Stepwise Conformational Transitions of Stimuli-Responsive Linear Ternary Heterografted Bottlebrush Polymers in Aqueous Solution

Stepwise conformational transitions of molecular bottlebrushes (MBBs) from extended wormlike to partially collapsed and stable globular in aqueous solution were realized with the use of two distinct stimuli-responsive polymers and poly(ethylene oxide) (PEO) as the side chains, where PEO served as a stabilizer when the responsive side chains became insoluble. Three dually responsive brush polymers, composed of either two distinct pH-responsive or two different thermoresponsive or one pH-responsive and one thermoresponsive polymers along with PEO randomly grafted on the backbone polymer, were synthesized by a click grafting-to method. Dynamic light scattering studies showed that all three brush polymers displayed two successive steplike size transitions in aqueous solutions in response to application of external stimuli, with overall size decreases of 27-29%. H-1 NMR spectroscopy analysis confirmed that the size transitions of MBBs originated from the pH-induced soluble-to-insoluble and/or the temperature-triggered lower critical solution temperature transitions of responsive side-chain polymers in the brushes. Atomic force microscopy revealed stepwise conformational changes from extended wormlike (with and without a pearl-necklace morphology) to partially collapsed (shrunken wormlike or wavelike or C-/S-shaped) and globular nano-objects stabilized by PEO side chains. As a demonstration of potential applications, Nile Red was encapsulated in the dually pH-responsive globular MBBs as a hydrophobic model drug, and its stepwise release was achieved upon gradually decreasing the pH.