Structural Studies of Polyesters. VII. Molecular and Crystal Structures of Racemic Poly(β-ethyl-β-propiolactone)

AbstractThe molecular and crystal structures of the synthesized racemic poly-(β-ethyl-β-propiolactone) [—CH(C2H5)CH2COO—]n were studied by X-ray diffraction. The unit cell is orthorhombic, P212121(D24), with a=9.32Å, b=10.02Å, and c(fiber axis)=5.56Å; two molecules pass through the unit cell. The molecule has a conformation shown by the Fischer projection for the case of the rectus polymer, giving a left-handed (2/1) helix: the O—C bond is nearly skew, 136°, and the C—C(O) bond is far from the gauche form, −21°. The internal rotation angle about the sequence O—CH—CH2—CH3is 64°. From the results of the X-ray analysis, it may be concluded that (1) the crystalline polymer prepared from the racemic monomer, β-ethyl-β-propiolactone, is isotactic and (2) the bulk sample of this racemic polymer consists of two kinds of crystallites, one composed only of left-handed helices of rectus polymer chains, the other only of right-handed helices of sinister polymer chains.