Chiral optical properties of a helical polymer synthesized from nearly racemic chiral monomers highly diluted with achiral monomers

In polyisocyanates composed only of randomly distributed (R) and (S) units, the chiral optical properties of the polymer are far out of proportion to the enantiomeric excess of the monomers. This highly disproportionate relat ionship, which arises from a majority-rule effect among these enantiomeric units on the helical sense of the backbone, is now found to be unaffected, within certain limits, by the overwhelming presence of achiral units random ly distributed along the chain. This experimental result can be explained q uantitatively by an analysis based on the one-dimensional random-field Isin g model, which shows that dilution of the chiral units with achiral units i ncreases the helical domain size in a manner that compensates for the dilut ion. In qualitative terms, since the random-field domain size is limited by the "objection" of the minority units to the helical sense dictated by the majority units, dilution of this "objection" acts to increase the domain s ize. As long as this domain size is not limited by the chain length or by t hermal fluctuations, the achiral dilution will not reduce the optical activ ity of the polymer.