Helix Inversion and Aggregation Behavior of Chiral Poly(N‐propargylamides)

Summary: N‐Propargyl‐2‐methyloctanamide, (1), N‐propargyl‐2‐ethyloctanamide, (2), and N‐propargyl‐2‐propyloctanamide, (3) were polymerized with (nbd)Rh+[η6‐C6H5B−(C6H5)3] as a catalyst to obtain the polymers with moderate molecular weights ( = 15 000–57 000). The 1H NMR spectra demonstrated that the polymers had a stereoregular structure (cis = 100%). The polymers were proven to take a helical conformation with an excess of one‐handed screw‐sense in CHCl3, which was supported by their intense CD signals and large specific rotations. The signs of Cotton effects of poly(2) and poly(3) were dependent on the solvents used for CD measurement, while that of poly(1) was independent of the solvents. In CHCl3/THF or CHCl3/toluene mixtures, the sign of Cotton effect of poly(2) was inverted with changing the solvent composition at 20 °C, and the Cotton effects of poly(2) in CHCl3/toluene = 70/30 and CHCl3/THF = 80/20 were largely influenced by temperature. From the filtration and light scattering experiments of poly(2), the CD sign inversion was proved to be the screw sense inversion. magnified image