Stimuli-Induced Star-Globule Shape Transitions of Dually Responsive Binary Heterografted Three-Arm Star Molecular Brushes in Aqueous Solution

This article reports on the synthesis of doubly responsive binary heterografted, three-arm star molecular brushes and the study of their stimuli-induced star-to-globule shape transitions in aqueous solution. The star brushes were prepared using the highly efficient copper­(I)-catalyzed azide–alkyne cycloaddition reaction to simultaneously graft an alkyne end-functionalized thermoresponsive polymer of ethoxydi­(ethylene glycol) acrylate (PDEGEA), with a lower critical solution temperature (LCST) of 9 °C, and a pH-responsive polymer of 2-(N,N-diethylamino)­ethyl methacrylate (PDEAEMA), with a pK_a of 7.4 at room temperature, onto an azide-bearing three-arm star backbone polymer. The star architecture of the formed brushes was confirmed by atomic force microscopy. The use of two different stimuli-responsive polymers as side chains allowed for the stabilization of collapsed brushes against aggregation when one side chain polymer became insoluble in water and for the formation of two distinct globular states with different polymers in the core. At temperatures below the LCST of PDEGEA, upon increasing the pH from 5.00 to 9.40, the brushes underwent a shape transition from an extended starlike to a collapsed globular state stabilized by the hydrated PDEGEA side chains. On the other hand, at pH values slightly lower than the pK_a of PDEAEMA, upon heating across the LCST of PDEGEA, the brushes also exhibited a star-to-globule shape transition, forming a globular state distinct from the one induced by pH changes.