STRUCTURE AND PACKING IN CRYSTALLINE ALIPHATIC POLYESTERS

Crystalline Aliphatic Polyesters show an interesting structural effect. Some adopt, like polyethylene and their polyamide counterparts, planar zigzag conformations. However many have conformations that contain kinks made up of pairs of gauche bonds of opposite sense separated by one or more trans bonds. The unit cells are of a variety of types including both orthorhombic and monoclinic structures. Thus this family provides excellent test cases for methods for predicting crystal structures and packing via conformational energy calculations. We have examined six of the aliphatic polyesters by using a molecular mechanics method that simultaneously minimizes the intramolecular energy and the intermolecular packing energy and takes advantage of a newly parameterized force field for the ester group. The polar interaction energies are represented by a mutual induction polarization model that explicitly calculates the internal field at each bond rather than invoking a macroscopic dielectric constant for mediating the electrostatic interactions. Good calculated values of unit cell parameters were obtained. The kink conformation containing examples are found to be stable compared to trans counterparts because the intramolecular energy penalty for kink formation is modest and because the kink conformations pack well for some kink locations. © 1990, American Chemical Society. All rights reserved.