Conformational Optical Activity in Polymers

Abstract

Structural order along a polymer chain involves in general monomolecular chirality, but even in highly isotactic polymers intramolecular compensation occurs due to equimolar amounts of sections with opposite handness. Predominance of one screw sense conformations can be induced by polymerizing optically active monomers, which incorporate in the chain an excess of asymmetric carbon atoms with a single absolute configuration. Whereas in low molecular weight compounds such an excess is proportional to the enantiomeric purity — and then to the magnitude of optical activity -, in polymers the asymmetric effect can be cooperatively transmitted along the chain. This effect in isotactic macromolecules can be achieved by chiral initiators or by copolymerizing chiral monomers with achiral comonomers. As the consequence, the dissymmetric conformational ordering of units from achiral monomers can provide a surplus of optical rotation (conformational optical activity). Synthetic approaches to polymers showing these particular properties and requisites for conformational optical activity are presented as well as experimental evidences deriving from measured and calculated chiroptical properties of typical polymers.