Proline-Based Block Copolymers Displaying Upper and Lower Critical Solution Temperatures

Novel dual thermoresponsive block copolymers displaying lower critical solution temperature (LCST) and upper critical solution temperature (UCST) were synthesized by reversible addition−fragmentation chain transfer (RAFT) polymerization of two proline-based monomers. Poly(N-acryloyl-l-proline methyl ester), poly(A-Pro-OMe), was selected as a thermoresponsive segment, whereas poly(N-acryloyl-4-trans-hydroxy-l-proline), poly(A-Hyp-OH), could be regarded as a water-soluble polymer. The block copolymer having suitable comonomer composition (A-Pro-OMe/A-Hyp-OH = 27/73) exhibited soluble−insoluble−soluble transition with lower (LCST = 19−21 °C) and upper (UCST = 39−45 °C) critical solution temperatures in acidic water. The comonomer composition of poly(A-Pro-OMe)-b-poly(A-Hyp-OH) and pH value in the aqueous solution were found to affect characteristic thermoresponsive behaviors. The temperature-dependent assembled structures and chiroptical properties were evaluated by dynamic light scattering (DLS) and circular dichroism (CD) measurements. Another type of dual thermosensitive block copolymers with blocks having two different LCSTs, poly(A-Pro-OMe)-b-poly(A-Hyp-OMe), were prepared by the methylation of the carboxylic acid groups in poly(A-Pro-OMe)-b-poly(A-Hyp-OH), and their temperature-dependent solution behaviors were investigated. To the best of our knowledge, this is the first report of the dual thermoresponsive system, which can be changed from a system exhibiting LCST and UCST into another one having two different LCSTs by a simple methylation reaction.