Understanding Polyproline’s Unusual Thermoresponsive\nProperties Using a Polyproline-Based Double Hydrophilic Block Copolymer

Polyproline\nis a unique thermoresponsive polymer characterized\nby large thermal and conformational hysteresis. This article employs\npolyproline-based double hydrophilic block copolymers (PNIPAM<sub><i>n</i></sub>-<i>b</i>-PLP<sub><i>m</i></sub>) to gain insight into polyproline’s thermoresponsive\nmechanism. The amine-terminated poly(<i>N</i>-isopropylacrylamide)\n(NH<sub>2</sub>-PNIPAM<sub><i>m</i></sub>) was used as the\nmacroinitiator for ring-opening polymerization of proline-NCA monomers,\nresulting in various block copolymers (PNIPAM<sub><i>n</i></sub>-<i>b</i>-PLP<sub><i>m</i></sub>) with\nvarying PLP block lengths. Block copolymers’ thermal phase\ntransitions were compared with their homopolymer counterparts using\nturbidimetry, variable-temperature NMR, dynamic light scattering,\nand circular dichroism spectroscopy. These experiments revealed that\nregardless of their compositions, all block copolymers exhibited a\ntwo-stage collapse (<i>T</i><sub>CP(PLP)</sub> > <i>T</i><sub>CP(PNIPAM)</sub>) during the heating cycle. In contrast,\nonly one clearing temperature (<i>T</i><sub>CL</sub>) was\nobserved during cooling. The observed clearing temperature is closely\ncorrelated to the clearing temperature of PNIPAM blocks, suggesting\nthe role of water-soluble PNIPAM blocks in resolving the PLP blocks.\nMoreover,\nthermal and conformational hysteresis related to the polyproline block\nis significantly suppressed in the presence of a PNIPAM block. Linking\nPNIPAM blocks has two significant effects on PLP segments’\nthermoresponsive behavior. For example, during the heating cycle,\nthe precollapsed PNIPAM chains (as <i>T</i><sub>CP(PNIPAM)</sub> < <i>T</i><sub>CP(PLP)</sub>) prevent orderly aggregation\nwithin the PLP block. Meanwhile, during the cooling cycle below the\nclearing temperature of the PNIPAM block, the PNIPAM chains impart\nwater solubility (as <i>T</i><sub>CL(PNIPAM)</sub> > <i>T</i><sub>CL(PLP)</sub>) to the collapsed PLP chains. Overall,\nthe PNIPAM block imparts water solubility and perturbs PLP chains\nto form the native aggregate structure, suppressing the hysteresis\neffect. Accordingly, the large thermal and conformational hysteresis\nassociated with native PLP chains appears to result from a noninterfering\naggregation above the critical temperature.