Effect of side chain structure on the conformation of poly(N-propargylalkylamide)

Achiral N-propargylalkylamides (1a-1g, HCequivalent toCCH(2)NHR) having various alkyl groups (R = CH3, C2H5, C3H7, i-C3H7, i-C4H9, n-C5H11, n-C7H15) were homopolymerized or copolymerized with a chiral comonomer, (R)-N-propargyl-3,7-dimethyloctanamide (2), in the presence of a Rh initiator to establish the relationship between the main-chain conformation and the structure of the pedant groups. H-1 NMR and viscosity measurements of the homopolymers revealed that the structure of the pendant groups markedly influences the rigidity of the polymer backbone and the stability of the helical conformation. The copolymerization using the achiral comonomers having linear or a-branched alkyl groups showed poor or no cooperative effects on the helical conformation, meaning that these polymers exist in a disordered state. On the other hand, a stable helical conformation, i.e., a long persistence length of the helical domain, was attainable for the polymer having beta-branched alkyl chains (poly(le), R = i-C4H9), which was evidenced by a clear, positive nonlinear relationship between the feed ratio of le to 2 and the optical rotation of the copolymers. T-TV-visible spectroscopic studies demonstrated that, in CHCl3, the helical and disordered main chains display absorption centered at 400 and 320 nm, respectively, which resulted in different colors in solution of the helical (yellow) and disordered (achromic) polymers. Thermochromism was achieved by the thermally induced reversible conformational change between helical and disordered states. The thermodynamic parameters (DeltaG(r), DeltaH(r), and DeltaS(r)) that govern the stability of the helical conformation of a copolymer were estimated by the temperature dependence of the populations of the helical and disordered states using UV-visible spectra.