p-n-Heptylphenylacetylene (pHepPA) was stereoregularly polymerized in n-hexane at 25 degrees C using [Rh(nbd)Cl](2) catalyst (nbd: norbornadiene) and NEt3, affording the purple-red Poly(R) in 97% yield. A 80 degrees 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 [(Hexa)Poly(R)(CC)]. The 80 degrees C heat treatment generated two tetragonal crystals: (Tetra)Poly(B)(CC) containing contracted cis-cisoid helices and (Tetra)Poly(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 degrees C in the solid phase, the (max) in the diffuse reflective UV-vis spectra of (Hexa)Poly(R)(CC) shifted from 482 to 560 nm. Additionally, an endothermic transition occurred in the (Hexa)Poly(R)(CC) differential scanning calorimetric trace at approximate to 80 degrees C. Therefore, these data corroborated the assertion that (Hexa)Poly(R)(CC) thermally converted to (Tetra)Poly(B)(CC) and (Tetra)Poly(B)(CT). (c) 2013 Wiley Periodicals, Inc.