Controlled Fabrication of Unimolecular Micelles as Versatile Nanoplatform for Multifunctional Applications

Unimolecular micelles (UMs) are nano-sized structures that are composed of single molecules with precise composition. Compared to self-assembled polymeric micelles, UMs possess ultra-stable property even in complex biological environment. With the development of controllable polymerization and coupling chemistry, the preparation of narrowly monodispersed UMs with precise morphology and size has been realized, which further facilitates their multifunctional applications. After brief introduction, state-of-the-art advances in the synthesis and applications of UMs are discussed with an emphasis on their bioapplications. It is believed that these UMs have great potential in future fabrication of multifunctional nanoplatforms.

Fluorescent Strategy for Direct Quantification of Arm Component in Mikto-Arm Star Copolymers

Fluorescent end-functional mikto-arm star copolymers were prepared by an “arm-first” approach mediated by a mixture of macro-RAFT agents. RAFT copolymerization of coumarin-POEGMA, boron-dipyrromethene (BODIPY)-PDMA and bisindolylmaleimide (BIM)-PNIPAM with different fluorophore-labeled...

Tuning small molecule release from polymer micelles: Varying H2S release through cross linking in the micelle core

Polymer micelles, used extensively as vehicles in the delivery of active pharmaceutical ingredients, represent a versatile polymer architecture in drug delivery systems. We hypothesized that degree of crosslinking in the hydrophobic core of amphiphilic block copolymer micelles could be used to tune the rate of release of the biological signaling gas (gasotransmitter) hydrogen sulfide (H2S), a potential therapeutic. To test this hypothesis, we first synthesized amphiphilic block copolymers of the structure PEG-b-P(FBEA) (PEG = poly(ethylene glycol), FBEA = 2-(4-formylbenzoyloxy)ethyl acrylate). Using a modified arm-first approach, we then varied the crosslinking percentage in the core-forming block via addition of a 'O,O'-alkanediyl bis(hydroxylamine) crosslinking agent. We followed incorporation of the crosslinker by 1H NMR spectroscopy, monitoring the appearance of the oxime signal resulting from reaction of pendant aryl aldehydes on the block copolymer with hydroxylamines on the crosslinker, which revealed crosslinking percentages of 5, 10, and 15%. We then installed H2S-releasing S-aroylthiooxime (SATO) groups on the crosslinked polymers, yielding micelles with SATO units in their hydrophobic cores after self-assembly in water. H2S release studies in water, using cysteine (Cys) as a trigger to induce H2S release from the SATO groups in the micelle core, revealed increasing half-lives of H2S release, from 117 ± 6 min to 210 ± 30 min, with increasing crosslinking density in the micelle core. This result was consistent with our hypothesis, and we speculate that core crosslinking limits the rate of Cys diffusion into the micelle core, decreasing the release rate. This method for tuning the release of covalently linked small molecules through modulation of micelle core crosslinking density may extend beyond H2S to other drug delivery systems where precise control of release rate is needed.

Synthetic route-dependent intramolecular segregation in heteroarm core cross-linked star polymers as Janus-like nanoobjects

Polymerization-induced intramolecular segregation can be realized during the “in–out” synthesis of heteroarm core cross-linked star polymers to facilitate well-defined microphase separation.

Flow-Assisted Switchable Catalysis of Metal Ions in a Microenvelope System Embedded with Core-Shell Polymers

Many efforts have been made on stimuli-responsive switchable catalysis to trigger catalytic activity over various chemical reactions. However, the reported light-, pH- or chemically responsive organocatalysts are mostly incomplete in the aspects of shielding efficiency and long-term performance. Here, we advance the flow-assisted switchable catalysis of metal ions in a microenvelope system that allows  the on-off catalysis mode on demand for long-lasting catalytic activity. Various metal-ion catalysts can be selectively embedded in a novel polymeric core-shell of the heteroarm star copolymer of poly(styrene) and poly(4-vinylpyridine) emanated from a polyhedral oligomeric silsesquioxane center. The immobilized core-shell polymer on the inner wall of a poly(dimethylsiloxane) envelope microreactor shows on-off switching catalysis between the expanded active mode and contracted protective mode under continuous flow of solvents or subsequent dry conditions. In particular, the preserved catalytic activity of toxic Hg²⁺ for oxymercuration was demonstrated even for 2 weeks without leaching, whereas the activity of moisture-sensitive Ru³⁺ ions for polymerization of methyl methacrylate was maintained even after 5 days from an open atmosphere. It is practical that the tight environment of the enveloped microfluidic system facilitates cyclic switching between the reaction-"on" and -"off" modes of such toxic, sensitive/expensive catalysts for long-term prevention and preservation.

RAFT polymerization to form stimuli-responsive polymers

Stimuli-responsive polymers respond to a variety of external stimuli, which include optical, electrical, thermal, mechanical, redox, pH, chemical, environmental and biological signals. This paper is concerned with the process of forming such polymers by RAFT polymerization.

Synthesis of Star Polymers by RAFT Polymerization as Versatile Nanoparticles for Biomedical Applications

The precise control of polymer chain architecture has been made possible by developments in polymer synthesis and conjugation chemistry. In particular, the synthesis of polymers in which at least three linear polymeric chains (or arms) are tethered to a central core has yielded a useful category of branched architecture, so-called star polymers. Fabrication of star polymers has traditionally been achieved using either a core-first technique or an arm-first approach. Recently, the ability to couple polymeric chain precursors onto a functionalized core via highly efficient coupling chemistry has provided a powerful new methodology for star synthesis. Star syntheses can be implemented using any of the living polymerization techniques using ionic or living radical intermediates. Consequently, there are innumerable routes to fabricate star polymers with varying chemical composition and arm numbers. In comparison with their linear counterparts, star polymers have unique characteristics such as low viscosity in solution, prolonged blood circulation, and high accumulation in tumour regions. These advantages mean that, far beyond their traditional application as rheology control agents, star polymers may also be useful in the medical and pharmaceutical sciences. In this account, we discuss recent advances made in our laboratory focused on star polymer research ranging from improvements in synthesis through to novel applications of the product materials. Specifically, we examine the core-first and arm-first preparation of stars using reversible addition–fragmentation chain transfer (RAFT) polymerization. Further, we also discuss several biomedical applications of the resulting star polymers, particularly those made by the arm-first protocol. Emphasis is given to applications in the emerging area of nanomedicine, in particular to the use of star polymers for controlled delivery of chemotherapeutic agents, protein inhibitors, signalling molecules, and siRNA. Finally, we examine possible future developments for the technology and suggest the further work required to enable clinical applications of these interesting materials.

Heteroarm core cross-linked star polymers via RAFT copolymerization of styrene and bismaleimide

Core cross-linked star polymer containing polystyrene and polylactide arms can be prepared by alternating RAFT copolymerization and self-assembles into superstructures.

Oximes as reversible links in polymer chemistry: dynamic macromolecular stars

We demonstrate the formation of oxime-functional macromolecular stars that are able to dissociate and reconstruct themselves upon application of a stimulus.

Constructing star polymersvia modular ligation strategies

Over recent years, modular ligation reactions—some of which adhere to the click criteria—have enabled the synthesis of a variety of star polymers via efficient polymer–polymer conjugations. The copper catalyzed azide–alkyne cycloaddition (CuAAC), Diels–Alder (DA), and Hetero Diels–Alder (HDA) reactions are reviewed here in detail for the facile generation of various macromolecular star topologies.