Computational design of polymers: poly(ester amide) and polyurethane

Abstract

A methodology for conformational analysis of polymers including both ester and amide groups in the repeating unit has been developed by exemplifying a poly(ester amide), poly(oxyethylene-iminosuccinyl) (abbreviated herein as PEA), and a polyurethane, poly(oxy-1,2-ethanediyloxycarbonylimino-1,2-ethanediyliminocarbonyl) (PU). The repeating units have been divided into two parts, each of which has been represented by a small model compound with the same bond sequence that the polymer includes. To determine which molecular orbital (MO) theory is appropriate for the models, B3LYP, M06-2X, and MP2 calculations were compared with NMR observations. Consequently, the M06-2X or MP2 electronic energy with the B3LYP geometry was shown to yield reliable conformer free energies. Both PEA and PU were found to prefer bent conformations with a number of gauche states owing to intramolecular N–H⋯O and C–H⋯OC attractions. The characteristic ratio and configurational entropy, calculated by the refined rotational isomeric state scheme with the MO energies, were, respectively, obtained as follows: 5.39 and 5.8 × 10⁻² cal K⁻¹ g⁻¹ (PEA); and 5.40 and 5.6 × 10⁻² cal K⁻¹ g⁻¹ (PU). Both polymers show equivalent characteristics, however, the ratios (f_U/f) of the internal energy contribution (f_U) to the total restoring force (f) in chain elasticity, evaluated from the temperature coefficient of the characteristic ratio, are greatly different: −0.13 (PEA); and 0.21 (PU). This means that the urethane chain is more likely to behave as an elastomer than the ester amide chain. The possibility that poly(ester amide)s and polyurethanes will become biodegradable is also discussed in terms of the primary structures.