Amplification of macromolecular helicity of poly(biphenylylacetylene)s composed of a small amount of chiral [5]helicene units

Abstract

AbstractA series of novel biphenylylacetylene (BPA)‐based chiral/achiral copolymers bearing a small amount of configurationally labile optically active [5]helicene pendants (5, 10, and 20 mol%) introduced at the 4′‐position of the biphenyl units was synthesized. An almost fully right (P)‐handed helical copolymer was obtained when copolymerized with 10 mol% of the (M)‐[5]helicene‐bound BPA monomer, indicative of a very strong sergeants and soldiers effect. The (M)‐helicene units of the copolymers are configurationally labile and gradually racemized over time. However, the copolymer with 10 mol% of the (M)‐[5]helicene‐bound BPA mostly maintained its preferred‐handed helicity even after the pendant helicene units were completely racemized, which can be ascribed to the static memory of the main‐chain helicity stabilized by the bulky yet racemic helicene units. Hence, the helicity memory of the corresponding helicene‐free homopolymer was lost rapidly. For comparison, an optically inactive copolymer containing 10 mol% of the racemic [5]helicene units was also prepared, and noncovalent helicity induction and subsequent memory of the helicity as well as its helix‐memory stability were investigated.