Nanotoroids Self-Assembled from Bottlebrush Copolymers

Toroids are cyclic, ring-shaped nanostructures with potential applications in topological materials, encapsulation, and separation. While nanotoroids naturally exist in biological systems (e.g., DNA toroids), their high-quality synthesis has been a long-standing challenge. Here, we report on the design of a bottlebrush copolymer that rapidly forms uniform nanotoroids (e.g., several minutes) in high yield and high fidelity through a robust solution-based self-assembly approach. The brushes are core-shell block copolypeptoids, which possess precisely tailored sequences and amphiphilicity and are structurally prearranged to adopt a flexible packing geometry and have high end-cap energy. These characteristics have promoted the formation of toroids through an intramicellar end-to-end coalescence mechanism. Our design is validated by various control experiments showing contrasting outcomes, where linear polypeptoids form nanosheets or cylinders and polymers with larger hydrophilic domains form worm-like or spherical micelles. Computer simulations replicate the assembly of toroidal structures and support the formation mechanism. Our strategy could be extended for the rational design of other complex nanostructures and soft materials.