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

Polyproline is a unique thermoresponsive polymer characterized by large thermal and conformational hysteresis. This article employs polyproline-based double hydrophilic block copolymers (PNIPAM_<i>n</i>-<i>b</i>-PLP_<i>m</i>) to gain insight into polyproline's thermoresponsive mechanism. The amine-terminated poly(<i>N</i>-isopropylacrylamide) (NH₂-PNIPAM_<i>m</i>) was used as the macroinitiator for ring-opening polymerization of proline-NCA monomers, resulting in various block copolymers (PNIPAM_<i>n</i>-<i>b</i>-PLP_<i>m</i>) with varying PLP block lengths. Block copolymers' thermal phase transitions were compared with their homopolymer counterparts using turbidimetry, variable-temperature NMR, dynamic light scattering, and circular dichroism spectroscopy. These experiments revealed that regardless of their compositions, all block copolymers exhibited a two-stage collapse (<i>T</i>_CP(PLP) > <i>T</i>_CP(PNIPAM)) during the heating cycle. In contrast, only one clearing temperature (<i>T</i>_CL) was observed during cooling. The observed clearing temperature is closely correlated to the clearing temperature of PNIPAM blocks, suggesting the role of water-soluble PNIPAM blocks in resolving the PLP blocks. Moreover, thermal and conformational hysteresis related to the polyproline block is significantly suppressed in the presence of a PNIPAM block. Linking PNIPAM blocks has two significant effects on PLP segments' thermoresponsive behavior. For example, during the heating cycle, the precollapsed PNIPAM chains (as <i>T</i>_CP(PNIPAM) < <i>T</i>_CP(PLP)) prevent orderly aggregation within the PLP block. Meanwhile, during the cooling cycle below the clearing temperature of the PNIPAM block, the PNIPAM chains impart water solubility (as <i>T</i>_CL(PNIPAM) > <i>T</i>_CL(PLP)) to the collapsed PLP chains. Overall, the PNIPAM block imparts water solubility and perturbs PLP chains to form the native aggregate structure, suppressing the hysteresis effect. Accordingly, the large thermal and conformational hysteresis associated with native PLP chains appears to result from a noninterfering aggregation above the critical temperature.