Positive Synergy between the Helical Poly(phenylacetylene) Backbones and the Helical L-Proline Oligopeptide Pendants for Enhanced Enantioseparation Properties

A series of optically active helical poly(phenylacetylene)s (PPA-Pro1, PPA-Pro3, PPA-Pro6, PPA-Pro9, and PPA-Pro12) bearing different chain lengths of L-proline oligopeptide in the side chains were obtained by polymerizing the corresponding novel phenylacetylene monomers. The monomer adopted a trans-rich helix structure when the L-proline oligopeptide chain length was longer, according to the optical activities and 2D-NMR analysis. The helical structure could be maintained and significantly influenced the polymers' helical conformation by introducing the L-proline oligopeptide to the pendants. By the way, the morphology of PPA-Pro3 was observed by atomic force microscope (AFM) on highly oriented pyrolytic graphite (HOPG), and the information on the helix direction, pitch, and chain arrangement was obtained. Also, the chiral separation properties of these polymer-based chiral stationary phases (CSPs) were investigated using high-performance liquid chromatography (HPLC). The poly(phenylacetylene)s showed enhanced enantioseparation properties toward various racemates depending on the longer chain length of the L-proline oligopeptide in the pendants and the positive synergy between the helical backbone and helical side chains. Particularly, PPA-Pro9 showed comparable or even superior enantioseparation properties for racemates 2 and 9 to four commercial columns (Daicel Chiralpak or Chiralcel AD, AS, OD, and OT), indicating that these poly(phenylacetylene)-based CSPs have potential practical values. This work presented here provides inspiration for the further development of CSPs based on a new paradigm.