Secondary cage recombination

Figure 5. Scheme showing (a) primary and (b) secondary cage recombination. 

For this reeuson, any si)ecific "polymer effects", if indeed they do occur, must be attributed to processes occurring outside the primary cage. Secondary cage recombination, for example, will be affected by the rate of diffusion in the polymer matrix. This might be expected to reduce the number of radicals which can escape the region associated their primary partners and become true "free" radicals. 

Wiles and co-workers have examined the role of peroxy-radicals in polyolefin photo-oxidation. They suggest that radical recombination processes have a high probability even if they escape the primary polymer cage. The occurrence of secondary cage-recombination processes was considered. Vasilenko et have also studied the role of free radicals in polyethylene photo-oxidation. 

Secondary cage recombination of peroxy radicals [698]. In a solid polymer, a pair of polymer peroxy radicals (POO 2) is trapped in the polymer matrix. When a radical pair, produced by photoinitiation, escapes the initial cage, the probability of its recombination remains high even after several propagation steps. This phenomenon, known as secondary cage recombination, has a pronounced effect on the kinetics of oxidation and on the distribution of kinetic chain lengths in the oxidation process. 

The methyl radical is highly reactive and the secondary reaction also decreases the chance of cage recombination. The photochemistry of x,a-dimethoxy-a-phenylaceto-phenone (DMPA) has been studied in the absence as well as in the presence of compoimds with double bonds Hageman et al. investigated its photochemistry... 

The cage effect described above is also referred to as the Franck-Rabinowitch effect (5). It has one other major influence on reaction rates that is particularly noteworthy. In many photochemical reactions there is often an initiatioh step in which the absorption of a photon leads to homolytic cleavage of a reactant molecule with concomitant production of two free radicals. In gas phase systems these radicals are readily able to diffuse away from one another. In liquid solutions, however, the pair of radicals formed initially are caged in by surrounding solvent molecules and often will recombine before they can diffuse away from one another. This phenomenon is referred to as primary recombination, as opposed to secondary recombination, which occurs when free radicals combine after having previously been separated from one another. The net effect of primary recombination processes is to reduce the photochemical yield of radicals formed in the initiation step for the reaction. 

In any event, in the solvent cage in which they are formed in the liquid phase, or for higher molecular weight alkenes condensed on surfaces, the two fragments formed by decomposition of the primary ozonide are held in close proximity and recombine to form a secondary ozonide ... 

The quantum yield for the primary photochemical process differs from that of the end product when secondary reactions occur. Transient species produced as intermediates can only be studied by special techniques such as flash photolysis, rotating sector devices, use of scavengers, etc. Suitable spectroscopic techniques can be utilized for their observations (UV, IR, NMR, ESR, etc.). A low quantum yield for reaction in solutions may sometimes be caused by recombination of the products due to solvent cage effect. 

It was proposed that the temperature dependence of polymer 5 arises from the temperature dependence of the kA step. Specifically, it was suggested that the polymer segments to which the radicals are attached are conformationally stressed. There are two possible modes for the newly formed radicals to relax and become separated They can rotate or recoil away from each other (Scheme 9). These secondary motions of the polymer arise from the relaxation of unfavorable bond conformations that are formed during the polymer casting process. The increased thermal energy facilitates the rotation and recoil relaxation processes, which effectively increases the rate constant for diffusion of the radicals out of the cage, kA. This leads to decreased radical-radical recombination and consequently an increase in photodegradation efficiency. 

As shown in Fig. 3-1, some radicals in separated radical pairs re-encounter their partners within the solvent cages, but others escape from the cages, forming "escape radicals". Since the time scales for the secondary recombination and the S-To conversion rates are 10 " 10 and 10 10 s, [3] respectively, the change in the S-To conversion rate by an external magnetic field and/or the HFC term can influence the yield of cage and escape products. 

The mean distance of the monomer swollen micelles is of the order of the diameter of the spurs (elementary reaction regions of secondary electrons, produced by the y-radiation). Therefore, possibly, a certain fraction of the radio-lytical radicals does not recombine in the Frank-Rabinovitch cage, but breaks into the micelles or particles and is trapped therein. 

In view of these solvent structure effects, it is convenient to classify the recombination events into primary and secondary processes. Primary recombination processes are those in which the recombination takes place before the atoms separate to a distance roughly equal to the first maximum in the mean potential (i.e., recombination in the solvent cage ). Secondary recombination involves the recombination of solvent-separated-atom pairs. 

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