ROTATIONAL CONFORMATIONAL ENERGETICS OF STIFF AROMATIC POLYIMIDES - EFFECTS OF EXCHANGE REPULSIONS, DIPOLE-MOIETY INTERACTIONS, AND PI-CONJUGATIONS

To study the conformational stability of aromatic polyimides which are highly thermally stable and retain their useful physical and electric al properties over 200 degrees C, we investigated four pristine compou nds (N-phenyl-1,8 -naphthalenedicarboximide, N-phenylmaleimide, N-2-py rrolyl maleimide, and N-3-pyrrolylmaleimide) using ab initio calculati ons. Among these, the polyimides having the last two pristine compound s have not been synthesized yet. The four compounds representing the a romatic polyimides were comprised of two adjoining rings of, respectiv ely, 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 gr oup (instead of a C-H group in 5) to interact with the C=O group of th e imide ring. The (internal rotation) angles (phi) between two rings o f the global minimum energy conformers for the systems of 6-6, 5-6, 5- 5, and 5-5' are predicted to be 90 degrees (perpendicular), 46 degrees , 0 degrees (coplanar), and 0 degrees, respectively. The conformationa l energetics are governed mainly by the competition between the exchan ge repulsions, dipole-moiety interactions, and pi conjugations. For ri ngs/6-6 and rings/5-6, the dipole interactions and pi conjugation effe cts in favor of coplanar structures are overwhelmed by the exchange re pulsions which make the coplanar structures unstable. On the other han d, for rings/5-5 and rings/5-5', the dipole-moiety interactions just o vercome the exchange repulsions, hence the rr conjugation effects woul d become relevant in accounting for the statibility of the coplanar co nformation. 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. (C) 1997 American Institute of Physics. [S0021 -9606(97)50747-1].