EFFECTS OF METHYL SUBSTITUTION ON THE STRUCTURE AND ROTATIONAL BARRIER IN THE RIGID-ROD POLYMER POLY(P-PHENYLENEBENZOBISTHIAZOLE)

X-ray crystallographic and ab initio molecular orbital analyses are pr esented for three model compounds of methyl-substituted poly(p-phenyle nebenzobisthiazole), PBZT, where monomethyl and dimethyl substitutents are located on the phenylene moiety. The barrier to phenylene rotatio n, a factor considered to be important for an understanding of the mec hanical, electronic, and nonlinear optical properties of PBZT and rela ted rigid-rod heterocyclic polymers, is calculated for each compound. Ortho substitution with a monomethyl group substantially lowers the ro tational barrier and profoundly changes the shape of the rotational po tential, whereas meta substitution has only a negligible effect. Discr epancies between experimental and theoretical phenyl torsion angles ar e attributed to crystal packing forces. Ab initio results differ quant itatively from semiempirical molecular orbital findings. Good agreemen t is observed between crystallographic and computed bond lengths and a ngles.