Conformational Transition between Random Coil and Helix of Poly(N-propargylamides)

Abstract

N-Propargylamides having pendent groups with different lengths (HC≡CCH 2 NHCOR, R = (CH 2 ) n H, n = 1-8) were polymerized in the presence of a Rh catalyst [(nbd)Rh + B - (C 6 H 5 ) 4 ; nbd = 2,5-norbornadiene] to obtain polymers with moderate molecular weight and high stereoregularity [poly(1)-poly(8)] in high yields. The conformational transition behavior of the resultant polymers was investigated by measuring UV-vis spectra in chloroform solution at different temperatures. Among the examined polymers, poly(5) and poly(6) took a stable helical conformation at relatively high temperatures, and their helical contents were the highest. Poly(2)-poly(4), which bear shorter alkyl pendent chains, did not exist in stable helical conformation owing to the lack of chain flexibility and the intermolecular hydrogen bonding. Poly(1) was not completely soluble in any solvents. Poly(7) and poly(8), which contain longer pendent chains, took helical conformation only at low temperatures because of the lower cis content of the polymer main chain. Both ΔH and AS for the conformational transition from random coil to helix assumed negative values, which also greatly depended on the length of the pendent groups. Whereas the helical conformation of poly(5) and poly(6) was readily generated in chloroform, neither THF nor toluene was favorable for helix formation.