Poly(3-dodecylthiophene) (P3DDT), with different amounts of head-to-head configuration defects, was characterized by ultraviolet-visible, H-1-NMR, photoluminescence, and Raman spectra. The heat-induced conformational sample changes were studied by electron paramagnetic resonance (EPR). For the study of these changes, a spin-probe technique was used, in which 5-doxyl-stearic acid methyl ester was applied as a spin probe. From the EPR spin-probe spectra, rotational correlation times and order parameters were calculated. The heat-induced conformational changes of P3DDT were accompanied by a monotonic decrease in the rotational correlation time up to approximately -33degreesC (240 K) and then by an increase in the range of the glass-transition temperatures, with the maximum being near room temperature and depending on the effective conjugation length. Afterward, the rotational correlation times had a decreasing tendency up to 90degreesC (363 K). (C) 2003 Wiley Periodicals, Inc.