Dually Responsive Shape-Changing Linear and Star Molecular Bottlebrushes with Bicomponent Side Chains

Molecular bottlebrushes (MBBs) can exhibit large conformational changes from wormlike to globular in solution in response to environmental stimuli. However, the instability of the collapsed state has prevented shape-changing MBBs from potential applications in, e.g., biomimetic catalysis and substance delivery. This dissertation work focused on dually responsive linear and star MBBs composed of bicomponent side chains in the form of either homografted diblock copolymer or binary heterografted polymeric side chains. When one polymer component collapsed, driving the shape changing of MBBs, another component served as a stabilizer. When both components in the side chains were stimuli-responsive, an additional level of control over the solution state of the brushes was achieved. The MBBs in this dissertation work were synthesized by a “grafting-to” method using the highly efficient Cu(I)-catalyzed azide-alkyne cycloaddition reaction.

Firstly, linear MBBs with diblock copolymer side chains composed of pH-responsive poly(2-(N,N-diethylamino)ethyl methacrylate) (PDEAEMA) as the inner block and thermoresponsive poly(methoxytri(ethylene glycol) acrylate) as the outer block was synthesized. These brushes underwent a unimolecular worm-to-globule shape transition in aqueous solution upon increasing the pH, where the PTEGMA block functioned as a stabilizer. The solution state of the brushes was then shown to be further controlled by the thermoresponsive property of PTEGMA. Subsequently, binary heterografted three-arm star MBBs consisting of two different homopolymers, pH-responsive PDEAEMA and thermoresponsive poly(ethoxydi(ethylene glycol) acrylate) (PDEGEA), as side chains were prepared and the formation of two distinct globular states with different polymers in the core from one brush system was demonstrated. A peculiar pearl-necklace brush morphology was observed from an aqueous phosphate buffer, which was further investigated using a linear version of MBBs with the same side chains. It was found that the bead size of the pearl-necklace morphology was strongly affected by buffer anions. Lastly, binary heterografted star MBBs composed of poly(ethylene oxide) and either poly(2-(N,N-dimethylamino)ethyl methacrylate) or PDEAEMA side chains were prepared and the pH-induced conformational changes were revealed. Both star brush systems were found to undergo star-to-globule shape transitions upon the addition of chaotropic anions. The results obtained from this dissertation work will open up opportunities for potential applications of stimuli-responsive shape-changing MBBs.