Synthesis of amino acid-derived vinyl polymers with precisely controlled hydropathy and their thermoresponsive behavior in water

Abstract

Controlling the thermoresponsive behavior of bio-based synthetic polymers is critical for the development of smart functional materials with various potential applications in biomedical and nanotechnological fields. Although the effects of polymer architectures, such as molecular weight and hydrophilic/hydrophobic balance, are being clarified, the rational design principles are not yet fully understood, particularly in vinyl-polymer systems, because of their structural diversity and complexity. Herein, the synthesis of amino acid-derived vinyl polymers with systematically different hydropathies and their thermal responses in water are reported. Thirteen distinct block and statistical random co/homopolymers are precisely synthesized via an ultra-rapid reversible addition–fragmentation chain transfer polymerization of N-acryloyl alanine (A) methyl ester (main component) and various N-acryloyl amino acid (X) methyl esters (X = G, S, K, L, F, V) (guest monomer). All polymers possess exactly the same total chain length (DP = 35) and composition (A/X = 30/5), and water-soluble polymers exhibit lower critical solution temperature behavior. Interestingly, a clear correlation between the hydropathy indices (HI) of amino acids X and the transition temperature (T_t) is observed; namely, T_t systematically reflects the hydrophobicity of the X units. Furthermore, the specific monomer sequence (i.e., the distribution of X units in the polymer chain) affects the thermal response, particularly when the guest amino acids have extremely high or low HI. These findings are important for the precise design of chemically diverse and complex stimuli-responsive bio-based polymers.