Conformational study of poly(p-phenylene) by molecular mechanics minimization

Molecular mechanics minimizations were performed on alkyl-substituted biphenyls to explore the effect of the position and length of side chains on the planarity of the phenyl rings. This approach was extended to poly(p-2,5-di-n-alkylphenylene)s. The structural behavior was investigated by systematically modifying the length of the main chain and the side chains. Studies of substituted biphenyls showed that the methyl substituent at 2, 2', 6, and 6' positions much affected the torsion angle between the phenyls. The torsion angle decreased and converged to an asymptotic value of similar to 70 degrees from trans as the length of side chains was increased in 2,2',5,5'-tetraalkylbiphenyls. The results of a single poly(p-2,5-di-n-alkylphenylene) chain clearly showed that the main chain and side chains cooperate on the molecular level to make a stable conformation. Poly(p-2,5-di-n-alkylphenylene)s with long side chains showed partial ordering of side chains with a plain plate structure, whereas those with short side chains did not show side-chain ordering and had cylindrical, or hairy-rod structure. The onset of side-chain ordering for this single-chain system was found to occur for a sexiphenylene unit with eight carbons in the side chains. The average torsion angles of poly(p-2,5-di-n-alkylphenylene)s decreased from 89 degrees to 61 degrees relative to trans as the length of side chains was increased from methyl to dodecyl.