Investigations of optically active polymeric chiral stationary phases
This thesis describes the application of optically active helical polymers to chiral stationary phases for high performance liquid chromatography. The use of porous graphitic carbon as a support for these phases is examined and its implication for the nature of the separation. In the first case 3,5-dimethylphenyl carbamate (CDMPC) was studied. This work continued the study by Grieb and Matlin. The use of porous graphitic carbon as a support for this polymer was examined. PGC is produced by Hypersil under the name Hypercarb. It is a porous carbon phase with virtually no surface functionalities. It was confirmed that a 25% w/w loading ofCDMPC produced using a batch coating method produced the optimum phase. The nature of the cellulose used to synthesise the CDMPC was also studied by gel permeation chromatography. It was found that Avicel cellulose (Merck) gave the best results. CDMPC has certain characteristics which make it an effective phase, these are; a) a-helical secondary structure and b) optical activity within the monomer unit. We decided to examine other polymers which possess these characteristics, in particular poly-L-Ieucine. It is believed that this polymer has an a-helical secondary structure and in its synthesis L-leucine is used as a single pure isomer. This polymer has been shown to be effective as an asymmetric organic catalyst. Poly-L-Ieucine was synthesised using condensation polymerisation with three methods of initiation using both L-Ieucine and N-carboxy anhydride L-Ieucine as monomers. N-carboxy anhydride L-Ieucine was initiated by ethylenediamine in solution and water via a humidity cabinet. L-Ieucine was polymerised using triphenyl phosphite, lithium chloride and N-methyl pyrrolidine. Poly-L-Ieucine containing a maximum of fifteen residues was synthesised using solid phase peptide synthesis techniques. These polymers were examined using MALDI-TOF and ESI mass spectrometry. The polymers synthesised from N-carboxy anhydride L-Ieucine were examined using viscometry. Comparing these results it was considered that the molecular weight of the water-initiated polymer was greater. This polymer was further modified by continuous extraction to remove lower molecular weight fragments. All of these polymers were coated onto the surface of PGC by evaporation. It was shown that a 20% w/w loading level was optimum for this type of phase. The optimum phase was found to be made from the water initiated polymer after continuous extraction. This phase was shown to be capable of the resolution ofa variety of racemic epoxides.