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

Block Co-PolyMOCs by Stepwise Self-Assembly

We report a stepwise assembly strategy for the integration of metal–organic cages (MOCs) into block copolymers (BCPs). This approach creates “block co-polyMOC” (BCPMOC) materials whose microscopic structures and mechanical properties are readily tunable by adjusting the size and geometry of the MOCs and the composition of the BCPs. In the first assembly step, BCPs functionalized with a pyridyl ligand on the chain end form star-shaped polymers triggered by metal-coordination-induced MOC assembly. The type of MOC junction employed precisely determines the number of arms for the star polymer. In the second step, microphase separation of the BCP is induced, physically cross-linking the star polymers and producing the desired BCPMOC networks in the bulk or gel state. We demonstrate that large spherical M₁₂L₂₄ MOCs, small paddlewheel M₂L₄ MOCs, or a mixture of both can be incorporated into BCPMOCs to provide materials with tailored branch functionality, phase separation, microdomain spacing, and mechanical properties. Given the synthetic and functional diversity of MOCs and BCPs, our method should enable access to BCPMOCs for a wide range of applications

Supported Au Nanoparticles with <i>N</i>‑Heterocyclic Carbene Ligands as Active and Stable Heterogeneous Catalysts for Lactonization

Attachment of <i>N</i>-heterocyclic carbenes (NHCs) on the surface of metal nanoparticle (NP) catalysts permits fine-tuning of catalytic activity and product selectivity. Yet, NHC-coated Au NPs have been seldom used in catalysis beyond hydrogenation chemistry. One challenge in this field has been to develop a platform that permits arbitrary ligand modification without having to compromise NP stability toward aggregation or leaching. Herein, we exploit the strategy of supported dendrimer-encapsulated metal clusters (DEMCs) to achieve aggregation-stable yet active heterogeneous Au NP catalysts with NHC ligands. Dendrimers function as aggregation-inhibitors during the NP synthesis, and NHCs, well-known for their strong attachment to the gold surface, provide a handle to modify the stereochemistry, stereoelectronics, and chemical functionality of the NP surface. Indeed, compared to “ligandless” Au NPs which are virtually inactive below 80 °C, the NHC-ligated Au NP catalysts enable a model lactonization reaction to proceed at 20 °C on the same time scale (hours). Based on Eyring analysis, proto-deauration is the turnover-limiting step accelerated by the NHC ligands. Furthermore, the use of chiral NHCs led to asymmetric induction (up to 16% enantiomeric excess) in the lactonization transformations, which demonstrates the potential of supported DEMCs with ancillary ligands in enantioselective catalysis

“Brush-First” Method for the Parallel Synthesis of Photocleavable, Nitroxide-Labeled Poly(ethylene glycol) Star Polymers

We describe the parallel, one-pot synthesis of core-photocleavable, poly­(norbornene)-<i>co</i>-poly­(ethylene glycol) (PEG) brush-arm star polymers (BASPs) via a route that combines the “graft-through” and “arm-first” methodologies for brush polymer and star polymer synthesis, respectively. In this method, ring-opening metathesis polymerization of a norbornene–PEG macromonomer generates small living brush initiators. Transfer of various amounts of this brush initiator to vials containing a photocleavable bis-norbornene cross-linker yielded a series of water-soluble BASPs with low polydispersities and molecular weights that increased geometrically as a function of the amount of bis-norbornene added. The BASP cores were cleaved upon exposure to UV light; the extent of photo-disassembly depended on the amount of cross-linker. EPR spectroscopy of nitroxide-labeled BASPs was used to probe differences between the BASP core and surface environments. We expect that BASPs will find applications as easy-to-synthesize, stimuli-responsive core–shell nanostructures

Using EPR To Compare PEG-<i>branch</i>-nitroxide “Bivalent-Brush Polymers” and Traditional PEG Bottle–Brush Polymers: Branching Makes a Difference

Attachment of poly­(ethylene glycol) (PEG) to polymeric nanostructures is a general strategy for sterically shielding and imparting water solubility to hydrophobic payloads. In this report, we describe direct graft-through polymerization of branched, multifunctional macromonomers that possess a PEG domain and a hydrophobic nitroxide domain. Electron paramagnetic resonance (EPR) spectroscopy was used to characterize microenvironments within these novel nanostructures. Comparisons were made to nitroxide-labeled, traditional bottle-brush random and block copolymers. Our results demonstrate that bivalent bottle-brush polymers have greater microstructural homogeneity compared to random copolymers of similar composition. Furthermore, we found that compared to a traditional brush polymer, the branched-brush, “pseudo-alternating” microstructure provided more rotational freedom to core-bound nitroxides, and greater steric shielding from external reagents. The results will impact further development of multivalent bottle-brush materials as nanoscaffolds for biological applications