Effects of phenyl-based pendent groups on helical conformation of poly(N-propargylamides)

Abstract

Five achiral N-propargylamide monomers with various phenyl-based substitutents, [HC ≡ CCH2NHCOR, R for M1: C6H4CH3; M2: C6H4CH2CH3; M3: C6H4(CH2)2CH3; M4: C6H4(CH2)3CH3; M5: C6H4C(CH3)3], were synthesized and polymerized with a rhodium catalyst, (nbd)Rh+B-(C6H5)4 (nbd = 2,5-norbornadiene). The corresponding five homopolymers were obtained in high yields of 90–95% and with moderate molecular weights (Mn ≥ 10 000). All the polymers possessed high cis contents (≥95%). Poly(1)–poly(3) exhibited UV-vis absorption peaks at approx. 350 nm, which indicates that the three polymers formed helical conformations, while no UV-vis absorption peaks could be observed in poly(4) and poly(5) in the wavelength range of 320–500 nm, demonstrating that these two polymers could not adopt helical structures under the examined conditions. To confirm the helical structures formed in poly(1)–poly(3), a chiral monomer, M6, was utilized to copolymerize with M2, which was used as the representative for M1−M3. M6 was utilized since its polymer could form stable helices under suited conditions. The resulting copolymers exhibited remarkable CD effects, however, the maximum wavelength in the copolymers varied remarkably, mainly depending on the composition of the copolymers. It is concluded that in the formation of ordered helical conformations, the substitutents of varied bulk led to different steric repulsion and varied synergic effects among the neighboring pendent groups.