Synthesis of amphiphilic and thermoresponsive ABC miktoarm star terpolymer via a combination of consecutive click reactions and atom transfer radical polymerization

Well-defined amphiphilic and thermoresponsive ABC miktoarm star terpolymer consisting of poly(ethylene glycol), poly(tert-butyl methacrylate), and poly(N-isopropylacrylamide) arms, PEG(-b-PtBMA)-b-PNIPAM, was synthesized via a combination of consecutive click reactions and atom transfer radical polymerization (ATRP). Click reaction of monoalkynyl-terminated PEG with a trifunctional core molecule bis(2-azidoethyl)amine, (N₃) ₂-NH, afforded difunctional PEG possessing an azido and a secondary amine moiety at the chain end, PEG-NH-N₃. Next, the amidation of PEG-NH-N₃ with 2-chloropropionyl chloride led to PEG-based ATRP macroinitiator, PEG(-N₃)-Cl. The subsequent ATRP of N-isopropylacrylamide (NIPAM) using PEG(-N₃)-Cl as the macroinitiator led to PEG(-N₃)-b-PNIPAM bearing an azido moiety at the diblock junction point. Finally, well-defined ABC miktoarm star terpolymer, PEG(-b-PtBMA)-b-PNIPAM, was prepared via the click reaction of PEG(-N ₃)-b-PNIPAM with monoalkynyl-terminated PtBMA. In aqueous solution, the obtained ABC miktoarm star terpolymer self-assembles into micelles consisting of PtBMA cores and hybrid PEG/PNIPAM coronas, which are characterized by dynamic and static laser light scattering, and transmission electron microscopy. On heating above the phase transition temperature of PNIPAM in the hybrid corona, micelles initially formed at lower temperatures undergo further structural rearrangement and fuse into much larger aggregates solely stabilized by PEG coronas.