Molecular-shape selective high-performance liquid chromatography: Stabilization effect of polymer main chain by alternating copolymerization

This work aims to clarify that stabilization or increased rigidity of polymer main chains as an organic stationary phase can lead the selectivity enhancement in high-performance liquid chromatography (HPLC). For this purpose, the alternating copolymer of octadecyl acrylate (ODA) with a cyclic monomer (N-octadecylmaleimide, OMI) as a rigid segment was synthesized and compared with the ODA homopolymer. Both of the polymer-grafted silicas (Sil-poly(ODA-alt-OMI) and Sil-poly(ODA), respectively) were prepared by radical polymerization on 3-mercaptopropyltrimethoxysilane-modified silica. The characterizations were carried out by elemental analyses, diffuse reflectance infrared Fourier transform (DRIFT), and solid-state 13C cross-polarization magic angle spinning nuclear magnetic resonance (CP-MAS-NMR) spectroscopies. Chromatographic behaviors were evaluated by the retention studies of shape-constrained isomers of polycyclic aromatic hydrocarbons (PAHs), isomers of tocopherol and carotenoids. Higher molecular-linearity selectivity toward PAHs was obtained on Sil-poy(ODA-alt-OMI) regardless of temperature changes but at temperature below 40 °C, Sil-poly(ODA) showed better planarity selectivity than that of Sil-poy(ODA-alt-OMI). As a result, the higher separation ability toward tocopherols and carotenoids was obtained on Sil-poy(ODA-alt-OMI). These results indicate that the stabilization in the polymer main chain by alternating copolymerization and the stabilization in the side chains by a disordered-to-ordered phase transition were effective to enhance the molecular-shape selectivity.