"Hierarchical comb brush architectures via sequential light-mediated controlled radical polymerizations"
Authors: Narupai, B., Poelma, J. E., Pester, C. W., McGrath, A. J., Toumayan, E. P., Luo, Y., Kramer, J. W., Clark, P. G., Ray, P. C. and Hawker, C. J.
A novel strategy for the synthesis and characterization of branched polymer brushes by sequential light-mediated controlled radical polymerizations is described. Initially, linear brushes are prepared by surface-initiated copolymerization of methyl methacrylate and 2-hydroxyethyl methacrylate (HEMA). In a subsequent step, the HEMA side chains are functionalized with initiating groups for secondary graft polymerization, leading to hierarchical, branched architectures. The increased steric bulk due to the polymer side chains results in a dramatic increase in film thickness when compared to the starting linear brushes. This strategy also allows chemical gradient and complex three-dimensional structures to be obtained by employing grayscale photomasks in combination with controlled radical polymerization.
"Improved self-assembly of poly(dimethylsiloxane-b-ethylene oxide) using a hydrogen-bonding additive."
Authors: Luo, Y., Kim, B., Montarnal, D., Mester, Z., Pester, C. W., McGrath, A. J., Hill, G., Kramer, E. J., Fredrickson, G. H. and Hawker, C. J.
Block copolymers with increased Flory–Huggins interaction parameters (χ) play an essential role in the production of sub-10 nm nanopatterns in the growing field of directed self-assembly for next generation lithographic applications. A library of PDMS-b-PEO block copolymers were synthesized by click chemistry and their interaction parameters (χ) determined. The highest χmeasured in our samples was 0.21 at 150 °C, which resulted in phase-separated domains with periods as small as 7.9 nm, suggesting that PDMS-b-PEO is a prime candidate for sub-10 nm nanopatterning. To suppress PEO crystallization, PDMS-b-PEO was blended with (l)-tartaric acid (LTA) which allows for tuning of the self-assembled morphologies. Additionally, it was observed that the order-disorder transition temperature (TODT) of PDMS-b-PEO increased dramatically as the amount of LTA in the blend increased, allowing for further control over self-assembly. To understand the mechanism of this phenomenon, we present a novel field-based supramolecular model, which describes the formation of copolymer-additive complexes by reversible hydrogen bonding. The mean-field phase separation behavior of the model was calculated using the random phase approximation (RPA). The RPA analysis reproduces behavior consistent with an increase of the effective χ in the PDMS-b-(PEO/LTA suprablock).
Authors: Hartmeier, B. F., Brady, M. A., Treat, N. D., Robb, M. J., Mates, T. E., Hexemer, A., Wang, C., Hawker, C. J., Kramer, E. J. and Chabinyc, M. L.
Ternary organic blends have potential in realizing efficient bulk heterojunction (BHJ) organic solar cells by harvesting a larger portion of the solar spectrum than binary blends. Several challenging requirements, based on the electronic structure of the components of the ternary blend and their nanoscale morphology, need to be met in order to achieve high power conversion efficiency in ternary BHJs. The properties of a model ternary system comprising two donor polymers, poly(3-hexylthiophene) (P3HT) and a furan-containing, diketopyrrolopyrrole-thiophene low-bandgap polymer (PDPP2FT), with a fullerene acceptor, PC61BM, were examined. The relative miscibility of PC61BM with P3HT and PDPP2FT was examined using diffusion with dynamic secondary ion mass spectrometry (dynamic SIMS) measurements. Grazing incidence small and wide angle X-ray scattering analysis (GISAXS and GIWAXS) were used to study the morphology of the ternary blends. These measurements, along with optoelectronic characterization of ternary blend solar cells, indicate that the miscibility of the fullerene acceptor and donor polymers is a critical factor in the performance in a ternary cell. A guideline that the miscibility of the fullerene in the two polymers should be matched is proposed and further substantiated by examination of known well-performing ternary blends. The ternary blending of semiconducting components can improve the power conversion efficiency of bulk heterojunction organic photovoltaics. The blending of P3HT and PDPP2FT with PC61BM leads to good absorptive coverage of the incident solar spectrum and cascading transport energy levels. The performance of this ternary blend reveals the impact of the miscibility of PC61BM in each polymer as a function of composition, highlighting an important factor for optimization of ternary BHJs.
