Internal rotation of ester linkage in phenyl benzoate and hydroxybenzoic acid dimer as models of aromatic polyesters using density functional theory

Abstract

Full Paper: The internal rotation of the ester linkage was reinvestigated more quantitatively by using the density functional theory (DFT) in order to understand the characteristic stiffness and extendedness of polymer chain found in aromatic polyesters. Phenyl benzoate (PB) and p-hydroxybenzoic acid (HBA) dimer (HB) were selected as models of aromatic polyesters. The relaxed potential energy surface (PES) scan was carried out along the internal rotation of three bonds (denoted as R, S and T, respectively) of the aromatic esters by using the hybrid DFT (B3LYP) with 6-31G* basis set. The rotation of S bond, which mainly determines the linearity of the molecule, leads to the trans- and cis-conformers of PB. Since the cis-conformer of PB is 7.69 kcal · mol-1 higher than the trans-conformer, the cis-conformer has little population at standard condition. HB does not have the cis-conformer. In addition, the chain persistence length of 364 Å is obtained by the rotation matrix formalism using the structural parameters of HB. These agree with the experimental understanding that poly(p-hydroxybenzoic acid) is the class of stiff and extended polymer.