No abstract

N-Propargylamides having pendent groups with different lengths (HCtCCH2NHCOR, R ) (CH2)nH, n ) 1-8) were polymerized in the presence of a Rh catalyst [(nbd)Rh + B -(C6H5)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 ∆H and ∆S 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.

N-Propargylamides having chiral centers at the α-carbon of the amide groups, 1−3, were polymerized with (nbd)Rh+[η 6-C6H5B-(C6H5)3] to afford polymers with moderate molecular weights (M n = 6000−32 000) in good yield. The 1H NMR spectra demonstrated that the polymers have stereoregular structures (cis = 100%). The polymers were proven to take a helical conformation with an excess of one-handed screw sense in CHCl3, which was supported by their intense CD effects and large optical rotations. It was confirmed that the helical structure was stabilized not only by the steric repulsion but also by the intramolecular hydrogen bonds between the pendant groups. CD spectroscopic study showed that the helical structure is more stable than that of the polymers without a branch at the α-position, which allowed the polymers to exist in the helical state in various solvents. The electronic absorption, CD effects, and optical rotations of the polymers closely correlated to the extent of the hydrogen bonding between the pendant amide groups.

HCtCCH2NHR) 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. 1 H 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 R-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 -branched alkyl chains (poly(1e), R ) i-C4H9), which was evidenced by a clear, positive nonlinear relationship between the feed ratio of 1e to 2 and the optical rotation of the copolymers. UV-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 (∆G r, ∆Hr, and ∆Sr) 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.

N-Propargylamides with one, two, and three phenyl groups at the R-position of the carboxyl group [HC≡CCH2NHCOR; 1, R ) C(CH3)2C6H5; 2, R ) CCH3(C6H5)2; 3, R ) C(C6H5)3] were polymerized with a rhodium catalyst, (nbd)Rh + B -(C6H5)4 (nbd ) 2,5-norbornadiene), to obtain the corresponding polymers in 85-91% yields. Poly(1) possessed a moderate molecular weight (Mn ) 6300) and was thoroughly soluble in chloroform and dichloromethane. On the other hand, poly(2) and poly(3) were not totally soluble in the solvents. The Mn's of chloroform-soluble parts were less than 3000. The secondary structure of these three polymers in chloroform was examined by UV-vis spectroscopy with varying temperature. It was found that only poly(1) could adopt helical conformation even at 60 °C. By the copolymerization of either monomer 2 or 3 with HC≡CNH2CO(CH2)4H ( 4), the solubility of the polymers was effectively improved, and the Mn's were remarkably increased. When the content of unit 4 in poly-(2-co-4)s was 25% and above, the copolymers could form helical conformation with different degrees, among which poly(20.40-co-40.60) showed the largest helicity. When the content of unit 4 of poly(3-co-4)s exceeded 95%, the copolymer took helical structure partly.