Dual pH and temperature responsive helical copolymer libraries with pendant chiral leucine moieties

Abstract

Reversible addition-fragmentation chain transfer (RAFT) polymerization was employed to afford two different chiral copolymer series having opposite chiroptical properties based on 2-(2-methoxyethoxy)ethyl methacrylate (MEO₂MA) and Boc-L/D-leucine methacryloyloxyethyl ester (Boc-L/D-Leu-HEMA) with varying co-monomer proportions. The random nature of the copolymers was indicated by the reactivity ratios of the two monomers for the two systems (MEO₂MA with Boc-L-Leu-HEMA and Boc-D-Leu-HEMA), and further supported by smooth variation of the single glass transition temperature (T_g) determined from differential scanning calorimetry (DSC). With an increasing amino acid content in the copolymer, the specific rotation and molar ellipticity values increased, as examined by polarimeter and circular dichroism (CD) spectroscopy, respectively. After Boc-group cleavage under acidic conditions, all the copolymers exhibited cationic nature, as confirmed by dynamic light scattering (DLS), as well as the dual thermo and pH-dependent solubility behaviour in aqueous solution. The temperature induced phase transition of these copolymers was investigated as a function of the copolymer compositions over the pH range from 5.5 to 6.5 using a UV-Vis spectrophotometer. Furthermore, the chiral recognition ability of the synthesized monomers and homopolymers towards 1,1′-bi-2-naphthol (rac-BINOL) was studied by ¹H NMR spectroscopy.