Abstract
Isotactic, syndiotactic and atactic isomers of oligomers of low-molecular-weight polystyrene were successfully resolved using a highly efficient C 18 column and and acetonitrile—methylene chloride gradient. Structural assignments for some of the collected trimer peaks were made by gas chromatography—mass spectrometry and 13C nuclear magnetic resonance analysis. After the initial separation, 27 different mobile-phase solvents, several reversed-phase bonded packings, and different column temperatures were tested in an effort to maximise the resolution of the stereoisomers. Peak separation( P i ) for each peak pair and chromatographic response function (CRF) for each chromatogram were calculated and compared. Only a few of the solvents tested produced stereoisomer separation. The sample solubility of polystyrene in the mobile-phase solvent appeared to be the best predictor for an optimum mobile phase. Only “weak” solvents, those solvents that could not easily dissolve polystyrene, gave good isomer separation when used as the mobile phase. Hansen's solubility model appears promising for optimizing the mobile-phase composition for separations of other solutes using reversed-phase high-performance liquid chromatography. A phenyl bonded phase, a C 8 bonded phase, and a synthesized perfluorinated bonded phase were compared to octadecylsilane. Only the C 8 column produced isomer separation. Decreasing the column temperature resulted in better resolution of the stereoisomers at the expense of longer retention times. Based on the high-performance liquid chromatographic data, it has been proposed that the mobile-phase affects both the conformations of the polystyrene stereoisomers and of the long-chain hydrocarbon bonded phases. The resolution of stereoisomers by capillary gas chromatography was found to be significantly less than that obtained by high-performance liquid chromatography with the C 8 and C 18 columns.
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