Primary cage recombination

We have recently been concerned about how a polymer environment might affect these two processes. The first question is whether the environment of a solid polymer will increase the amount of primary cage recombination of a dissociating radical pair. Studies were made [21] of the quantum yield for photolysis of benzoyl peroxide in two solvents, benzene and toluene, and in solid polyethylene and polystyrene films ... 

This puts the final radicals at least 4.5 A apart outside the primary cage. As a result the radical pairs do not recombine. In looking for reactive structures that could serve as photolabile side groups, this was an important consideration because cage recombination is an important factor in viscous media such as those used in the polymerization of polyfunctional acrylates. 

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. 

Here, the n=0 term of the right-hand side of Eq. (11-52) has a value of Xp s represents the probability of the geminate recombination within the primary cage. 

In the light of our investigations in isooctane, however, the recombination process within pairs of primary peroxy radicals is not a true termination (see Scheme II) Macroperacid radicals and freely mobile HOO radicals are expected to be formed from the primary tetroxide recombination cage by interaction with 0 (processes (2b) and (3) in Scheme III). 

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. 

Photosubstitution.—The primary process in the photosubstitution of [Cr(CO)s] and of [W(CO)8] by pyridine has been studied, in particular to account for the quantum deficit (for example, d>A=0.66 0.05 for [py] = 2.4x 10 M and [Cr(CO)6l = 3.5 X 10- M 292 nm irradiation in cyclohexane). Results in viscous solvents eliminated cage recombination as a possibility, and excited singlet to excited triplet intersystem crossing was shown to occur with unit efficiency so that the triplet state must dissociate and the deficit is due to a radiationless decay which competes with dissociation. 

As a result of this primary stage, radical cations XXIX, NO and nitrate-anion are formed. The dissociating decomposition of radical cations and subsequent cage recombination of the primary stage products give compounds XXX. These compounds, being apparently intermediates, are converted into stable acylarylaminoxyl radicals XXVn due to reversible dissociation with detachment of NO ... 

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