Synthesis and Structure in Solution of Poly[(−)-menthyl propiolate] as a New Class of Helical Polyacetylene

Polymerization of (−)-menthyl propiolate (MtPr) by [(nbd)RhCl]2 and MoOCl4−n-Bu4Sn gave polymers in moderate yields. The 1H NMR spectrum of the polymer formed with the Rh catalyst (PMtPr-Rh) showed a signal characteristic of the cis olefinic proton, meaning the high stereoregularity (cis) of PMtPr-Rh. In contrast, no clear signal attributable to the cis-olefinic proton was detected in the 1H NMR spectrum of the polymer produced with the Mo catalyst (PMtPr-Mo), which indicates the geometrically irregular structure of PMtPr-Mo. PMtPr-Rh displayed much larger [α]D and molar ellipticity [θ] than those of PMtPr-Mo, MtPr, and (−)-menthyl acrylate. This leads to conclusions that the main chain of PMtPr-Rh exists in a helical conformation with an excess of one-handed screw sense and that induction of the helical conformation requires the cis-transoidal structure of the main chain. No serious decrease in magnitude of CD effects of PMtPr-Rh was observed upon heating the polymer solution at 110 °C, indicating the high thermal stability of the helix of PMtPr-Rh in solution.