Conformations of Perfluoropoly(oxyethylene) from Ab Initio Electronic Structure Calculations on Model Molecules

Abstract

The conformational characteristics of perfluoropoly(oxyethylene) (PFPOE) chains have been investigated in detail, based upon ab initio electronic structure calculations on the model molecules perfluorodiethyl ether and perfluoro-1,2-dimethoxyethane, and have also been compared with those of poly(oxyethylene) (POE) and poly(tetrafluoroethylene) (PTFE). The C-C-O-C bond exhibits split trans conformations, similar to those for the C-C-C-C bond in PTFE, which are strongly favored over the gauche state by 2.5 kcal/mol. In contrast, O-C-C-O bond shows no splitting for the trans or gauche conformations due to the absence of pendant fluorine atoms on the oxygen atoms, and favors a gauche conformation over the trans state by about 0.2 kcal/mol, according to MP2 level ab initio calculations using a D95+∗ basis set. This oxygen gauche effect, coupled with an expanded C-O-C skeletal angle of 122°, results in a relatively small predicted random coil dimension for PFPOE with a characteristic ratio of C∞ ≈ 3.9 at 300 K. Within the limits of ab initio values for the conformational energies and geometries, the predicted value is in good agreement with experimental results of about 4.9. © 1995, American Chemical Society. All rights reserved.