A novel type of optically active helical polymers:: Synthesis and characterization of poly(N-propargylureas)

This article presents two novel artificial helical polymers, substituted polyacetylenes with urea groups in side chains. Poly(4) and poly(5) can be obtained in high yields (>= 97%) and with moderate molecular weights (11,000-14,000). Poly(4) contains chiral centers in side chains, and poly(5) is an achiral polymer. Both of the two polymers adopted helical structures under certain conditions. More interestingly, poly(4) exhibited large specific optical rotations, resulting from the predominant one-handed screw sense. The helical conformation in poly(5) was stable against heat, while poly(4) underwent conformational transition from helix to random coil upon increasing temperature from 0 to 55 degrees C. Solvents had considerable influence on the stability of the helical conformation in poly(4). The screw sense adopted by the helices was also largely affected by the nature of the solvent. Poly(4-co-5)s formed helical conformation and showed large optical rotations, following the Sergeants and Soldiers rule. By comparing the present two polymers (with one -N-H groups) with the three polymers previously reported (with two -N-H groups in side chains), the nature of the hydrogen bonds formed between the neighboring urea groups played big roles in the formation of stable helical conformation. (c) 2008 Wiley Periodicals, Inc.