Synthesis and Self-Assembly Behavior of Six-Armed Block Copolymers with pH- and Thermo-Responsive Properties

A series of amphiphilic six-armed copolymers, containing poly(e-caprolactone) (PCL)and poly(2-(diethylamino) ethyl methacrylate) (PDEAEMA or PDEA) as the arm and cyclotriphosphazene as the core, were obtained by a combination of ring-opening polymerization (ROP) and atom transfer radical polymerization (ATRP). The star block S(PCL-b-PDEAEMA) copolymers were characterized by H-1 NMR spectroscopy and size exclusive chromatography (SEC). These copolymers could self-assembly into micelles in an aqueous solution at low pH with a low critical aggregation concentration (CAC) in the range of 2 to 10 mg/L, which depend on the composition of the copolymers. The original aggregates could further form larger micelles and/or vesicles by adjusting the external stimulus, which was examined by dynamic light scattering (DLS) and transmission electron microscopy (TEM). The lower critical solution temperature (LCST) of the star copolymers were determined to be 49.7 and 26.3 degrees C at pH 6.5 and 7.4, respectively, demonstrating a pH-dependent thermo-response.