Helical Disubstituted Polyacetylenes: Synthesis and Chiroptical Properties of Poly(phenylpropiolate)s

Disubstituted polyacetylenes with helical chirality have been rarely prepared due to the involved synthetic difficulty, and we here report a facile polymerization system for the synthesis of such polymers. Two groups of chiral acetylenes, i.e., C₆H₅C≡CCO₂R* {R* = [(1S)-endo]-(-)-borneyl (1), (1R,2S,5R)-(-)-menthyl (5), cholesteryl (6)} and C₆H₅C≡CCO₂C ₆H₄CO₂R* [R* = borneyl (2), menthyl (3), cholesteryl (4)], are prepared by esterifications of phenylpropiolic acids with borneol, menthol, and cholesterol. Polymerizations of 1-4 are effected by WCl₆-Ph₄Sn, giving poly(phenylpropiolate)s P1-P4 with high molecular weights in moderate yields. The structures and properties of the polymers are characterized and evaluated by IR, UV, NMR, CD, TGA, and SEM analyses. All the polymers are stable: neither decreases in their molecular weights nor changes in their spectra are detected after the polymers have been stored on shelf for ∼3 years, and no weight losses are recorded when the polymers are heated to ∼300 °C. Although the polymers do not possess regioregular Z or conformations, the polyacetylene backbones are induced to helically rotate by the chiral pendants, as verified by the strong Cotton effects in the backbone absorption region of the polymers (molar ellipticity up to 102 300 deg cm² dmol^-1). The polymers exhibit helical thermochromism, with their chain helicity being continuously and reversibly tunable by temperature change. The helical polymers are capable of self-assembling, as demonstrated by the formation of twisted ribbons upon diffusing a THF solution of P3 into hexane.