Switchable and Self-Learning Enantioseparation Based on Ultrafast Helix Induction and Memory of a Helical Polyacetylene

We now report the ultrafast helix induction of an optically inactive poly(biphenylylacetylene) (PBPA) with a helix inducer and subsequent memory of the helicity, which is completed within several minutes. Using living block polymerization followed by chain-end-selective immobilization of a PBPA onto silica gel, we have successfully developed a novel immobilized chiral stationary phase (CSP) that retains a high conformational freedom and exhibits markedly enhanced chiral recognition. By continuously injecting a helix inducer followed by racemic analytes, we achieved highly efficient, operationally simple, and rapidly reversible switching of enantiomer separation. During this study, we discovered the unprecedented partial separation of a nonracemic analyte on an immobilized CSP composed of a racemic PBPA. Due to the ultrafast helicity induction and its static memory in a PBPA of the CSP, the original dynamically racemic PBPA folds into an excess one-handed helix during chromatography solely through interaction with a nonracemic analyte. Hence, the PBPA acquires a resolving capability through a unique “self-learning” mechanism, and the nonracemic analyte can be eventually partially separated into enantiomers. This separation mechanism entirely differs from the classical self-disproportionation of enantiomers on achiral stationary phases and represents a new paradigm in chiral separation.