Boronic Acid-Terminated Polymers: Synthesis by RAFT and Subsequent Supramolecular and Dynamic Covalent Self-Assembly

Well-defined homopolymers and block copolymers with boronic acid-functional end groups were prepared by reversible addition - fragmentation chain transfer (RAFT) polymerization and subsequently investigated for the potential to undergo model supramolecular and dynamic covalent self-assembly in aqueous and organic media. By employing a boronic acid-functionalized RAFT chain transfer agent, end-functional polymers were prepared without recourse to postpolymerization functionalization. The polymerizations of styrene, N,N-dimethylacrylamide, and N-isopropylacrylamide all led to homopolymers of controlled molecular weight that were capable of subsequent chain extension to form well-defined block copolymers. The amphiphilic and stimuli-responsive block copolymers were capable of self-assembly into micellar aggregates. Quantitative retention of the boronic acid chain termini during polymerization was spectroscopically demonstrated, and two types of dynamic covalent transformations involving these moieties were investigated. The formation of boronic esters with a model 1,2-diol proved to be a highly efficient process in nonaqueous media. Additionally, Lewis base-promoted trimerization of the boronic acid end groups led to three-arm star polymers with boroxine junction points. Both of these covalent interactions were readily reversible upon the introduction of water, suggesting the potential application of these polymers to construct dynamic covalent macromolecular assemblies.