p-n-Heptylphenylacetylene (pHepPA) was stereoregularly polymerized in n-hexane at 25 °C using [Rh(nbd)Cl]2 catalyst (nbd: norbornadiene) and NEt3, affording the purple-red Poly(R) in 97% yield. A 80 °C heat treatment transformed Poly(R) to the black Poly(B). The Poly(R) X-ray diffraction (XRD) pattern revealed a hexagonal crystal structure comprising contracted cis-cisoid helices [HexaPoly(R)CC]. The 80 °C heat treatment generated two tetragonal crystals: TetraPoly(B)CC containing contracted cis-cisoid helices and TetraPoly(B)CT containing stretched cis-transoid helices. The helical diameters before and after heat treatment were estimated using XRD and were consistent with molecular mechanics calculations (MMFF94 force field method). When heated at 80 °C in the solid phase, the λmax in the diffuse reflective UV–vis spectra of HexaPoly(R)CC shifted from 482 to 560 nm. Additionally, an endothermic transition occurred in the HexaPoly(R)CC differential scanning calorimetric trace at ∼80 °C. Therefore, these data corroborated the assertion that HexaPoly(R)CC thermally converted to TetraPoly(B)CC and TetraPoly(B)CT. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 5177–5183
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