Design of helical poly(N-propargylamides) that switch the helix sense with thermal stimuli

Copolymerization of chiral N-propargylamides, (S)-N-propargyl-2-methyldecanamide (1), and either (R)-N-propargyl-3,7-dimethyloctanamide (2) or (IS)-N-propargyl-3-oxo-2-oxa-4,7,7-trimethyl2-bicyclo[2.2]heptan-1-acetamide (3) were conducted with (nbd)Rh+[eta(6)-C6H5B-(C6H5)(3)] catalyst to obtain the corresponding copolymers with moderate molecular weights (Mn = 19 000-56 000) in good yields. The CD spectroscopic study showed that these two types of copolymers with various compositions formed helical structure and underwent a helix-helix transition driven by temperature change. In all cases, the optical activity became zero at certain temperatures, which depended on copolymer composition. The temperature dependence of helix sense of the copolymers allowed the determination of thermodynamic parameters. The helical structure of poly(1-co-2), which has long alkyl pendants, was affected by external stimuli more than poly(1-co-3) was. The thermodynamic parameters of helix inversion were also influenced by solvent polarity.