Shape-Changing Bottlebrush Polymers

Abstract

Bottlebrush polymers (BBPs), composed of relatively short polymeric side chains densely grafted on a polymer backbone, exhibit many unique characteristics and hold promise for a variety of applications. This Perspective focuses on environmentally induced shape-changing behavior of BBPs at interface and in solution, particularly worm/star−globule shape transitions. While BBPs with a single type of homopolymer or random copolymer side chains have been shown to undergo pronounced worm-to-globule shape changes in response to external stimuli, the collapsed brushes are unstable and prone to aggregation. By introducing a second, solvophilic polymer into the side chains, either as a distinct type of side chain or as the outer block of block copolymer side chains, the collapsed brushes not only are stabilized but also create unimolecular micellar nanostructures, which can be used for, e.g., encapsulation and delivery of substances. The current challenges in the design, synthesis, and characterization of stimuli-responsive shape-changing BBPs are discussed.

Molecular polymer bottlebrushes in nanomedicine: therapeutic and diagnostic applications

“…In this context, MPBs can act as building blocks for supramolecular or colloidal self-assembly to provide shape-shifting nanomaterials. 46,117,196,197 Supramolecular features are underrepresented in brush design, [198][199][200] but may incorporate valuable repair or timescale properties. MPBs comprising a supramolecular backbone may exhibit comparable in vivo performance but benefit from the ability disassemble over time, thus combating long-term exposure of nanomaterials to organs.…”

“…MPBs can be designed to respond to biologically relevant triggers, whereby the MPBs or conjugates/assemblies thereof react to pH, temperature, ionic environment, or light. 45,46 These attributes are especially powerful in the areas of nanomedicine, where the intention is to control and predict performance of therapeutics through nanoparticle design, in an endeavour to develop safer and more efficient patient treatments.…”

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

“…Molecular bottlebrushes (MBBs), also termed bottlebrush polymers, are a special type of graft copolymers in which relatively short polymer chains are covalently grafted on a long backbone polymer with a sufficiently high grafting density that the backbone is forced to take on stretched conformations. − These architecturally complex polymers have attracted considerable attention in the past years, and their potential applications have been demonstrated in a wide variety of areas, including drug delivery, , photonic crystals, − supersoft elastomers, biological tissue-inspired advanced organic materials, and surface coating for lubrication . Due to the dense grafting of macromolecular side chains, MBBs exhibit many intriguing characteristics and behavior, including high persistence length, no backbone entanglement, large conformational changes (i.e., shape transitions), and unusual crystal habit. − , Among these, stimuli-triggered shape transitions of MBBs are particularly interesting. ,, While bottlebrush polymers adopt a cylindrical or wormlike shape under conditions favorable for side chains (e.g., in good solvents), they can exhibit pronounced and abrupt shape changes and take on different conformations (e.g., spherical) when the interactions of side chains with the environment become unfavorable. Shape-changing MBBs may find applications in areas such as delivery of substances, regulation of molecular interactions, and templated synthesis of nanomaterials with controlled morphologies. ,, …”

“…Due to the dense grafting of macromolecular side chains, MBBs exhibit many intriguing characteristics and behavior, including high persistence length, no backbone entanglement, large conformational changes (i.e., shape transitions), and unusual crystal habit. − , Among these, stimuli-triggered shape transitions of MBBs are particularly interesting. ,, While bottlebrush polymers adopt a cylindrical or wormlike shape under conditions favorable for side chains (e.g., in good solvents), they can exhibit pronounced and abrupt shape changes and take on different conformations (e.g., spherical) when the interactions of side chains with the environment become unfavorable. Shape-changing MBBs may find applications in areas such as delivery of substances, regulation of molecular interactions, and templated synthesis of nanomaterials with controlled morphologies. ,, …”

“…Stimuli-induced shape transitions of MBBs from wormlike to globular or from starlike to disklike were demonstrated at interfaces. − Sheiko and co-workers showed that linear and star MBBs with homografted poly­( n -butyl acrylate) (P n BA) side chains underwent reversible rod-to-sphere and star-to-disk shape transitions, respectively, at the air–water interface by lateral compression and decompression. , MBBs with stimuli-responsive polymers as the side chains can undergo shape transitions in solution in response to environmental stimuli, which are more relevant to potential applications. A variety of stimuli-responsive polymers, including thermosensitive water-soluble polymers, pH- and light-responsive polymers, as well as polyelectrolytes, have been employed to construct stimuli-responsive MBBs. ,,− Li et al reported that thermosensitive MBBs with poly­( N -isopropylacrylamide) (PNIPAm) side chains collapsed from a wormlike to a globule shape upon heating from below to above the lower critical solution temperature (LCST) of PNIPAm . The collapsed globular PNIPAm MBBs, however, were unstable at high temperatures; the brush molecules were found to aggregate and eventually precipitate out from water.…”