Synthesis, liquid crystallinity, and chiroptical properties of sterol-containing polyacetylenes

Poly(phenylacetylene)s and poly(1-alkyne)s containing chiral sterol pendant groups with molecular structures of -[HC=C-C6H4-CO2-R]n-, -[HC=C-C6H4-O(CH2)10-CO2-R]n- and -[HC=C(CH2) mCO2-R]n-, (where R = cholesterol, stigmasterol, ergosterol and m = 2, 3, 8} are designed and synthesized. The monomers are prepared by esterifications of acetylenic acids with cholesterol, stigmasterol, and ergosterol and exhibit cholestericity at high temperatures. Polymerizations of the monomers are effected by WCl6-Ph4Sn, MoCl5-Ph4Sn, and organorhodium catalysts, giving high molecular weight (Mw up to 8.0 × 105) polymers in high yields (up to 99%). The structures and properties of the polymers are characterized and evaluated by IR, NMR, TGA, DSC, POM, X-ray, UV, and CD analyses. All the polymers are thermally stable (greater than or equal to 300 °C). Polymers with long flexible alkyl chains form smectic and cholesteric phases at elevated temperatures. With an increase in the spacer length in poly(1-alkyne)s, the packing arrangements of the mesogenic pendants in the mesophases change from bilayer or mixed mono- and bilayer into homogeneous monolayer structures. Few poly(phenylacetylene)s show CD bands in the absorption region of the polyacetylene backbones, revealing that the main chains are helically rotating with a preferred screw sense.