CNDO studies on nonplanar conformations in some cis- and trans-polybenzobisoxazoles and polybenzobisthiazoles

W. J. Welsh, J. E. Mark

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16 Scopus citations


While essentially rigid in the axial direction, the rod-like polymers cis- and trans- polybenzobisoxazole (PBO) and polybenzobisthiazole (PBT) do exhibit conformational flexibility with respect to rotations about the bonds between alternating phenylenes and heterocyclic groups. Since preparation of high-strength materials from these polymers requires a high degree of alignment, this flexibility should be important in this regard. CNDO/2 molecular orbital calculations were therefore carried out to obtain the conformational-energy profiles of related model compounds. The cis- and trans-PBO models prefer the coplanar conformation while the barrier to the perpendicular conformation is about 8.4 kJ mol-1. The trans-PBT model prefers 0 = 20° with barriers to the coplanar and perpendicular conformations of about 2.1 and 25.0 kJ mol-1, respectively. iModel compounds of cis-PBT are bowed in the crystalline state. The bond-strain energies apparently responsible for bowing are estimated by comparing the CNDO/2 energy of the observed "bowed" molecule to that of a ficticious planar form. The "bowed" form is more stable by a large amount, indicating that in-plane bond-angle and bond-length distortions are not sufficient to relieve bond strain. Bowing hence appears to be a necessary alternative to these mechanisms for relieving the considerable strain within the planar form.

Original languageEnglish (US)
Pages (from-to)1119-1124
Number of pages6
JournalJournal of Materials Science
Issue number4
StatePublished - Apr 1983
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • General Materials Science
  • Mechanics of Materials
  • Mechanical Engineering


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