Synthesis and characterization of L-leucine containing chiral vinyl monomer and its polymer, poly(2-(methacryloyloxyamino)-4-methyl pentanoic acid)

A chiral monomer containing L-leucine as a pendant group was synthesized from methacryloyl chloride and L-leucine in presence of sodium hydroxide at 4 °C. The monomer was polymerized by free radical polymerization in propan-2-ol at 60 °C using 2,2'-azobis isobutyronitrile (AIBN) as an initiator under nitrogen atmosphere. The polymer, poly(2-(Methacryloyloxyamino)-4-methyl pentanoic acid) is thus obtained. The molecular weight of the polymer was determined to be: Mw is 6.9 x 10³ and Mn is 5.6 x 10³. The optical rotation of both chiral monomer and its polymer varies with the solvent polarity. The amplification of optical rotation due to transformation of monomer to polymer is associated with the ordered conformation of chiral monomer unit in the polymeric chain due to some secondary interactions like H-bonding. The synthesized monomer and polymer exhibit intense Cotton effect at 220 nm. The conformation of the chain segments is sensitive to external stimuli, particularly the pH of the medium. In alkaline medium, the ordered chain conformation is destroyed resulting disordered random coils. The ordered coiling conformation is more firmly present on addition of HCl. The polymer exhibits swelling-deswelling characteristics with the change of pH of the medium, which is reversible. The Cotton effect decreases linearly with the increase of temperature which is reversible on cooling.