Synthesis of twin‐tail tadpole‐shaped hydrophilic copolymers and their thermo‐responsive behavior

Twin‐tail tadpole‐shaped hydrophillic copolymers composed of cyclic poly(ethylene gycol) (PEG) and two linear poly( N ‐isopropylacrylamide) (PNIPAM) chains have been successfully synthesized by the combination of single‐electron‐transfer living radical polymerization and click chemistry under high concentration. Click cycloaddition reaction occurred between linear PNIPAM‐ b ‐PEG‐ b ‐PNIPAM with two azide groups at block junctions and dipropargyl oxalylate with high yield and efficiency. The resulting intermediates and the targeted polymers were characterized by proton nuclear magnetic resonance, fourier transform infrared spectroscopy, and gel permeation chromatography. The thermal phase transition behaviors of twin‐tail tadpole‐shaped polymers and their linear precursors were investigated by temperature‐dependent turbidity measurements, micro differential scanning calorimetry, and laser light scattering. The twin‐tail tadpole‐shaped polymers possess higher critical solution temperature (LCST) and smaller average aggregate size compared with their linear precursors with the same molecular weight. The above differences in the thermal phase transition behaviors should be due to the repulsive forces caused by the ring topology, which prohibited the intermolecular association.