The road to chiral amplification in polymers originated in Italy

Abstract

The fact that stereoregular polymers produced by Ziegler-Natta polymerization methods could be crystallized and studied by X-ray diffraction methods lead to the discovery that these polymers adopted helical conformations in the crystalline solid. In addition to the nearly perfect asymmetric synthesis demonstrated by the highly isotactic stereoregularity produced by the catalyst, the conformation adopted in the crystalline state also involves chirality. The large number of units polymerized produced though gives rise to a cryptochiral material in which the mirror helical conformations are necessarily equally populated and therefore the measurement of optical activity can never be a consideration in these experiments. At that time though Natta's students began to prepare stereoregular vinyl polymers with chiral nonracemic pendant groups and discovered that the optical activities of such materials in solution showed nonlinear relationships between enantiomeric content and optical activity. This phenomenon arising from the effect of the chiral pendants on the population of the helical senses of the polymer backbone, acted to demonstrate that these stereoregular polymers adopted helical conformations in solution comparable to those investigated in the solid state. In recent times this effect has reached an extreme in the polyisocyanates which show a much larger sensitivity to chiral effects than found for the vinyl polymers although the underlying forces at work are identical to those studied earlier by the Italian School. Chirality 9:424–427, 1997. © 1997 Wiley-Liss, Inc.