Crystal structure of form III of isotactic poly‐1‐butene

The crystal structure of form III of isotactic poly‐1‐butene, (poly(1‐ethylethylene)), has been determined by X‐ray powder diffraction techniques, owing to the impossibility of obtaining fiber spectra on this form. The polymer crystallizes in the orthorhombic system, with lattice dimensions: a = 12,38±0,08 Å; b = 8,88±0,06 Å; c = 7,56±0,05 Å; (direction coinciding with the helix axis). The unit cell contains eight monomeric units and the space group is P212121. The structural determination allows to ascertain a conformation 4/1 for the polymer helix, different from those of the other polymorphs I and II. The structure consists of anticlinic isomorphous helices which are packed in an approximately hexagonal coordination. The discrepancy factor R', evaluated by comparing the calculated and observed intensities, is equal to 0,104.