 are used as the plasma gases, and that where inert gases such as Ar and He arc used. In the former, the direct reaction of activated gas with the polymer surface occurs, while in the latter, formation of free radicals and subsequent reactions such as cross-linking (via the recombination of free radicals and incorporation of oxygen from the extraneous environment) arc the dominant processes. The APG plasma process is limited to the latter process since only He can be used as the plasma gas. 

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