Conformation Shift Switches the Chiral Amplification of Helical Copoly(phenylacetylene)s from Abnormal to Normal Sergeants-and-Soldiers Effect

Optically active helical random copoly(phenylacetylene)s were prepared from structurally similar monomers, (+)-3-hexadecylcarbamoyl-5-[((S)-1-phenylethyl)carbamoyl]phenylacetylene and 3-hexadecylcarbamoyl-5-(benzylcarbamoyl)phenylacetylene, under the catalysis of Rh[C(C6H5)=C(C6H5)(2)](nbd)(4-FC6H4)(3)P (nbd = 2,5-norbornadiene). The chiral amplification followed either normal or abnormal sergeants-and-soldiers rule depending on the solvent nature. In apolar solvents (i.e., CHCl3 and THF), contracted cis-cisoid M- and P-helices were selectively induced for the copolymers containing chiral units below or above 74 mol %, respectively. In polar solvent (i.e., CHCl3/CH3OH, 70/30, v/v), a cis-cisoid to cis-transoid transition occurred and stretched M-helices were dominantly formed, the optical activity of which scaled up nonlinearly with increasing chiral component. This unusual phenomenon was rationalized by the distinct competing interactions between the vicinal chiral/achiral and chiral/chiral unit pairs in the contracted and stretched helices according to the modified Ising model. It offers a promising design strategy to control macromolecular helicity.