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.