Eunhyung Nam

Tertiary amine catalyzed photo-induced controlled radical polymerization of methacrylates

A novel tertiary amine catalyst (TAC) and trithiocarbonate (TTC) synergistic photo-induced controlled radical polymerization of methacrylates in the absence of conventional photo-initiators, metal-catalysts, or dye sensitizers, has been realized under mild UV irradiation (λmax ≈ 365 nm), yielding polymethacrylates with low molecular weight distributions and excellent end-group fidelity.

A novel solid state photocatalyst for living radical polymerization under UV irradiation

This study presents the development of a novel solid state photocatalyst for the photoinduced controlled radical polymerization of methacrylates under mild UV irradiation (λmax ≈ 365 nm) in the absence of conventional photoinitiators, metal-catalysts or dye sensitizers. The photocatalyst design was based on our previous finding that organic amines can act in a synergistic photochemical reaction with thiocarbonylthio compounds to afford well controlled polymethacrylates under UV irradiation. Therefore, in the current contribution an amine-rich polymer was covalently grafted onto a solid substrate, thus creating a heterogeneous catalyst that would allow for facile removal, recovery and recyclability when employed for such photopolymerization reactions. Importantly, the polymethacrylates synthesized using the solid state photocatalyst (ssPC) show similarly excellent chemical and structural integrity as those catalysed by free amines. Moreover, the ssPC could be readily recovered and re-used, with multiple cycles of polymerization showing minimal effect on the integrity of the catalyst. Finally, the ssPC was employed in various photo-“click” reactions, permitting high yielding conjugations under photochemical control.

Photocontrolled Cargo Release from Dual Cross-Linked Polymer Particles.

Burst release of a payload from polymeric particles upon photoirradiation was engineered by altering the cross-linking density. This was achieved via a dual cross-linking concept whereby noncovalent cross-linking was provided by cyclodextrin host-guest interactions, and irreversible covalent cross-linking was mediated by continuous assembly of polymers (CAP). The dual cross-linked particles (DCPs) were efficiently infiltrated (∼80-93%) by the biomacromolecule dextran (molecular weight up to 500 kDa) to provide high loadings (70-75%). Upon short exposure (5 s) to UV light, the noncovalent cross-links were disrupted resulting in increased permeability and burst release of the cargo (50 mol % within 1 s) as visualized by time-lapse fluorescence microscopy. As sunlight contains UV light at low intensities, the particles can potentially be incorporated into systems used in agriculture, environmental control, and food packaging, whereby sunlight could control the release of nutrients and antimicrobial agents.

Fabrication of ultra-thin polyrotaxane-based films via solid-state continuous assembly of polymers

Surface-confined ultra-thin polyrotaxane (PRX)-based films with tunable composition, surface topology and swelling characteristics were prepared by solid-state continuous assembly of polymers (ssCAP). The PRX-based films supported cell attachment, and their degradation in biological media could be tuned. This study provides a versatile nano-coating technology with potential applications in biomedicine, including tissue engineering and medical devices.

Continuous assembly of polymers via solid phase reactions

The continuous assembly of polymers in the solid state via ring-opening metathesis polymerization (ssCAPROMP) is reported as an effective method for the fabrication of smooth, surface confined, cross-linked nanostructured films. Macrocross-linkers, polymers pre-functionalized with polymerizable pendent groups, were first deposited onto initiator-modified substrates via spin-coating, followed by film cross-linking via ssCAPROMP. The film thickness is tunable by adjusting the reaction time, and multilayered films can be achieved through reinitiation steps, generating complex and unique film architectures with nanometer precision. The technique developed herein allows for controlled and directional growth of cross-linked thin films from the substrate surface in the solid state.

Spatial-controlled nanoengineered films prepared via rapid catalyst induced cross-linking

A new top-down approach to generate stable nanoscale films via catalyst induced cross-linking (CIC) is demonstrated. Polymers with various compositions and bearing pendent norbornene groups (defined as macrocross-linkers) are initially spin-coated onto substrates to form nanometre-thick films; when the films are brought into contact with a catalyst solution, ring-opening metathesis polymerization (ROMP)-mediated cross-linking efficiently occurs to lock the film into place. CIC provides a new paradigm for the fabrication of stable nanoscale films and provides an alternative to traditional methods that use external stimuli (e.g., heat or light) to trigger film cross-linking. The process requires short cross-linking times (<3 min) to generate covalently bonded and stable nanoscale films. This facile nanoengineering approach allows for the creation of complex multi-layered and multi-compositional patterned films, enables excellent control over film properties such as thickness and swellability, and provides access to nanoscale free-standing polymer sheets. To highlight the versatility of the CIC approach, cross-linked, nanostructured and stratified multi-layered films with tunable film thickness were prepared from norbornene functionalised poly(oligo(ethylene glycol) methacrylate), poly(ethylene glycol) and poly(3-hexylthiophene) macrocross-linkers. CIC proceeds at low catalyst concentrations and allows the catalyst solution to be recycled multiple times, as demonstrated through repetition of 10 individual CIC cycles, making the process economical, scalable and applicable to advanced manufacturing techniques. Furthermore, the technique can be used to produce patterned films through selective exposure of specific regions of the polymer film to the catalyst solution. The CIC approach mediated by ROMP is highly efficient, rapid, robust and versatile, providing new opportunities in film assembly, and complementing existing nanoscale film fabrication methodologies.