Crystal Structure of Cyclic Dimer of Poly(butylene terephthalate)

Abstract

The crystal structure of a cyclic dimer from poly(butylene terephthalate) was determined. The dimer crystallized in a monoclinic space group P21/c, with a=11.006(1), b=11.926(1), c=8.489(1) Å and β=95.742(6)°; Dc=1.320, Dm=1.31 Mg m−3, Z=2. The final R was 0.052 for 1778 observed data (Fo≥2σ(Fo)). The molecule has crystallographically imposed \\bar1 molecular symmetry, so that the two bridging tetramethylene chains are oriented anti to each other and the aromatic moieties constitute necessarily a pair of parallel rings. Each benzene ring is bent to a small extent into a boat shape with its bow and stern displaced inward toward the other ring, attesting to the presence of intramolecular steric strain. The tetramethylene carbon chain exhibits abnormal torsional angles far from 60° (gauche) or 180° (trans), the independent torsion angles being 84.8°, −152.6°, and −90.9°, respectively. A significant shortening of the Csp3-Csp3 bonds of the tetramethylene part is observed in the molecule as generally found in the n-methylene dibenzoate derivatives.