Dynamic-covalent star nanostructures prepared from functional block copolymers obtained by reversible addition-fragmentation chain transfer (RAFT) polymerization

Traditionally, polymeric systems have relied on strong, irreversible, non-labile linkages for maximum strength and durability. However in the past few years, there has been growing interest in polymers containing dynamic linkages that can be cleaved and reassembled under specific environmental conditions. The possibility of inducing a macroscopic response in polymeric materials by reversible cleavage/formation of physical/chemical bonds at the molecular level has triggered significant research effort. This, combined with significant developments in the field of controlled/living polymerizations has led to utilization of both non-covalent and dynamic-covalent linkages for design of block copolymers with cleavable junctions, reversible side-chain functionalization of linear polymers, "self-healing" and shape-memory polymers, and self-assembly of polymeric nanostructures.;Macromolecular star architectures, with several polymer chains connected to a central core, are interesting due to their well-defined and compact structure, a high density of functional groups, and unique properties in solution and in the melt. Incorporation of dynamic linkages into star polymers provides a characteristic of dynamics (e.g., potential for controlled degradability) to these branched macromolecules. While there is precedent to self-assembled macromolecular stars being constructed via supramolecular interactions and dynamic-covalent bonds, the ability of the reversible linkages to afford reversible topological switching between the highly branched star architecture and the individual linear arms has not been thoroughly investigated.;The research discussed here demonstrates the "arm-first" route to star polymers by dynamic-covalent crosslinking of well-defined functional block copolymers synthesized via RAFT polymerization. Three different types of dynamic-covalent chemistries, namely boronic ester formation, Diels-Alder cycloaddition and disulfide chemistry were utilized for synthesis of the core-crosslinked stars. Thus, well-defined block copolymers with a boronic acid, furan, or an anhydride functional reactive block/segment and a non-reactive (inert) block/segment were prepared via RAFT polymerization. Core-crosslinked stars were obtained by reaction of the reactive segments of the block copolymers with multifunctional 1,2-/1,3-diol, bismaleimide, or cystamine dihydrochloride crosslinkers. Various aspects of the star-assembly, such as effect of polymer concentration and polymer:crosslinker stoichiometry on the rate and extent of star formation were investigated The reversibility of the stars induced by specific stimuli (i.e., addition of a competing mono/multifunctional diol in case of the boronic ester linkages, heating/cooling in case of the Diels-Alder linkages and reduction/oxidation in case of disulfide linkages) was further studied over multiple cycles. The ability to switch the macromolecular architecture between stars and linear chains can possibly be further extended to other complex macromolecular architectures. Furthermore, dynamic-covalent chemistry can also be used to induce "macromolecular metamorphosis" or transformation from one complex architecture to another (e.g., star to brush), possibly allowing subtle changes in macroscopic properties in solution or bulk for interesting future applications.