Chirality, polymers

Interest in optically active polymers arose from analogy with macromolecules of biological origin. In addition, there was the hope to obtain new information to clarify the stereochemical features of synthetic polymers this, in fact, did come about. Attempts to direct the course of polymerization using chiral reagents had been made already prior to the discovery of stereospecific polymerization. It was only after the 1950s, however, that the problem of polymer chirality was tackled in a rational way. The topic has been reviewed by several authors (251-257). In this section I shall try to illustrate three distinct aspects the prediction of chirality in macromolecular systems, the problems regarding the synthesis of optically active polymers, and polymer behavior in solution. 

Bickait (258) relates to magnitudes of optical activity well below the detection limits of the available instmments. It plays a role in the examination of polymer chirality and will be discussed later. Our attention will be focused on those polymers that display observable rotation and are, therefore, necessarily chiral. A clear separation among these classes of polymers—stochastically achiral, cryp-tochiral, and chiral—can be made with reference to a suitable model. 

Further discussion and comparison of the various models will be made in the section dealing with polymer chirality. 

Schlitzer, D. S. Novak, B. M. Trapped Kinetic States, Chiral Amplification and Molecular Chaperoning in Synthetic Polymers Chiral Induction in Polyguanidines through Ion Pair Interactions, J. Am. Chem. Soc. 1998, 120, 2196-2197. 

We found that the CD sign of the chiral aggregates around the CD reversal point (THF/methanol 55%/45%) is switchable, depending on the solvent addition order either Method I (methanol added into THF solution of the polysilane) or Method II (THF solution of the polysilane added into methanol), as shown in Fig. 21. This solvent polarity dependent CD switching may be related to the polymeric seed chirality in the initial stage of the polymer chirality amplification process, which can be detected as Cotton CD signal active polymer aggregates. 

R Li, sparteine or chiral polymer (chiral) Tetr Asym 6 1507 (1995)... 

Synthetic polymers can be prepared to contain chirality as is the case for cellulose and other natural polymers. Chirality can be introduced into the monomer before polymerization to yield the chiral polymer. Alternatively polymerization of an achiral monomer in the presence of some chiral catalyst yields the chiral polymer. Polymethacrylates exhibiting chirality due to single-handed helicity have been prepared via polymerization in the presence of a chiral catalyst. These materials are used in liquid chromatography primarily under low-pressure conditions and have shown good resolution for compounds capable of hydrogen-bond formation. 

Allender CJ, Brain KR, Heard CM (1997) Binding cross-reactivity of Boc-phenyl-alanine enantiomers on molecularly imprinted polymers. Chirality 9(3) 233-237... 

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