Helix oscillation of polyacetylene esters detected by dynamic 1H NMR, IR, and UV-vis methods in solution

Abstract

An aliphatic substituted acetylene, n-heptyl propiolate, was stereoregularly polymerised using a catalyst, [Rh(nbd)Cl]₂, at 40 °C in methanol to obtain the corresponding helical polymer, PnHepP. The changes in the line shapes and splitting patterns of the ¹H NMR spectra were interpreted consistently as representing restricted rotation around the ester O–C^ε bond in –O–C^εH^ε₂(R)– (R = n-hexyl alkyl chains), rather than helix inversion, with the aid of a 3-site jump model. A two-line peak corresponding to the ester methylene protons of –O–CH₂– observed at 30 °C suggested the formation of three rotamers designated as A, B, and C based on the presence of contracted helix and stretched helix forms, each of which has an intrinsic helical pitch with a helical cis–cisoid structure. Furthermore, an accordion-like helix oscillation (HELIOS) along the main chain axis was proposed to explain the temperature dependence of spectral changes observed in the ¹H NMR, UV-vis, and IR spectra. The temperature dependence of the UV-vis and ¹H NMR spectra of PnHepP corroborated the presence of contracted and stretched one-handed helix sense polymers in solution.