The conformational properties of a series of rigid-rod and stiff chain polymers have been investigated using semiempirical molecular orbital calculations. Poly(2,5-benzimidazole) (ABPBI), poly(2,5-benzoxazole) (ABPBO), poly(2,6-benzothiazole) (ABPBT), poly(p-phenylene benzobisimidazole) (PPBI), poly(p-phenylene benzobisoxazole) (PBO) and poly(p-phenylene benzobisthiazole) (PBZT) have been studied. For ABPBO and both cis and trans PBO, the results indicate that a coplanar arrangement of the heterocyclic and phenyl rings gives the lowest energy. For all the other polymers considered, non-coplanar conformations were found to have the lowest energies for isolated molecules. Molecular mechanics calculations were also used to investigate the conformational properties of cis-PBO and trans-PBZT having various substituents on the phenyl ring. Substituents considered included methyl, ethyl, t-butyl, hydroxyl, phenyl, and benzthiazole groups. The results show that most substituents have surprisingly little influence on the conformations of the molecules. Small substituents, as well as large, planar substituents, were uniform in their lack of inducing a specificity of conformation in the molecules. Bulky groups, especially t-butyl, and those introducing interactions of a more specific nature, such as coulombic or hydrogen-bonding, play a greater role in making specific conformations more energetically favourable. In most cases, the substituent causes a rotation of the backbone rings away from coplanarity, but once the steric interactions are relieved, there is considerable latitude for further rotation. In most cases, the conformational freedom is such that packing considerations could significantly influence the conformations of the materials in the solid state. © 1990.
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