THE STRUCTURE OF GAMMA-POLYPIVALOLACTONE- A COMBINED ANALYSIS OF SINGLE-CRYSTAL ELECTRON-DIFFRACTION DATA AND POWDER X-RAY-DIFFRACTION PROFILES WITH THE RIETVELD METHOD

The crystal structure of the gamma-phase of polypivalolactone (gamma-PPVL) has been determined from electron diffraction data and tested with the Rietveld method against X-ray powder diffraction profiles. The electron diffraction study is based on single-crystal patterns obtained from melt-crystallized solution-cast thin films, while the Rietveld analysis was performed on data from melt-crystallized samples containing comparable amounts of alpha- and gamma-phase PPVL. The crystal class is orthorhombic (space group P212121), and two molecules with the same chirality and opposite directions pass through the unit cell having lattice dimensions a = 8.23, b = 11.28, c ( chain repeat) = 6.02 Angs.. Both h k 0 and tilted-specimen electron diffraction data were used in the refinement, carried out in the kinematic approximation. The analysis based on these data and on powder profiles coherently indicates that the refined structural model has a main-chain conformation intermediate between the one found in the α-phase and the minimum-energy conformation, while differing significantly from both