Core-Shell Copolymers with Brush-on-Hyperbranched Arm Architecture: Synthesis, Dual Thermoresponsive Behaviors, and Nanocarriers

A series of core-shell copolymers with brush-on-hyperbranched arm architecture were synthesized and characterized in detail. These copolymers were prepared via the combination of cationic ring-opening polymerization (CROP) and azide-alkyne “click” chemistry. The primary hyperbranched polyether core (HPEHO) was synthesized via the CROP of 3-ethyl-3-(hydroxymethyl)oxetane (EHO). Subsequently, the outer layer of glycidyl azide polymer (GAP) linear arms was prepared through the CROP of epichlorohydrin (ECH), followed by azidation of the chlorine atoms. Poly(ethylene glycol) (PEG) was last grafted onto the GAP chains via “click” reaction to obtain an amphiphilic core-shell structure. These core-shell copolymers exhibited a lower critical solution temperature (LCST)-type behavior in aqueous solutions. The LCST cloud point (CP) temperature not only decreased with an increasing polymer concentration but also was dependent on the molecular architecture that it increased upon increasing the PEG brush lengths and GAP arm lengths. More interestingly, while the free PEG chains were completely soluble in THF, an upper critical solution temperature (UCST) transition was observed for these core-shell copolymers in THF, and the CP shifted toward higher temperatures with increasing GAP linear arm lengths and concentrations. Last, the PEG outer shell and hydrophobic interior polyether core made these copolymers decent nanocarriers for hydrophobic cargo molecules in aqueous solutions.