Synthesis and Behavior of Stimuli-Responsive Shape-Changing Linear Molecular Bottlebrushes

Molecular bottlebrushes are composed of polymer side chains densely grafted to a macromolecular backbone. The main focus of this dissertation work is the synthesis and behavior of linear molecular brushes that can undergo unimolecular size and shape changes in response to environmental stimuli in dilute aqueous solution. Chapter 1 provides an introduction to molecular bottlebrushes and stimuli-responsive polymers.

Chapter 2 presents the development of a robust method for the synthesis of well-defined molecular brushes by a “grafting to” approach using copper-catalyzed azide-alkyne cycloaddition “click” reactions. Well-defined, new azide-containing backbone polymers with different lengths and high degrees of azide functionality were synthesized. Using alkyne end-functionalized poly(ethylene oxide) (PEO) as model side chain polymer, homografted molecular brushes with nearly quantitative grafting densities were prepared when a molar ratio of approximately 1 : 2 for backbone monomer units to side chains was adopted, and the grafting density can be readily tuned by adjusting this ratio.

Chapters 3 and 4 extend the methodology to stimuli-responsive molecular bottlebrushes. Homografted molecular brushes were prepared with side chains composed of either thermosensitive poly(di(ethylene glycol) ethyl ether acrylate) (PDEGEA), thermo- and light-responsive poly((di(ethylene glycol) methyl ether acrylate)-co-(o-nitrobenzyl acrylate)) (P(DEGMA-co-NBA)), or pH-responsive poly(N,N-diethylaminoethyl methacrylate) (PDEAEMA). High grafting density brushes were achieved, and their dramatic size changes were observed in response to applied stimuli in aqueous solution. Chapter 4 details the preparation of binary heterografted molecular brushes containing PEO and stimuli-responsive PDEGEA, PDEAEMA, or PNBA side chains randomly distributed along the backbone. Compared with homografted brushes, the collapsed state was stabilized by PEO side chains. These brushes underwent size and shape transitions from an extended worm-like state to a collapsed, roughly spherical state, and we demonstrated that this shape-changing can be exploited to control binding between biotin-containing molecular brushes and avidin.

Chapter 5 focuses on stimuli-responsive hydrogels based on tertiary amine-containing ABA triblock copolymers containing a PEO central block and thermo- and pH-sensitive outer blocks. The effect of different tertiary amines on thermally induced sol-gel transitions of moderately concentrated aqueous solutions was investigated. Chapter 6 provides a summary this dissertation work and future prospects.