Dynamically Stable Helices of Poly(N-propargylamides) with Bulky Aliphatic Groups

Abstract

N-Propargylamides with bulky pendent groups [HC⋮CCH2NHCOR, 9: R = CH2C(CH3)3, 10: R = C(CH3)3, 11: R = C(CH3)2CH2CH2CH3, 12: R = CH(CH2CH3)2, 13: R = CH(CH2CH2CH3)2] were polymerized with a rhodium catalyst, (nbd)Rh+B-(C6H5)4 (nbd = 2,5-norbornadiene), to obtain the polymers in 80−92% yields. Poly(11) and poly(12) possessed moderate molecular weights (Mn ≥ 10 000) and were totally soluble in a few solvents including chloroform. On the other hand, the Mn values of poly(9), poly(10), and poly(13) were no more than 5000, and these polymers were not completely soluble in any solvents. The conformational transition behavior of these polymers was examined by temperature-variable UV−vis spectroscopy in chloroform solution, which revealed that poly(10)−poly(13) could form dynamically stable helical conformation even at 60 °C. By copolymerizations of monomers 9 and 10 with monomer 4, HC⋮CCH2NHCO(CH2)4H, the solubility of the polymers was effectively improved and almost all the copolymers totally dissolved in chloroform, while the molecular weights of the copolymers increased up to 18 600−45 000. Moreover, the helix contents of poly(40.63-co-90.37) and poly(40.40-co-100.60) were the highest among the two series of (co)polymers, respectively. It is concluded that the copolymerization of 9 and 10 with 4 effectively decreased the steric repulsion between the crowded side chains, which probably allowed the copolymers to take helical conformation efficiently.