DIRECT DETERMINATION OF POLYMER CRYSTAL-STRUCTURES BY ELECTRON CRYSTALLOGRAPHY ISOTACTIC POLY(1-BUTENE), FORM (III)

Three-dimensional electron diffraction intensity data from form (III) of isotactic poly(1-butene) have been collected by tilting solution- and epitaxially grown thin microcrystals. The orthorhombic space group is P2(1)2(1)2(1) with cell constants a = 12.38, b = 8.88 and c = 7.56 angstrom, the latter being the chain repeat. The crystal structure was determined by direct methods, employing the tangent formula (QTAN) and initially 106 phase values were found for 125 unique observed reflections. Refinement by Fourier techniques gave a structure very similar to that proposed in an earlier powder X-ray diffraction study. After rejecting three reflections during the refinement process, the final R factor for 122 reflections is 0.26 and the r.m.s. phase error to the earlier model is only 4-degrees, with no deviations found for the centrosymmetric reflections. The structure consists of infinite chains, each having approximate 4(1)-helical symmetry with the chains coiled about the true crystallographic 2(1) axes at (1/4,0,z) and (3/4,1/2,z).