Molecular orbital conformational energy calculations of the aromatic heterocyclic poly(5,5′‐bibenzoxazole‐2,2′ diyl‐1,4‐phenylene) and poly(2,5‐benzoxazole)

Abstract

AbstractInterest in potential high‐performance polymers, leading to characterization and development of the rodlike poly(p‐phenylene benzobisoxazoles) (PBO) and poly(p‐phenylene benzobisthiazoles) (PBT), has recently been extended to a related group of polymers referred to as AAPBO, ABPBO, AAPBT, and ABPBT. In this study, geometry‐optimized CNDO/2 molecular orbital calculations have been carried out on AAPBO and ABPBO model compounds to determine conformational energies as a function of rotation about each type of rotatable bond within the repeat units. For AAPBO, which contains two types of rotatable bonds per repeat unit, the bond between the benzoxazole group and p‐phenylene group prefers the coplanar conformation with a barrier to free rotation of 2.1 kcal mol−1, while the bond between the benzoxazole groups prefers a conformation approximately 60 degrees away from coplanarity with a barrier to coplanarity and to free rotation of 3.6 kcal mol−1. For ABPBO, which contains only the former type of rotatable bond per repeat unit, the coplanar conformations were preferred with a barrier to free rotation of 1.6 kcal mol−1. These results are in excellent agreement with the results of both theoretical and experimental studies on the structurally analogous PBO. They are also consistent with the liquid crystalline behavior found for ABPBO but not for AAPBO.