Irreversible helix rearrangement from Cis‐transoid to Cis‐cisoid in poly(p‐n‐hexyloxyphenylacetylene) induced by heat‐treatment in solid phase

Abstract

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

Contracted helix to stretched helix Rearrangement of an aromatic polyacetylene prepared inn-hexane with [Rh(norbornadiene)Cl]2-triethylamine catalyst

“…However, to the best of our knowledge, a nonpolar, aprotic, and nondonor solvent such as n Hex has never been used in this type of polymerization. The decrease of the molecular weight of Poly( B ) observed after the thermal treatment at 80 °C is substantially correlated to such a stiffness of π‐conjugated polymer which induces the main‐chain scission . (a)…”

“…The helical pitch of an alkoxy‐substituted poly(phenylacetylene) (PPA) can be controlled using external stimuli. For example, thermal treatment of the solids or exposure to solvent vapor converts the stretched cis‐transoid helices to contracted cis‐cisoid helices, and this transformation is accompanied by a drastic color change from bright yellow to dark red . However, the morphology of these polymers and the mechanism of their solid‐phase transformations have not been thoroughly investigated despite the polymers' many interesting physicochemical properties: oxygen permeability, nonlinear optical behavior, electrical conductivity, time and temperature memory effects, enantioselectivity, and responsiveness to external stimuli …”

“…These interferences occurred because gels, large quantities of irregular sequences, for example, head‐to‐head or tail‐to‐tail bond units, or unpaired electrons were present in the polymers. The unpaired electrons were generated through the rotational scission of cis CC bonds by thermal cis ‐to‐ trans isomerization, compression, (b) and the polymerization itself . (c), …”

“…Furthermore, determining whether the contracted cis‐cisoid helix ( contracted helix CC ) is generated as a stable structure is important. The SPA contracted helix CC was considered as a missing helix for a long time and was only recently identified in solution and solid phases . (a) Recently, we reported that the helical backbones of achiral and chiral aliphatic polyacetylene esters exhibit an interesting “ accordion‐like helix oscillation ( HELIOS ) of the main chain” in solution .…”

Stimuli-Responsive Polyacetylenes and Dendronized Poly(phenylacetylene)s

“…32 The former polymer has a broad absorption centered at 445 nm and cis-transoidal structure according to X-ray analysis, while the latter has an absorption around 575 nm and cis-cisoidal structure. The former polymer is irreversibly converted to the latter by heat treatment, which suggests a thermally irreversible rearrangement from an unstable, stretched cis-transoidal helix to a stable, contracted cis-cisoidal helix.…”