Secondary cage effect

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. 

Most of the energy associated with an incident x-ray or y-ray is absorbed by ejected electrons. These secondary electrons are ejected with sufficient energy to cause further ionizations or excitations. The consequences of excitations may not represent permanent change, as the molecule may just return to the ground state by emission or may dissipate the excess energy by radiationless decay. In the gas phase, excitations often lead to molecular dissociations. In condensed matter, new relaxation pathways combined with the cage effect greatly curtail permanent dissociation. Specifically in DNA, it is known that the quantum yields for fluorescence are very small and relaxation is very fast [6]. For these reasons, the present emphasis will be on the effects of ionizations. 

The highly excited states of molecules produced by high-energy radiation that arc chemically important are mainly the ionic states because of the rapidity of internal conversion processes. Primary excitation is relatively unimportant while secondary excitation is quite common. In the condensed phases energy dissipation is very rapid because of colli-sional deactivation, the cage effect, and excitation energy transfer processes all of which act to negate the chemical effects of secondary excitation,... 

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. 

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. 

So, in essence, all four types of degradation proceed through free radical reactions. Depending on the conditions, they can be relatively harmless when a cage effect is possible or there can be a whole sequence of secondary and tertiary reactions which lead to polymer degradation. 

The factor f, called initiator efficiency, takes into account that not all the primary radicals R effectively initiate polymer chains some can be lost due to the so-called cage effect. This implies secondary reactions of the radicals within a cage of solvent surrounding the initiator [5] (the effect can be more pronounced at high conversions/viscosities due to diffusion limitations). The values of / usually lie in the range 0.3-0.8. 

Once the initiating radical is formed, there is competition between addition to the monomer and all other possible secondary reactions. A secondary reaction, such as a recombination of fragments, can be caused by the cage effect of the solvent molecules. Other reactions can take place between a radical and a parent initiator molecule. This can lead to formation of different initiating species. It can, however, also be a dead end as far as the polymerization reaction is concerned. 

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. 

This reaction takes place at 60°C in a benzene solution, but not all the radicals produced may go on to initiate chain growth. Secondary reactions can occur between the radicals produced because of the confining effect of solvent molecules (the cage effect). Primary recombination can occur when the two benzoyloxy radicals produced are unable to diffuse away from each other fast enough... 

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