The conformational properties of polyoxypropylene chains - A critical correlation of theory and experiment

The conformational properties of polyoxypropylene (POP) chains are greatly affected by their tacticity and by the preference for gauche rotational states about the C-C skeletal bond, known as the ''gauche-oxygen'' effect. The latter feature is not reproduced by rotational-isomeric-state (RIS) calculations based on conventional, semiempirical conformational-energy functions, and has therefore led, in the literature, to the somewhat arbitrary ''fitting'' of conformational-energy parameters, in order that a chosen RIS model will reproduce the experimentally-measured values of, for example, the mean-square unperturbed end-to-end distance, [r(2)], and the populations of trans and gauche C-C bond conformers. In addition, an explanation of the existing literature describing the measurements of [r(2)] and bond-conformer populations reveals that, due to the effects of variations in the degrees of isotacticity introduced during the polymerization of propylene oxide, the properties of POP samples cannot be related unambiguously to specific chain tacticities. As a result, the assumption used by various authors, that the experimental values of [r(2)] and the bond-conformer populations used to parameterize theoretical models of the POP chain, using RIS theory, refer to wholly isotactic chain-structures, is probably in error. At present, experimental values of [r(2)] are best reproduced, without the arbitrary fitting of experimental parameters, by using an RIS model based on a forcefield which neglects Coulombic interactions. The isolated-chain RIS calculations which include Coulombic interactions do not adequately account for the screening of the in-chain electrostatic charges, which is likely to occur to some extent for the polymer in bulk or in solution. The forcefields also need to be improved, to take more realistic account of the properties of ether-oxygen atoms. In addition, the conformationally-related properties of POP samples with a range of defined tacticities need to be measured experimentally.