Nanobowls from Amphiphilic Core–Shell Cyclic Bottlebrush Polymers

Abstract

Cyclic bottlebrush polymers are macrocycles that contain pendent side-chain polymers on nearly every repeat unit. We leveraged a method to generate cyclic bottlebrush polymers utilizing ring-expansion polymerization (REP) followed by subsequent grafting-from via atom-transfer radical polymerization (ATRP) to produce core−shell cyclic bottlebrush polymers and investigated their self-assembly behavior in water. The bottlebrush polymers were comprised of a cyclic backbone with side chains composed of a hydrophobic polystyrene block and a hydrophilic poly(acrylic acid) block. A two-step morphological transformation from spheres to porous spheres to nanobowls (spheres with a single large pore) was observed with solvent exchange from tetrahydrofuran to water. In contrast, under identical experimental conditions, an analogous linear bottlebrush polymer with a similar backbone and side chain degrees of polymerizations did not aggregate into nanobowls but rather exhibited a porous sphere morphology. These unusual morphologies and their propensities to transform can be attributed to changes in the internal viscosity of the aggregates during the solvent exchange process. Additionally, the rate of solvent exchange was found to influence the propensity for shape transformation to occur.

Polymeric Bowl‐Shaped Nanoparticles: Hollow Structures with a Large Opening on the Surface

“…Although BNPs are not commonly observed in polymer selfassembly compared with micelles or vesicles, amphiphilic polymers with different topological structures including random copolymers, [27,92,93] star polymers, [29,69,94] graft copolymers, [95][96][97] and hyperbranched copolymers [40,98] can also self-assemble into BNPs at specific circumstances. For instance, Eisenberg et al [27] prepared BNPs by the self-assembly of a random copolymer of poly(styrene-co-methacrylic acid) [P(S-co-MAA)].…”

Functionalized Cyclic Polymers and Network Gels

“…A cyclic polymer with the same chemical composition and similar molecular weight as its linear counterpart exhibits different solution, , bulk, , and surface properties. , Cyclic polymers are emerging as exciting materials in medical applications such as drug delivery − due to their lower intrinsic viscosities, ,− ability to prevent nanoparticle aggregation and elevated micelle cloud point temperatures, and prolonged blood circulation. , …”

Ring Expansion Alkyne Metathesis Polymerization

“…Other thermodynamic differences include higher entanglement molecular weights and increased crystallization rates and nucleation density . Increased conjugation lengths for cyclic polymers also lead to clear differences in photophysical properties including enhanced fluorescence, excited-state lifetimes, and a higher refractive index. , Showing promise in medical applications for drug delivery, − in solution, cyclic polymers exhibit lower intrinsic viscosities, , prevent nanoparticle aggregation, and have elevated micelle cloud point temperatures, resulting in prolonged blood circulation. , …”