Single- and double-stranded helical polymers: synthesis, structures, and functions.

Biological macromolecules, such as DNA and proteins, possess a unique and specific ordered structure, such as a right-handed double helix or a single alpha-helix. Those structures direct the sophisticated functions of these molecules in living systems. Inspired by biological helices, chemists have worked to synthesize polymers with controlled helicity, not only to mimic the biological helices but also to realize their functions. Although numerous synthetic polymers that fold into a single-handed helix have been reported, double-stranded helical polymers are almost unavailable except for a few oligomers. In addition, the exact structures of most helical polymers remain obscure. Therefore, the development of a conceptually new method for constructing double-stranded helical polymers and a reliable method for unambiguously determining the helical structures are important and urgent challenges in this area. In this Account, we describe the recent advances in the synthesis, structures, and functions of single- and double-stranded helical polymers from our group and others and provide a brief historical overview of synthetic helical polymers. We found unique macromolecules that fold into a preferred-handed helix through noncovalent bonding interactions with specific chiral guests. During the noncovalent helicity induction process, these guest molecules significantly amplified chirality in a dynamic helical polymer. During the intensive exploration of the helicity induction mechanism, we observed an unusual macromolecular helical memory in dynamic helical polymers. Furthermore, we found that rigid-rod helical poly(phenylacetylene)s and poly(phenyl isocyanide)s showing a cholesteric or smectic liquid crystal self-assemble to form two-dimensional crystals with a controlled helical conformation on solid substrates upon exposure to solvent vapors. We visualized their helical structures including the helical pitch and handedness by atomic force microscopy (AFM). We propose a modular strategy to construct complementary double helices by employing chiral amidinium-carboxylate salt bridges with m-terphenyl backbones. The double-stranded helical structures were characterized by circular dichroism in solution and X-ray diffraction of the crystals or the direct AFM observations. Serendipitously, we found that oligoresorcinols self-assemble into well-defined double helices resulting from interstrand aromatic stacking in water. These oligoresorcinols bound cyclic and linear oligosaccharides in water to form rotaxanes and hetero-double helices, respectively. The examples presented in this Account demonstrate the notable progress in the synthesis and structural determination of helical polymers including single- and double-stranded helices. Not only do we better understand the principle underlying the generation of helical conformations, but we have also used the knowledge of these unique helical structures to develop novel helical polymers with specific functions.