Optically active polyacetylene: Synthesis and helical conformation of a poly(phenylacetylene) carryingL-alanyl-L-alanine pendants

The development of biomimetic helical polymers is a topic of current interest. Studies of these simple unnatural systems may help us to understand complex natural systems and to explore efficient strategies for assembling molecular components into hierarchical architectures. Helical polymers with π conjugation along the main chain are of particular interest because of their high-tech applications as chiral stationary phases, optical polarizing films, asymmetric electrodes, and so forth. Polyacetylene is the best known conjugated polymer and exists in an excess of one-handed helical conformation when appropriate chiral pendants are attached.1–4 Yashima et al.5 prepared helical polyacetylenes bearing poly(γ-benzyl-L-glutamate) and poly(L-glutamic acid) moieties. Masuda et al.6 synthesized chiral copolymers with amino acid side groups and found that the helix content could be controlled through the variation of the concentrations of the chiral monomers. Attracted by the academic and practical aspects, we have also worked on the synthesis of optically active polyacetylenes with naturally occurring building blocks of amino acids, saccharides, and nucleobases.7–10 The chain helicity of the polymers can be continuously and reversibly tuned by simple external stimuli such as the solvent and pH. Amphiphilic polyacetylenes, in response to changes in their environments, spontaneously assemble into a variety of morphological structures, such as helices, vesicles, honeycomb patterns, and mollusk shapes.11–13 To enrich the research field of helical polyacetylenes, in this communication, we report the first synthesis of a chiral poly(phenylacetylene) containing dipeptide appendages, and we present its chiroptical properties.