"PET/CT Imaging of Chemokine Receptors in Inflammatory Atherosclerosis Using Targeted Nanoparticles"
Authors: Luehmann, H. P., Detering, L., Fors, B. P., Pressly, E. D., Woodard, P. K., Randolph, G. J., Gropler, R. J., Hawker, C. J., Liu, Y.
Atherosclerosis is inherently an inflammatory process that is strongly affected by the chemokine/chemokine receptors axes regulating the trafficking of inflammatory cells at all stages of the disease. Of the chemokine receptor family, some specifically up-regulated on macrophages play a critical role in plaque development and may have the potential to track plaque progression. However, the diagnostic potential of these chemokine receptors has not been fully realized. Based on our previous work using a broad spectrum peptide antagonist imaging 8 chemokine receptors together, the purpose of this study was to develop a targeted nanoparticle for sensitive and specific detection of these chemokine receptors in both a mouse vascular injury model and a spontaneously developed mouse atherosclerosis model. Methods: The viral macrophage inflammatory protein-II (vMIP-II) was conjugated to a biocompatible poly(methyl methacrylate)-core/polyethylene glycol-shell amphiphilic comb-like nanoparticle through controlled conjugation and polymerization before radiolabeling with 64Cu for PET imaging in an ApoE-/- mouse vascular injury model and a spontaneous ApoE-/- mouse atherosclerosis model. Histology, immunohistochemistry, and real-time reverse transcription polymerase chain reaction (RT-PCR) were performed to assess the plaque progression and up-regulation of chemokine receptors. Results: The chemokine receptors targeted 64Cu-vMIP-II-Comb showed extended blood retention and improved biodistribution. PET imaging showed specific tracer accumulation at plaques in ApoE-/- mice, confirmed by competitive receptor blocking studies and assessment in wild-type mice. Histopathological characterization showed the progression of plaque including size and macrophage population, corresponding to the elevated concentration of chemokine receptors and more importantly increased PET signals. Conclusion:This work provides a useful nanoplatform for sensitive and specific detection of chemokine receptors to assess plaque progression in mouse atherosclerosis models.
"Particles with Tunable Porosity and Morphology by Controlling Interfacial Instability in Block Copolymer Emulsions"
Authors: Ku, K.H., Shin, J. M., Klinger, D., Jang, S. G., Hayward, R. C., Hawker, C. J., Kim, B. J.
A series of porous block copolymer (BCP) particles with controllable morphology and pore sizes was fabricated by tuning the interfacial behavior of BCP droplets in oil-in-water emulsions. A synergistic adsorption of polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) BCPs and sodium dodecyl sulfate (SDS) to the surface of the emulsion droplet induced a dramatic decrease in the interfacial tension and generated interfacial instability at the particle surface. In particular, the SDS concentration and the P4VP volume fraction of PS-b-P4VP were key parameters in determining the degree of interfacial instability, leading to different types of particles including micelles, capsules, closed-porosity particles, and open-porosity particles with tunable pore sizes ranging from 10 to 500 nm. The particles with open-porosity could be used as pH-responsive, high capacity delivery systems where the uptake and release of multiple dyes could be achieved.
Authors: Lawrence, J., Lee, S. H., Abdilla, A., Nothling, M. D., Ren, J. M., Knight, A. S., Fleischmann, C., Li, Y., Abrams, A. S., Schmidt, B. V. K. J., Hawker, M. C., Connal, L. A., McGrath, A. J., Clark, P. G., Gutekunst, W. R., Hawker, C. J.
