PEGylated <i>N</i>‑Heterocyclic Carbene Anchors Designed To Stabilize Gold Nanoparticles in Biologically Relevant Media

<i>N</i>-Heterocyclic carbenes (NHCs) have emerged as versatile ligands for surface functionalization. Their ease of synthesis and ability to form strong bonds with a range of substrates provide a unique complement to traditional surface modification methods. Gold nanoparticles (NPs) are a particularly useful class of materials whose applications intimately depend on surface functionalization. Here we report the development of PEGylated-NHC ligands for Au-NP surfaces and the first example of NHC-functionalized NPs that are compatible with biologically relevant conditions. Our PEGylated-NHC-Au-NPs are stable toward aggregation in aqueous solutions in the pH range of 3–14, in <250 mM electrolyte solutions, at high and low temperatures (95 and −78 °C), in cell culture media, and in aqueous H₂O₂ solutions. This work demonstrates for the first time that NHCs can serve as anchors for water-soluble Au-NPs and opens the door to potential biomedical applications of NHC surface anchors

Visible-Light-Controlled Living Radical Polymerization from a Trithiocarbonate Iniferter Mediated by an Organic Photoredox Catalyst

Living radical polymerization of acrylates and acrylamides from trithiocarbonate iniferters using a compact fluorescent lamp (CFL) bulb and 10-phenylphenothiazine as an organic photoredox catalyst is reported. With this system, chain growth can be efficiently switched between “on” and “off” in response to visible light. Polymer molar masses increase linearly with conversion, and narrow molar mass distributions are obtained. The excellent fidelity of the trithiocarbonate-iniferter enables the preparation of triblock copolymers from macro-iniferters under the same visible-light mediated protocol, using UV light without a photoredox catalyst or under traditional thermally induced RAFT conditions. We expect that the simplicity and efficiency of this metal-free, visible-light-mediated polymerization will enable the synthesis and modification of a range of materials under mild conditions

Sub-10 nm Self-Assembly of Mesogen-Containing Grafted Macromonomers and Their Bottlebrush Polymers

We explore the morphology and phase behavior of branched diblock macromonomers and their polymers. A series of macromonomers was synthesized based on a disubstituted norbornene. The first branch consists of polydimethylsiloxane (PDMS) while the second branch is a quasi-mesogenic structure incorporating one or more cyanobiphenyl (CB) moieties. Bottlebrush polymers with varying degrees of polymerization were prepared by “graft-through” ring-opening metathesis of the macromonomers. The molecules in the resulting library of macromonomers and bottlebrush polymers self-assemble to form classically observed microphase-separated structures, including spheres, hexagonally packed cylinders, bicontinuous gyroid, and lamellae. The systematic variation of molecular structure, molecular weight of each branch, and degree of polymerization of the polymers results in a diverse set of structures and properties. We report the observation of well-ordered lamellae and cylinders with <i>d</i>-spacings as low as 6.1 and 8.0 nm, respectively. The system displays an asymmetric phase diagram, with large deviations from the canonical phase behavior of linear coil–coil diblocks. Hexagonally packed cylinders and lamellae are observed at remarkably small mass fractions of the mesogen-containing block of 0.07 and 0.21, respectively. The samples are highly birefringent, and polarized optical microscopy revealed the formation of well-developed textures in microphase-separated states formed by cooling samples through the order–disorder transition. The textures are reminiscent of the classic fan-like or focal-conic textures observed in small molecule liquid crystal mesophases, highlighting the formation of unusually large and well-ordered grains of the microphase-separated PDMS and CB microdomains. Apparent crystallization of the CB units in systems with two or three CB moieties per monomer results in distortion of the microphase-separated structure. The small <i>d</i>-spacings and large grain sizes observed here highlight the versatility and potential utility of this molecular architecture for designing and engineering new functional materials

Topological Structure of Networks Formed from Symmetric Four-Arm Precursors

Gels formed by coupling two different four-arm star polymers lead to polymer networks with high strength and low spatial heterogeneity. However, like all real polymer networks, these materials contain topological defects which affect their properties. In this study, kinetic graph theory and Monte Carlo simulation are used to investigate the structure and cyclic defects formed via A–B type end-linking of symmetric tetra-arm star polymer precursors. While loops constituting of odd number of junctions are forbidden by precursor chemistry, the amount and the correlation of secondary loops are found to increase with decreasing precursor concentration. This suppresses gelation, and the delay of gel point is quantitatively predicted by the topological simulations. Furthermore, comparison with network formed with asymmetric bifunctional–tetrafunctional precursors revealed that the behavior of loops consisting of 2<i>n</i> junctions in the symmetric system is analogous to the behavior of loops consisting of <i>n</i> junctions in the asymmetrical system, suggesting analogies between chemically distinct networks

Addressable Carbene Anchors for Gold Surfaces

New strategies to access functional monolayers could augment current surface modification methods. Here we present addressable <i>N</i>-heterocyclic carbene (ANHC) anchors for gold surfaces. A suite of experimental and theoretical methods was used to characterize ANHC monolayers. We demonstrate grafting of highly fluorinated polymers from surface-bound ANHCs. This work establishes ANHCs as viable anchors for gold surfaces

Kinetic Monte Carlo Simulation for Quantification of the Gel Point of Polymer Networks

