Previous investigators have shown that statistical mechanical averages for configuration-dependent physical properties of long unperturbed polyoxyethylene chains are sensitive to the gauche-trans energy difference for rotation about C-C bonds. Agreement between theory and experiment could be obtained only by significant adjustment of this energy away from values predicted by semiempirical conformational energy computations. The present work examines the success of MM2 in evaluating conformational properties of long unperturbed polyoxyethylene chains. Calculations are performed which identify the rotational isomers, and their energies, for the indicated bonds in CH30CH2CH20-CH,-CH,-OCH,CH,OCH,.
These energies are used to assign statistical weights utilized in the configuration partition function for a rotational isomeric state chain with symmetric threefold interdependent rotations. The customary generator matrix scheme is employed to evaluate the mean-square unperturbed end-to-end distance, mean-square unperturbed dipole moment, and their temperature coefficients. Contrary to computational schemes employed previously, MM2 is found to provide an estimate of the gauche-trans energy difference for rotation about C-C which is in harmony with the known dimensions and dipole moments of the unperturbed polymer. MM2 also provides good estimates for most of the other parameters required in the rotational isomeric state treatment. A notable exception is provided by the gauche-trans energy difference for rotation about the C-0 bond. This energy difference is overestimated by MM2.
INTRODUCTION'
Rotational isomeric state theory has enjoyed considerable success in rationalization of configuration-dependent physical properties of polymers unperturbed by long-range interactions.2 The strategy employed requires formulation of a configuration parti tion function which incorporates all important short-range interactions. Configuration-dependent physical properties are then computed using appropriately constructed generator matrices which incorporate statistical weights from the configuration partition function along with pertinent structural information, e.g., bond lengths, bond angles, dihedral angles for rotational states, bond dipole moments, and bond polarizability tensors.
Polyoxyethylene is among the macromolecules that have been widely investigated from the viewpoint of rotational isomeric state theory. Configuration-dependent physical properties -___