Control of One-Handed Helicity in Polyacetylenes: Impact of an Extremely Small Amount of Chiral Substituents.

Controlling the one-handed helicity in synthetic polymers is crucial for developing helical polymer-based advanced chiral materials. We now report that an extremely small amount of chiral biphenylylacetylene (BPA) monomers (ca. 0.3-0.5 mol %) allows complete control of the one-handed helicity throughout the polymer chains mostly composed of achiral BPAs. Chiral substituents introduced at the 2-position of the biphenyl units of BPA positioned in the vicinity of the polymer backbones contribute to a significant amplification of the helical bias, as interpreted by theoretical modeling and simulation. The helical structures, such as the helical pitch and absolute helical handedness (right- or left-handed helix) of the one-handed helical copolymers, were unambiguously determined by high-resolution atomic force microscopy combined with X-ray diffraction. The exceptionally strong helix-biasing power of the chiral BPA provides a highly durable and practically useful chiral material for the separation of enantiomers in chromatography by copolymerization of an achiral functional BPA with a small amount of the chiral BPA (0.5 mol %) due to the robust helical scaffold of the one-handed helical copolymer.

Catalytic One-Handed Helix Induction and Subsequent Static Memory of Poly(biphenylylacetylene)s Assisted by a Small Amount of Carboxy Groups Introduced at the Pendants.

A dynamically racemic helical copolymer composed of an achiral biphenylylacetylene (BPA) bearing methoxymethoxy groups at the 2,2'-positions and 1 mol % of an achiral BPA carrying 2-carboxy-2'-methoxymethoxy groups at the biphenyl pendants was found to fold into an excess one-handed helix with significant amplification of the helicity in the presence of a small amount of optically active amines. The induced macromolecular helicity was retained ("memorized") after removal of the chiral amines. The copolymer had a significant sensitivity for detecting the chirality of chiral amines with a sensitivity more than 10000-fold higher than that of the corresponding homopolymers with no carboxy group, thus showing Cotton effects even in the presence of a 0.01 equiv of an optically active amine. The effects of the substituents at the 4'-position of the biphenyl pendants of the copolymers and the structures of the chiral amines on the macromolecular helicity induction were also investigated.