Radical Pair Processes in Bulk Polymers

DYNAMICS OF RADICAL PAIR PROCESSES IN BULK POLYMERS 

A detailed description of bulk polymers as hosts for geminal radical pairs and their precursors is also beyond the scope of this chapter. For general sources of information about photochemical and photophysical processes in bulk polymers, we recommend the classic book by Guillet as well as the book edited by Winnik, the journal Polymer Degradation and Stability (incorporating the defunct journal. 

Carbon-Centered Free Radical and Radical Cations. Edited by Malcolm D. E. Forbes Copyright 2010 John Wiley Sons, Inc. 

The reactions analyzed here (and used to generate the initial radical pairs in the polymer media) can be separated into two distinct categories those that involve lysis of one molecule into two radicals (such as the Norrish Type 1, photo-Fries, and photo-Claisen reactions) and those that require bimolecular processes in which a part of one molecule is abstracted by another (e.g., H-atom abstractions from a phenol or an amine by the lowest energy triplet state of benzophenone). Each reaction produces either singlet or triplet radical pairs and, thus, allows the influence of spin multiplicity on radical pair reaction rates to be separated somewhat from other influences, such as the natures of the polymer matrices and the radical structures. Different methods for extracting rates of processes for the radicals from both static and dynamic data will be discussed.  

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Structure and mechanism in photochemical reactions. The reactions of geminal radical pairs created in bulk polymers are presented by Chesta and Weiss in Chapter 13. Of the many possible chemical reactions for such pairs, they are organized here by polymer and reaction type, and the authors provide solid rationalizations for the observed product yields in terms of cage versus escape processes. Chapter 14 contains a summary of the editor s own work on acrylic polymer degradation in solution. Forbes and Lebedeva show TREPR spectra and simulations for many main-chain acrylic polymer radicals that cannot be observed by steady-state EPR methods. A discussion of conformational dynamics and solvent effects is also included. 

Clearly, there are many benefits to investigating radical processes in bulk polymers because the media constitute a viscous space in which the processes suffered by radicals (and, especially, pairs of radicals) are slowed significantly, allowing them to be observed more easily. We have provided an example in which decarbonylation can be used as a clock over time domains that are appropriate in polymeric media, but would be too slow in many fluid solvents. A great deal of basic scientific information about the dynamics of polymeric radicals can be derived as well. 

Decarboxylation of excited singlets of aromatic esters (Eq. 13.6) is a concerted process, and it can account for a significant fraction of the reaction by appropriately substituted aryl esters, especially in bulk polymers and other media that are capable of imposing conformational constraints on guest molecules.Because photoinduced decarboxylation occurs before lysis of the aryl esters, it does not influence the rates at which the A B singlet pair react. For that reason, the relative yields of decarboxylation products need not be considered in analyses of the radical pairs unless their formation precludes sufficient radical pair production for their easy direct or indirect detection. 

In each of the aryl esters discussed above, the acyl radical formed upon lysis of the excited singlet state of the ester loses CO very slowly at the temperatures of the irradiations. At 296K, the rates of loss of CO by acetyl and propanoyl radicals in the gas phase are 4.0 and 2.1 x 10 s respectively." As aresult, no products from decarbonylation and rearrangement are expected" (or have been found) when either of the NA or NM isomers is irradiated in liquid solvents or bulk polymers, and kinetic information from photoproducts alone is limited to relative rates of radical pair processes (Scheme 13.3). For example, if no Fries products from 1-NA or 1-NM emanate from reencounter of radicals that have escaped from their initial cages, [2-AN]/[4-AN]/[l-NOL] = /c2a/ 4a/ nol-... 

Although aspects of the processes suffered by radical pairs in polymeric media have been investigated for many years, and information about those processes is of fundamental importance to industry and new technologies, the field is still in its infancy. Few studies have delved into the detailed mechanisms by which radical pairs move and interact in the confining environments of bulk polymers. 

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