Two acrylamides containing proline and hydroxyproline moiety, N-acryloyl-l-proline (A-Pro-OH) and N-acryloyl-4-trans-l-proline (A-Hyp-OH), were polymerized by reversible addition-fragmentation chain transfer (RAFT) process to afford well-defined amino acid based polymers. Two chain transfer agents (CTAs), benzyl dithiobenzoate (CTA 1) and benzyl 1-pyrrolecarbodithioate (CTA 2), were compared for the direct polymerization of these monomers without protecting chemistry. With 2,2′-azobis(isobutyronitrile) as an initiator, the dithiocarbamate-type RAFT agent (CTA 2) was efficient for the controlled synthesis of poly(A-Pro-OH)s, which can be regarded as a weak polyelectrolyte. Controlled character of the polymerization of A-Hyp-OH, which has a carboxylic acid and a hydroxyl group in the monomer unit, in the presence of CTA 1 was confirmed by the formation of narrow polydispersity products and the linear relationship between the molecular weight and conversion. Water-soluble poly(A-Hyp-OH)s with number-average molecular weights between 8.2 × 103 and 2.21 × 104 and relatively low polydispersities were obtained, depending on the monomer/CTA ratio. Their methylated samples, poly(A-Pro-OMe) and poly(A-Hyp-OMe), and random copolymers showed characteristic thermal phase transitions in aqueous solutions.

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.

Novel pH- and thermo-responsive block copolymers comprising proline-based segments were synthesized by reversible addition−fragmentation chain transfer (RAFT) polymerization. Poly(N-acryloyl-l-proline methyl ester), poly(A-Pro-OMe), was selected as a thermo-responsive segment, whereas poly(N-acryloyl-l-proline), poly(A-Pro-OH), was employed as a weak anionic polyelectrolyte. The chain extension from the dithiocarbamate-terminated poly(A-Pro-OH) to A-Pro-OMe could be well controlled under suitable conditions, and it provided block copolymers with as-designed chain structures and low polydispersities. The characteristic pH- and thermo-responsive properties of the block copolymers were then investigated in aqueous solutions. By varying the pH, salt concentration, and comonomer composition, the lower critical solution temperature (LCST) can be tuned between 18 and 55 °C. The chiroptical properties and assembled structures of the resulting block copolymers in aqueous solutions were evaluated by circular dichroism (CD) and dynamic light scattering (DLS) measurements.

Many photothermal materials have been developed in recent years to achieve efficient photothermal therapy. Although larger-sized materials are preferred to ensure a better photothermal performance, upsizing hampers the cellular uptake of these materials. To overcome this dilemma, we proposed an active control system to manipulate the assembly of magnetic composite particles (MCPs) at the cellular level. Herein, MCPs of different sizes (small, medium, and large) were synthesized, and their surfaces were modified with a polymer. These MCPs were then cultured with HeLa cells, and their uptake and mobility were investigated. Small-sized MCPs (d v = 206 nm) and medium-sized MCPs (d v = 312 nm) could be introduced into HeLa cells. However, a smaller amount of the large-sized MCPs was introduced as compared to the other MCPs. Using a direct current magnetic field [DCMF (B = 150 mT)], medium-sized MCPs were observed to quickly assemble in cells; however, redispersal did not occur after the DCMF was turned off. To improve the particle dispersibility, polyethylene glycol and polyethyleneimine (PEI) were used to modify the medium-sized MCPs. Coating the MCPs with PEI (a polymer molecular weight of 25,000 or 270,000) improved their dispersibility in phosphate-buffered saline and cellular uptake. In addition, the redispersal of assembled PEI-coated MCPs was observed after the DCMF was turned off. The photothermal conversion efficiency of the MCPs was also improved using the DCMF. Consequently, the PEI-coated MCPs were first introduced into cells in a dispersed state, and their assembly was induced at the subcellular scale by applying a DCMF, and the assembly was spontaneously redispersed by switching off the DCMF. Such a remote control could improve the cytotoxicity of MCPs under near-infrared laser irradiation.