Polymerization of p‐n‐hexyloxyphenylacetylene (pHPA) by using a [Rh(norbornadine)Cl]2‐triethylamine catalyst was carried out at room temperature to afford stereoregular helical poly(p‐n‐hexyloxyphenylacetylene)s (PpHPAs). When ethanol and n‐hexane were used as polymerization solvents, a bright yellow PpHPAs, poly(Y) with Mn = 8.5 × 104 and its purple red polymer, poly(R) with Mn = 5.3 × 104 were obtained in 95% yields and 84% yields, respectively. Diffuse reflective UV–vis spectra of poly(Y) and poly(R) in solid phase showed different broad absorption peaks at 445 and 575 nm, respectively. X‐Ray diffraction patterns of poly(Y) and poly(R) showed typical columnar structures assignable to cis‐transoid and cis‐cisoid structures, respectively, which were also supported by molecule mechanics calculation. Poly(Y) was irreversibly transformed to a reddish‐black polymer, poly(Y‐B), which columnar diameter was nearly the same as that of poly(R). Further, poly(Y) showed an exothermic peak in the differential scanning calorimetry trace at 80 °C for 1 h in N2 gas. Thus, these findings suggest a thermally irreversible rearrangement from an unstable cis‐transoid form, poly(Y) with a stretched cis‐transoid helix to a stable cis‐cisoid form, poly(R), with a contracted cis‐cisoid helix in the solid phase to give poly(Y→B) with the cis‐cisoid form. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012

A chiral substituted acetylene, (s)-2-octyl propiolate, was stereoregularly polymerized using a catalyst, [Rh(nbd)Cl]2, at 40 °C in methanol to give the corresponding helical polymer, Ps2OcP. The changes of (1)H and (13)C NMR spectra in line shapes and splitting patterns were consistently interpreted in terms of restricted rotation around the ester O-*C bond, ~O-*C(ε)H(ε)(R)~, R = a branched CH(ε)3 in the ester side chains rather than the helix inversion with the aid of a 3-site jump model. Three peaks due to the branched methyl H(ε) proton and its C(η) carbon observed at 0 °C suggested the formation of three rotamers called A, B, and C, based on the presence of the contracted helix and stretched helix forms that have an intrinsic helical pitch. Furthermore, an accordion-like helix oscillation (HELIOS) along the main chain axis was proposed to explain the temperature dependence spectral changes observed in (1)H and (13)C NMR, UV-vis, and circular dicromism (CD) spectra. The temperature dependence UV-vis and CD spectra of Ps2OcP corroborate the presence of contracted and stretched one-handed helix sense polymers in solution in which the helical pitches and their persistence lengths depend on the temperature.

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