UV Irradiation-Induced Shell Cross-Linked Micelles with pH-Responsive Cores Using ABC Triblock Copolymers

A triblock copolymer, poly(ethylene glycol)- b -poly(glycerol monomethacrylate)- b -poly(2-(diethylamino)ethyl methacrylate) (PEG−PGMA−PDEA), was synthesized via atom transfer radical polymerization (ATRP) by successive polymerization of glycerol monomethacrylate (GMA) and 2-(diethylamino)ethyl methacrylate (DEA) using a PEG-based ATRP macroinitiator. Reacting the obtained triblock copolymer with varying amounts of cinnamoyl chloride in anhydrous pyridine yielded PEG−(PCGMA- co -PGMA)−PDEA triblock copolymer with photo-cross-linkable moieties, where PCGMA is poly(3-cinnamoyl glycerol monomethacrylate) and the mean degree of cinnamoylation ranges from 5 to 50 mol % relative to the PGMA block. All PEG−(PCGMA- co -PGMA)−PDEA triblock copolymers molecularly dissolve in aqueous media at acidic pH; upon addition of NaOH, micellization occurred above pH 7−8 to form three-layer “onionlike” micelles comprising PDEA cores, PCGMA- co -PGMA inner shells, and PEG outer coronas. The pH-induced micellization kinetics of PEG 113 −(CGMA 0.5 - co -GMA 0.5 ) 50 −DEA 65 triblock copolymers was investigated by stopped-flow light scattering upon a pH jump from 3 to 10, and compared to that of PEG 113 −PGMA 50 −PDEA 65 . Facile cross-linking of the PCGMA- co -PGMA inner shell was then conducted via UV irradiation. The PDEA cores of the resulting shell cross-linked (SCL) micelles exhibited reversible pH-responsive behavior. The extent of pH-induced swelling/shrinking and the colloidal stability of SCL micelles were mainly determined by the extent of cross-linking. The dissociation kinetics of the triblock copolymer micelles before and after shell cross-linking was also investigated employing the stopped-flow technique. It was found that SCL micelles prepared at higher degrees (>20 mol %) of cross-linking exhibited excellent colloidal stability to external pH changes.