Helical Nitrogen-Containing Polymers: From Monomer Stereochemistry to Macroscopic Function

Chiral and stereoregular nitrogen-containing polymers are accessible through a comparatively small number of controlled polymerizations. The present review discusses different monomer classes that carry nitrogen both in the monomer and in the polymer backbone, namely aziridines, 2-oxazolines, N -carboxyanhydrides, lactams, carbodiimides, isocyanates, and aromatic amides, together with the polyamines, polypeptides, polyamides, polyamidosaccharides, polyguanidines, and polyaramids obtained from them. Only linear homo- and copolymers of the same monomer class are considered, whereby the chirality originates either from the monomer itself, from a stereospecific initiator or catalyst, or from an enantioselective end-group. For each class it is examined how this stereochemical information is transferred to the chain tacticity, to the helical conformation, and to the supramolecular organization, and which macroscopic properties follow from it, i.e., crystallinity, thermal properties, chiroptical response, and mechanical anisotropy. Cases in which this relationship is weak, or contradicted by the reported data, are addressed as well. On this basis, the applications reported so far, among them enantioselective membrane separations, asymmetric organocatalysis, antifreeze and antimicrobial polypeptides, and targeted biomedical carriers, are assessed, and the gaps that still prevent a predictive design are identified.