Conformational analysis of poly(butylene terephthalate) (PBT) has been carried out by the refined rotational isomeric state scheme combined with ab initio molecular orbital (MO) calculations and NMR experiments on its model compound, tetramethylene glycol dibenzoate (TetMGDB). Two conformers of TetMGDB, showing intramolecular phenyl-phenyl close contacts, yielded so small free energies at the MP2 level that the overestimated MP2 stabilization for the ππ attraction was compensated by addition of the hemi-MP3 term. The altered MO energies satisfactorily reproduced the NMR and dipole moment data on TetMGDB. The energy correction was also applied to poly(trimethylene terephthalate) (P), and configurational properties of PBT and P were evaluated from the altered energies. Fluorescence emissions observed from poly(ethylene terephthalate) (PET), P, PBT, and their model compounds were well correlated with the spacer conformations. Configurational, thermal, and optical properties of the aromatic polyesters of industrial importance are discussed herein in terms of their conformational characteristics.
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