Resolution through Photochemical Methods

As for some of the work described above, it was shown that the growing chain itself (as opposed to the initiator) was able to carry out the resolution [39], Importantly from the point of view of the preparative separation of enantiomers, there was a description of how enantiopure material could be isolated from the polymer and the enantioenriched residual material. 

It is fair to say that polymerization processes suitable for the synthetic separation of enantiomers are rare, particularly in any preparative sense, which is perhaps surprising as the obvious differences in sizes of polymer versus monomer make the separation process potentially easy. The scarcity of examples may in part be because the interests of the investigators are focused more on the polymers themselves than resolution processes. Nevertheless, there are a handful of examples (such as those above) where it is clear that a polymerization can very effectively select out an enantiomer from a racemic solution, sometimes mediated by the growing polymer itself independently of any additive. Perhaps, more cases will be discovered in the coming years, in line with recent theoretical predictions of ways in which polymerization might be stereoselective [40-42]. 

There has been a sustained interest in the possibility of absolute asymmetric synthesis using physical fields rather than reagents [43]. It has been known for more than a century that circularly polarized light (GPL) can effect the selective transformation (usually photodecomposition/photodestruction) of one enantiomer 

The responsiveness of the irradiated molecule to the CPL is more formally known as the g (or anisotropy) factor the relative difference of the extinction coefficients exhibited by an enantiopure compound to right and left CPL [49-51]. Thus, the success of the method is dependent on a basic physical property of the molecule as well as the conditions used [52, 53], meaning that, Hke aU the other processes described in this book, the appHcabihty of the method of the separation of enantiomers varies depending on the structure of the molecule under consideration and the conditions (such as solvent) that may be employed. 

As for the more usual chemical means of resolutions described earher in this book, there are ways around the statistical Hmitation of a kinetic resolution. It is theoretically possible for a process to involve racemization of the exited states of the chiral compound to give a photostationary state - a steady enantiomeric excess. These processes, sometimes caRed photoderacemizations or photoresolutions axe rare, partly because of the need to ensure that the excited state does not undergo some other reaction but instead reverts cleanly to the ground state. This combination 

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