Polyacetylene prepared by the Durham precursor route can be obtained either in a poorly-crystalline form by thermal conversion of a precursor film cast from solution onto a substrate, or in a well-aligned and highly crystalline form if a free-standing precursor film is stretched during the transformation reaction. We find that measurements sensitive to the length of defect-free polyene chain show similar 'long-chain' characteristics in both stretch-aligned films and in Shirakawa polyacetylene. These include the dispersion of the resonant-Raman spectra with excitation energy and the ESR linewidth for the neutral spin defects present in materials. In contrast, the unstretched films show the properties expected of chains frequently interrupted by defects. We consider that these are mostly chain bends and chain twists, and show that these conformational defects do not break bond-alternation symmetry on the chain and therefore do not prevent the creation of soliton-like defect states during chemical doping. The evidence for this comes from analysis of the dispersion of the resonant-Raman spectra (the dispersion is very weak), and from the optical absorption spectra of doped films. The peak in optical absorption in unstretched films is at 2.3-2.5 eV, and the 'mid-gap' absorption is at 1 eV; both these energies are significantly higher than in Shirakawa material. © 1986.