Influence of Weak Attractive Interactions on Structures and Properties of Poly(trimethylene terephthalate)

Conformational analysis of poly(trimethylene terephthalate) (PTT) has been carried out by the refined rotational isomeric state (RIS) scheme with ab initio molecular orbital (MO) calculations for a model compound, trimethylene glycol dibenzoate (TMGDB). Bond conformations of the O-CH(2)-CH(2)-CH(2)-O part of TMGDB were determined from (1)H and (13)C NMR experiments for unlabeled and (13)C-labeled TMGDBs. The MO calculations exactly reproduced not only the experimental bond conformations but also the dipole moment and molar Kerr constant observed from TMGDB dissolved in nonpolar solvents. Comparison of the MO calculations with and without electron correlations revealed that a variety of intramolecular attractions, such as pi-pi interactions, electrostatic attractions, dipole-dipole interactions, and C=O center dot center dot center dot H-C and C-O center dot center dot center dot H-C close contacts, are formed in TMGDB and considerably stabilize its folding conformers with g(+/-)g(-/+) sequences in the central CH(2)-CH(2) bonds. The folding conformers may be sources of dimer fluorescence characteristic of TMGDB and PTT. Configurational properties and thermodynamic quantities of PTT were derived from the refined RIS calculations with the MO energies established for TMGDB. Structures and properties of PTT and poly(ethylene terephthalate) were compared and discussed mainly in terms of their intramolecular attractive interactions.