A Helix in Helices:  A Helical Conjugated Polymer That Has Helically Arranged Hydrogen-Bond Strands

The secondary structure of Rh-based stereoregular cis-poly(N-propargylamides) was proposed on the basis of the UV−vis, CD, and IR spectra, the wormlike touched-bead model theory, and a semiempirical molecular orbital calculation. The IR spectra demonstrated the formation of well-ordered intramolecular hydrogen bonding between the pendant amide groups. The relatively red-shified absorption and the intense CD effects indicate that the main chain twists only slightly from coplanarity to exist in a helical conformation with a large helix pitch. Analyzing the intrinsic viscosity of a polymer, (±)-poly(N-propargyl-2-ethylhexanamide), on the basis of the wormlike touched-bead model theory, showed a large helix pitch per monomer unit (h, 0.2 nm), which also supports the above characteristics that the main chain loosely twists. A semiempirical calculation with the AM1 method showed that the helical conjugated backbone is surrounded by two helically arranged hydrogen-bond strands.