Separation and characterization of oligomers by reversed-phase high-performance liquid chromatography; a study on well-defined oligothiophenes

Reversed-phase high-performance liquid chromatography (RP-HPLC) was used fo r the separation of 3-hexylthiophene oligomers in the range of 3 to 30 mono meric units, while systematically varying stationary and mobile phases. A s et of different columns was chosen, covering a broad range of silica types, pore sizes and bonding chemistry. Mobile phases of tetrahydrofuran (THF) c ombined with water, acetonitrile (ACN) or methanol (MeOH) were used. Althou gh differences between columns were small, a higher selectivity correlated with a lower hydrophobicity parameter from the Galushko column test. The mo del of Jandera, based on the linear solvent strength model of Snyder, was u sed to describe the retention of the oligomers in gradient mode. This gave information about selectivities on different stationary phases similar to t he hydrophobicity parameter. Contrary to the stationary phase, the mobile p hase had a major influence on the selectivity. The THF-water combination ga ve much higher selectivities compared to THF combined with MeOH pr ACN. Usi ng the aqueous mobile phase even enabled separation of different isomers. D etermination of thermodynamic parameters for the model compounds showed tha t retention of the different isomers was mainly determined by the orientati on of the side chains at both ends of the chain. An additional repeating un it in the middle of the polymer backbone gave a similar contribution to ret ention, irrespective of the orientation of its side chain. Three model isom ers were separated by preparative RP-HPLC and identified by proton nuclear magnetic resonance spectroscopy. The combination of subsequent preparative size-exclusion chromatography, RP-HPLC and matrix-assisted laser desorption ionization time-of-flight mass spectrometry enabled the identification of the two major oligomeric series in the sample as the regioregular product w ith one bromine end group and, in smaller amounts, a regioirregular product with two bromine end groups. (C) 2001 Elsevier Science B.V. All rights res erved.