Selectivity and the separation of isomers of low molecular weight polystyrenes

The separation of diastereoisomers of polystyrene has been investigated on carbon-clad zirconia (CCZ), C-18, and diamond C-18 stationary phases using eluents of widely different properties. Eluents were chosen according to their selectivity, solvent polarity index, and polymer solubility constants. Acetonitrile mobile phases in combination with CCZ produced the best diastereoisomer separations, however, when methanol was employed as the mobile phase, the separation profiles were very similar, except that retention was greatly increased. Retention and isomer separation decreased when isopropanol was employed. Surprisingly, some diastereoisomer separation was still apparent using the CCZ column and a 99% hexane/1% isopropanol mobile phase, although none was observed for the same solvent mixture on a C-18 column. Indeed, significant diastercoisomer separation was only obtained on the C-18 column-using acetonitrile as the mobile phase, but the resolution was far inferior to that observed on the CCZ column. The diamond C18 surface, which is a hybrid of C-18 and CCZ, gave separations that were intermediate between those of the parent surfaces, with enhanced molecular weight selectivity in comparison with CCZ, but slightly less discrimination on the basis of stereochemistry. Polystyrenes eluted unretained if either dichloromethane or tetrahydrofuran were used as the mobile phases on all stationary phases.