Conformational Study of Poly(N-propargylamides) Having Bulky Pendant Groups

N-Propargylamides having chiral centers at the α-carbon of the amide groups, 1−3, were polymerized with (nbd)Rh⁺[η⁶-C₆H₅B^-(C₆H₅)₃] to afford polymers with moderate molecular weights (M_n = 6000−32 000) in good yield. The ¹H NMR spectra demonstrated that the polymers have stereoregular structures (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 effects and large optical rotations. It was confirmed that the helical structure was stabilized not only by the steric repulsion but also by the intramolecular hydrogen bonds between the pendant groups. CD spectroscopic study showed that the helical structure is more stable than that of the polymers without a branch at the α-position, which allowed the polymers to exist in the helical state in various solvents. The electronic absorption, CD effects, and optical rotations of the polymers closely correlated to the extent of the hydrogen bonding between the pendant amide groups.