Synthesis, self-assembling properties, and atom transfer radical polymerization of an alkylated L-phenylalanine-derived monomeric organogel from silica: A new approach to prepare packing materials for high-performance liquid chromatography

The monomer N'-octadecyl-N-alpha-(4-vinyl)-benzoyl-L-phenylalaninea- mide (4) based on L-phenylalanine has been simply but effectively synthesized, and its self-assembling properties have been investigated. FTIR and a variable-temperature H-1 NMR spectroscopic investigation demonstrated that the aggregation of compound 4 in various organic solvents is due to the formation of intermolecular hydrogen bonds among the amide moieties. UV/Vis measurements indicated that the multiple pi-pi interactions of the phenyl groups also contribute to the self-assembly. As was observed by 13C cross-polarization magic-angle spinning (CP/MAS) NMR and variable-temperature H-1 NMR measurements, the ordered alkyl chains also played an important role in the molecular aggregation by van der Waals interactions. Compound 4 was polymerized by surface-initiated atom transfer radical polymerization from porous silica gel to prepare a packing material for HPLC. The results of thermogravimetric analysis showed that a relatively large amount of polymer was grafted onto the silica surface. The organic phase on silica was in a noncrystalline solid form in which the long alkyl chain exists in a less-ordered gauche conformation. Analysis of chromatographic performance for polyaromatic hydrocarbon samples showed higher selectivity than conventional reversed-phase HPLC packing materials.