Synthesis and chiral recognition of optically active hydrogels containing helical polymer chains

Abstract

Copolymerizations of two novel N-propargylamide monomers [HCCCH₂NHCOR, in M1, R: CHCH₂; in M2, R: CH₂CH₂CH₂CHCH₂] with the other two N-propargylamide monomers (M3: achiral monomer; M4: chiral monomer), which were synthesized previously, were carried out in varied monomer feed ratios in the presence of (nbd)Rh⁺B⁻(C₆H₅)₄, with the aim to prepare optically active polymers simultaneously possessing helical polymer backbones and pendent polymerizable vinyl groups. Among the four homopolymers derived from M1–M4, polymer 1 and polymer 2 cannot dissolve completely in any of the usual solvents, while polymer 3 and polymer 4 are known to form stable helical conformations. The expected copolymers were obtained in high yields (≥95%) and exhibited good solubility in several organic solvents. The obtained copolymers were found to adopt stable helical structures under the investigated conditions. The as-prepared helical copolymers bearing vinyl groups were further employed as macromonomers to copolymerize with N-isopropylacrylamide (NIPAM) in chloroform at 55 °C using 2,2′-azobis(isobutyronitrile) as an initiator and N,N′-methylenebisacrylamide as a cross-linking agent. The copolymerizations took place smoothly, quantitatively providing hydrogels containing helical polymer chains preformed in the first copolymerization step. The helical structures remained in the hydrogels according to circular dichroism spectroscopy measurements. The hydrogels preferentially adsorbed D-tryptophan and (R)-(+)-1-phenylethylamine in the corresponding two enantiomer pairs, demonstrating the potential applications of the novel optically active hydrogels in the areas of chiral recognition and chiral resolution.