Rotational conformational energetics of stiff aromatic polyimides: Effects of exchange repulsions, dipole-moiety interactions, and π-conjugations

Abstract

To study the conformational stability of aromatic polyimides which are highly thermally stable and retain their useful physical and electrical properties over 200 °C, we investigated four pristine compounds (N-phenyl-1,8-naphthalenedicarboximide, N-phenylmaleimide, N-2-pyrrolylmaleimide, and N-3-pyrrolylmaleimide) using ab initio calculations. Among these, the polyimides having the last two pristine compounds have not been synthesized yet. The four compounds representing the aromatic polyimides were comprised of two adjoining rings of, respectively, 6-6, 5-6, 5-5, and 5-5′, where each number denotes the number of atoms in each ring-skeleton, and 5′ represents a ring having an N–H group (instead of a C–H group in 5) to interact with the C=O group of the imide ring. The (internal rotation) angles (φ) between two rings of the global minimum energy conformers for the systems of 6-6, 5-6, 5-5, and 5-5′ are predicted to be 90° (perpendicular), 46°, 0° (coplanar), and 0°, respectively. The conformational energetics are governed mainly by the competition between the exchange repulsions, dipole–moiety interactions, and π conjugations. For rings/6-6 and rings/5-6, the dipole interactions and π conjugation effects in favor of coplanar structures are overwhelmed by the exchange repulsions which make the coplanar structures unstable. On the other hand, for rings/5-5 and rings/5-5′, the dipole–moiety interactions just overcome the exchange repulsions, hence the π conjugation effects would become relevant in accounting for the statibility of the coplanar conformation. It is worthwhile investigating these new types of coplanar polyimides for possible developments of new types of polyimides with useful electrical, optical, and physical properties.