A versatile strategy is reported for the multigram synthesis of discrete oligomers from commercially available monomer families, e.g., acrylates, styrenics, and siloxanes. Central to this strategy is the identification of reproducible procedures for the separation of oligomer mixtures using automated flash chromatography systems with the effectiveness of this approach demonstrated through the multigram preparation of discrete oligomer libraries (Đ = 1.0). Synthetic availability, coupled with accurate structural control, allows these functional building blocks to be harnessed for both fundamental studies as well as targeted technological applications.
Authors: Areephong, J.; Mattson, K. M. ; Treat, N. J. ; Poelma, S. O.; Kramer, J. W.; Sprafke, H. A.; Latimer, A. A.; Read de Alaniz, J.; Hawker, C. J.
Triazine-based unimolecular initiators are shown to mediate the controlled radical polymerization of several monomer classes, yielding polymers with low dispersities, targeted molecular weights, and active chain ends. We report the modular synthesis of structurally and electronically diverse triazine-based unimolecular initiators and demonstrate their ability to efficiently control the radical polymerization of modified styrene monomers. Copolymerizations of styrene with butyl acrylate or methyl methacrylate were conducted to highlight the monomer family tolerance of this system. Notably, in the case of methyl methacrylate and styrene, up to 90 mol% methyl methacrylate comonomer loadings could be achieved while maintaining a controlled polymerization, allowing the synthesis of a range of block copolymers. This class of triazine-based mediators has the potential to complement current methods of controlled radical polymerization and marks an important milestone in ongoing efforts to develop initiators and mediators with high monomer tolerance that are both metal and sulfur-free.
"Chemoselective Radical Dehalogenation and C–C Bond Formation on Aryl Halide Substrates Using Organic Photoredox Catalysts"
Authors: S. O. Poelma, G. L. Burnett, E. H. Discekici, K. M. Mattson, N. J. Treat, Y. Luo, Z. M. Hudson, S. L. Shankel, P. G. Clark, J. W. Kramer, C. J. Hawker, J. Read de Alaniz
Despite the number of methods available for dehalogenation and carbon–carbon bond formation using aryl halides, strategies that provide chemoselectivity for systems bearing multiple carbon–halogen bonds are still needed. Herein, we report the ability to tune the reduction potential of metal-free phenothiazine-based photoredox catalysts and demonstrate the application of these catalysts for chemoselective carbon–halogen bond activation to achieve C–C cross-coupling reactions as well as reductive dehalogenations. This procedure works both for conjugated polyhalides as well as unconjugated substrates. We further illustrate the usefulness of this protocol by intramolecular cyclization of a pyrrole substrate, an advanced building block for a family of natural products known to exhibit biological activity.
Authors: Kaila M. Mattson, Christian W. Pester, Will R. Gutekunst, Andy T. Hsueh, Emre H. Discekici, Yingdong Luo, Bernhard V. K. J. Schmidt, Alaina J. McGrath, Paul G. Clark, and Craig J. Hawker
A light-mediated method for the facile removal of polymer end groups that are common to controlled radical polymerization techniques is presented. This metal-free strategy is general, being effective for chlorine, bromine, and thiocarbonylthio moieties as well as a number of different polymer families (styrenic, acrylic, and methacrylic). In addition to solution reactions, this process is readily translated to thin films, where light mediation allows the straightforward fabrication of hierarchically patterned polymer brushes.
Authors: Christian W. Pester, Benjaporn Narupai, Kaila M. Mattson, David P. Bothman, Daniel Klinger, Kenneth W. Lee, Emre H. Discekici, Craig J. Hawker
Solution-exchange lithography is a new modular approach to engineer surfaces via sequential photopatterning. An array of lenses reduces features on an inkjet-printed photomask and reproduces arbitrarily complex patterns onto surfaces. In situ exchange of solutions allows successive photochemical reactions without moving the substrate and affords access to hierarchically patterned substrates.