INTERNAL-ROTATION BARRIERS - AB-INITIO CALCULATIONS ON SUBSTITUTED ETHYL BENZOATES AND BENZOIC-ACIDS AS MODELS FOR POLYESTER FLEXIBILITY

The rotational barriers of several ortho and para substituted ethyl be nzoates (EB) and benzoic acids (BA), used as models for internal rotat ion in PET polymers, have been calculated at the SCF and MP2 levels. S ince there were no experimental data available on internal rotations i n EB and BA, barriers to rotation in benzaldehyde and three similar co mpounds were calculated as benchmarks and compared with experimental d ata. Calculated barriers in the benzaldehydes were within 1 kcal/mol o f the experimental values. The tentative conclusion from the rotation barrier results on model compounds is that PET-type polymers made from high-rotation-barrier monomers like ortho-hydroxy-terephthalic acid s hould have larger internal barriers to rotation than unsubstituted PET , leading to more rigid polymers with higher glass transition temperat ures, T-g, and an altered degree of crystallinity. We found an unexpec tedly low barrier to rotation of 2.3 kcal/mol for ortho X = CH3, compa red to about 5 kcal/mol for ortho X = H. Based on these model studies, we would expect that a PET-type polymer made from ortho-methyl substi tuted terephthalate would be more flexible, would have a lower glass t ransition temperature and different degree of crystallinity with accom panying change in void space. (C) 1998 Elsevier Science B.V. All right s reserved.