Accurate prediction of the gel point for real polymer networks is a long-standing challenge in polymer chemistry and physics that is extremely important for applications of gels and elastomers. Here, kinetic Monte Carlo simulation is applied to simultaneously describe network topology and growth kinetics. By accounting for topological defects in the polymer networks, the simulation can quantitatively predict experimental gel point measurements without any fitting parameters. Gel point suppression becomes more severe as the primary loop fraction in the networks increases. A topological homomorphism theory mapping defects onto effective junctions is developed to qualitatively explain the origins of this effect, which accurately captures the gel point suppression in the low loop limit where cooperative effects between topological defects are small

Application of ¹H DOSY for Facile Measurement of Polymer Molecular Weights

To address the practical issues of polymer molecular weight determination, the first accurate polymer weight-average molecular weight determination method in diverse living/controlled polymerization via DOSY (diffusion-ordered NMR spectroscopy) is reported. Based on the linear correlation between the logarithm of diffusion coefficient (log <i>D</i>) and the molecular weights (log <i>M</i>_w), external calibration curves were created to give predictions of molecular weights of narrowly dispersed polymers. This method was successfully applied to atom transfer radical polymerization (ATRP), reversible addition–fragmentation chain transfer (RAFT), and ring-opening metathesis polymerization (ROMP), with weight-average molecular weights given by this method closely correlated to those obtained from GPC measurement

Main-Chain Zwitterionic Supramolecular Polymers Derived from <i>N</i>‑Heterocyclic Carbene–Carbodiimide (NHC–CDI) Adducts

Polyzwitterions have found extensive applications in biological and materials sciences. Despite this success, most polyzwitterions have nondegradable polyolefin backbones with pendant zwitterionic groups. Transcension of this structural paradigm via the formation of main-chain zwitterionic supramolecular polymers could lead to readily processable, as well as self-healing and/or degradable, polyzwitterions. Herein, we report the synthesis and characterization of poly­(azolium amidinate)­s (PAzAms), which are a new class of supramolecular main-chain polyzwitterions assembled via the formation of <i>N</i>-heterocyclic carbene–carbodiimide (NHC–CDI) adducts. These polymers exhibit a wide range of tunable dynamic properties due to the highly structure-sensitive equilibrium between the NHC–CDI adduct and its constituent NHCs and CDIs: e.g., PAzAms derived from <i>N</i>-aryl-<i>N′</i>-alkyl CDIs are dynamic at lower temperatures than those derived from <i>N</i>,<i>N′</i>-diaryl CDIs. We develop a versatile synthetic platform that provides access to PAzAms with control over the main-chain charge sequence and molecular weight. In addition, block copolymers incorporating PAzAm and poly­(ethylene glycol) (PEG) blocks are water soluble (>30 mg mL^–1) and self-assemble in aqueous environments. This work defines structure–property relationships for a new class of degradable main-chain zwitterionic supramolecular polymers, setting the stage for the development of these polymers in a range of applications

Logic-Controlled Radical Polymerization with Heat and Light: Multiple-Stimuli Switching of Polymer Chain Growth via a Recyclable, Thermally Responsive Gel Photoredox Catalyst

Strategies for switching polymerizations between “ON” and “OFF” states offer new possibilities for materials design and fabrication. While switching of controlled radical polymerization has been achieve using light, applied voltage, allosteric effects, chemical reagents, pH, and mechanical force, it is still challenging to introduce multiple external switches using the same catalyst to achieve logic gating of controlled polymerization reactions. Herein, we report an easy-to-synthesize thermally responsive organo-/hydro-gel that features covalently bound 10-phenylphenothiazine (PTH). With this “Gel-PTH”, we demonstrate switching of controlled radical polymerization reactions using temperature “LOW”/“HIGH”, light “ON”/“OFF”, and catalyst presence “IN”/“OUT”. Various iniferters/initiators and a wide range of monomers including acrylates, methacrylates, acrylamides, vinyl esters, and vinyl amides were polymerized by RAFT/iniferter and ATRP methods using Gel-PTH and a readily available compact fluorescent light (CFL) source. In all cases, polymer molar masses increased linearly with conversion, and narrow molar mass distributions were obtained. To further highlight the utility of Gel-PTH, we achieved “AND” gating of controlled radical polymerization wherein various combinations of three stimuli were required to induce polymer chain growth. Finally, block copolymer synthesis and catalyst recycling were demonstrated. Logic-controlled polymerization with Gel-PTH offers a straightforward approach to achieve multiplexed external switching of polymer chain growth using a single catalyst without the need for addition of exogenous reagents

Counting Secondary Loops Is Required for Accurate Prediction of End-Linked Polymer Network Elasticity

To predict and understand the properties of polymer networks, it is necessary to quantify network defects. Of the various possible network defects, loops are perhaps the most pervasive and yet difficult to directly measure. Network disassembly spectrometry (NDS) has previously enabled counting of the simplest loopsprimary loopsbut higher-order loops, e.g., secondary loops, have remained elusive. Here, we report that the introduction of a nondegradable tracer within the NDS framework enables the simultaneous measurement of primary and secondary loops in end-linked polymer networks for the first time. With this new “NDS2.0” method, the concentration dependences of the primary and secondary loop fractions are measured; the results agree well with a purely topological theory for network formation from phantom chains. In addition, semibatch monomer addition is shown to decrease both primary and secondary loops, though the latter to a much smaller extent. Finally, using the measured primary and secondary loop fractions, we were able to predict the shear storage modulus of end-linked polymer gels via real elastic network theory (RENT)