"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.
Authors: James R. Hemmer, Saemi O. Poelma, Nicolas Treat, Zachariah A. Page, Neil D. Dolinski, Yvonne J. Diaz, Warren Tomlinson, Kyle D. Clark, Joseph P. Hooper, Craig Hawker, and Javier Read de Alaniz
A class of tunable visible and near-infrared donor−acceptor Stenhouse adduct (DASA) photoswitches were efficiently synthesized in two to four steps from commercially available starting materials with minimal purification. Using either Meldrum’s or barbituric acid “acceptors” in combination with aniline-based “donors”, an absorption range spanning from 450 to 750 nm is obtained. Additionally, photoisomerization results in complete decoloration for all adducts, yielding fully transparent, colorless solutions and films. Detailed investigations using density functional theory, nuclear magnetic resonance, and visible absorption spectroscopies provide valuable insight into the unique structure−property relationships for this novel class of photoswitches. As a final demonstration, selective photochromism is accomplished in a variety of solvents and polymer matrices, a significant advantage for applications of this new generation of DASAs.
Authors: Michael A. Brady, Sung-Yu Ku, Louis A. Perez, Justin E. Cochran, Kristin Schmidt, Thomas M. Weiss, Michael F. Toney, Harald Ade, Alexander Hexemer, Cheng Wang, Craig J. Hawker, Edward J. Kramer, and Michael L. Chabinyc
Conjugated block copolymers provide a pathway to achieve thermally stable nanostructured thin films for organic solar cells. We characterized the structural evolution of poly(3-hexylthiophene)-block-poly(diketopyrrolopyrrole–terthiophene) (P3HT-b-DPPT-T) from solution to nanostructured thin films. Aggregation of the DPPT-T block of P3HT-b-DPPT-T was found in solution by small-angle X-ray scattering with the P3HT block remaining well-solvated. The nanostructure in thin films was determined using a combination of wide and small-angle X-ray scattering techniques as a function of processing conditions. The structure in solution controlled the initial nanostructure in spin-cast thin films, allowing subsequent thermal annealing processes to further improve the ordering. In contrast to the results for thin films, nanostructural ordering was not observed in the bulk samples by small-angle X-ray scattering. These results suggest the importance of controlling solvent induced aggregation in forming nanostructured thin films of conjugated block copolymers.
Authors: Page, Z. A., Bou Zerdan, R., Gutekunst, W. R., Anastasaki, A., Seo, S., McGrath, A. J., Lunn, D. J., Clark, P. G. and Hawker, C. J.
A new di-tert-butyl acrylate (diTBA) monomer for controlled radical polymerization is reported. This monomer complements the classical use of tert-butyl acrylate (TBA) for synthesis of poly(acrylic acid) by increasing the density of carboxylic acids per repeat unit, while also increasing the flexibility of the carboxylic acid side-chains. The monomer is well behaved under Cu(II)-mediated photoinduced controlled radical polymerization and delivers polymers with excellent chain-end fidelity at high monomer conversions. Importantly, this new diTBA monomer readily copolymerizes with TBA to further the potential for applications in areas such as dispersing agents and adsorbents.
Authors: Anastasaki, A., Willenbacher, J., Fleischmann, C., Gutekunst, W., Hawker, C. J.
The versatile and high yielding functionalization of polymer end groups is a critical tool for controlling material properties and/or for successful post polymerization reactions. In this report, bromine-terminated poly(methyl acrylate) derivatives are used as a model system for identifying conditions leading to quantitative transformation of the end group. A wide range of small molecules and associated reactions for the introduction of specific acidic, basic, hydrophilic or hydrophobic functionality are described. Analysis by SEC, 1H NMR and MALDI-ToF-MS provides evidence for full conversion of the end group. The user-friendly nature of these procedures serve as a powerful strategy for the synthesis of end functionalized polymers.
"A synthetic strategy for the preparation of sub-100 nm functional polymer particles of uniform diameter"
Authors: Killops,K. L.; Rodriguez, C. G.; Lundberg, P.; Hawker, C. J.; Lynd, N.l A.
An amphiphilic block copolymer surfactant is used to impart peripheral surface functionality to polymer nanoparticles synthesized via emulsion polymerization. Particles ranged in size from ca.55 nm by SEM (ca. 82 nm by DLS) to just over 200 nm. Particles displaying latent functionality were readily functionalized directly after polymerization using a fluorescent dye.
"A robust platform for functional microgels via thiol–ene chemistry with reactive polyether-based nanoparticles"
Authors: Fleischmann, C.; Gopez, J.; Lundberg, P.; Ritter, H.; Killops, K. L.; Hawker, C. J.; and Klinger, D.
We herein report the development of crosslinked polyether particles as a reactive platform for the preparation of functional microgels. Thiol–ene crosslinking of poly(allyl glycidyl ether) in miniemulsion droplets – stabilized by a surface active, bio-compatible polyethylene glycol block copolymer – resulted in colloidal gels with a PEG corona and an inner polymeric network containing reactive allyl units. The stability of the allyl groups allows the microgels to be purified and stored before a second, subsequent thiol–ene functionalization step allows a wide variety of pH- and chemically-responsive groups to be introduced into the nanoparticles. The facile nature of this synthetic platform enables the preparation of microgel libraries that are responsive to different triggers but are characterized by the same size distribution, surface functionality, and crosslinking density. In addition, the utilization of a crosslinker containing cleavable ester groups renders the resulting hydrogel particles degradable at elevated pH or in the presence of esterase under physiological conditions.
"Phase behavior of electrostatically complexed polyelectrolyte gels using an embedded fluctuation model"
Authors: Audus, D. J.; Gopez, J. D.; Krogstad, D. V.; Lynd, N. A.; Kramer, E. J.; Hawker, C. J.; Fredrickson, G. H.
Nanostructured, responsive hydrogels formed due to electrostatic interactions have promise for applications such as drug delivery and tissue mimics. These physically cross-linked hydrogels are composed of an aqueous solution of oppositely charged triblocks with charged end-blocks and neutral, hydrophilic mid-blocks. Due to their electrostatic interactions, the end-blocks microphase separate and form physical cross-links that are bridged by the mid-blocks. The structure of this system was determined using a new, efficient embedded fluctuation (EF) model in conjunction with self-consistent field theory. The calculations using the EF model were validated against unapproximated field-theoretic simulations with complex Langevin sampling and were found consistent with small angle X-ray scattering (SAXS) measurements on an experimental system. Using both the EF model and SAXS, phase diagrams were generated as a function of end-block fraction and polymer concentration. Several structures were observed including a body-centered cubic sphere phase, a hexagonally packed cylinder phase, and a lamellar phase. Finally, the EF model was used to explore how parameters that directly relate to polymer chemistry can be tuned to modify the resulting phase diagram, which is of practical interest for the development of new hydrogels.