Complementary Nucleobase Interactions Drive the Hierarchical Self-Assembly of Core–Shell Bottlebrush Block Copolymers toward Cylindrical Supramolecules
Spyridon Varlas, Zan Hua, Joseph R. Jones, Marjolaine Thomas, Jeffrey C. Foster, Rachel K. O’Reilly
Abstract
The self-assembly of amphiphilic block copolymers has facilitated the preparation of a wide variety of nano-objects of diverse morphology. Ready access to these nanostructures has opened up new possibilities in catalysis, sensing, and nanomedicine. In comparison, the self-assembly of large building blocks (i.e., amphiphilic bottlebrush polymers) has received less attention, owing in part to the relatively more challenging synthesis of these macromolecules. Bottlebrush amphiphiles can self-assemble into uniquely stable spherical nanostructures and can also produce dynamic cylinders with lengths modulated by environmental conditions, motivating further research in this area. Herein, we report the synthesis of core–shell bottlebrush polymers (BBPs) containing complementary nucleobase functionalities via a combination of ring-opening metathesis polymerization (ROMP) and reversible addition–fragmentation chain transfer (RAFT) polymerization, using a “grafting-from” approach, and their hierarchical self-assembly in aqueous media. Mixtures of BBPs containing thymine or adenine units in their core blocks were found to self-assemble into higher-order cylindrical supramolecules upon heating above a critical temperature. This temperature was demonstrated to correspond to the lower critical solution temperature (LCST) of the corona-forming poly(4-acryloylmorpholine) block, providing evidence for a unique one-dimensional BBP assembly mechanism. Moreover, the formation of extended supramolecular assemblies was preferentially observed when both thymine- and adenine-functionalized BBPs were present in equimolar concentrations, pointing toward an alternating, isodesmic mechanism of organization occurring via nucleobase interactions located at their chain termini. We anticipate that these discoveries will provide the basis for future studies regarding BBP self-assembly, especially with regard to the formation of stimuli-responsive anisotropic nanostructures.