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

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 center dot center dot center dot O and C-H center dot center dot center dot O=C 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 x 10(-2) cal K-1 g(-1) (PEA); and 5.40 and 5.6 x 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.