Crystalline structure of isotactic polyvinylcyclobutane from fiber diffraction spectra

The crystalline structure of isotactic poly(vinylcyclobutane), claimed to be tetragonal, has been revised on the basis of fiber diffraction spectra. The chain symmetry is 41, as already assessed, but the conformation is different; (the identity period is 6.91 angstroms and not 6.60 angstroms). The space group is the orthorhombic P212121 with lattice constants a = 17.14 angstroms, b = 8.99 angstroms, and c = 6.91 angstroms. The excellent quality of the oriented samples allowed a full structure analysis and refinement. Integrated diffraction intensities, from a stretched fiber, measured with a Fuji image plate detector, have been utilized in a constrained refinement (the helix was assumed strictly of 41 symmetry and the C-C bond lengths were set to 1.54 angstroms) obtaining a final R index of 15%. The refined chain bond angles are 103 and 107°, and the chain torsion angles are 165 and 273°.