Accessing pluripotent materials through tempering of dynamic covalent polymer networks.

Pluripotency, which is defined as a system not fixed as to its developmental potentialities, is typically associated with biology and stem cells. Inspired by this concept, we report synthetic polymers that act as a single "pluripotent" feedstock and can be differentiated into a range of materials that exhibit different mechanical properties, from hard and brittle to soft and extensible. To achieve this, we have exploited dynamic covalent networks that contain labile, dynamic thia-Michael bonds, whose extent of bonding can be thermally modulated and retained through tempering, akin to the process used in metallurgy.

Doubly Threaded Slide-Ring Polycatenane Networks.

Crosslinking in polymer networks leads to intrinsic structural inhomogeneities that result in brittle materials. Replacing fixed covalent crosslinks with mobile ones in mechanically interlocked polymers (MIPs), such as in slide-ring networks (SRNs) in which interlocked crosslinks are formed when polymer chains are threaded through crosslinked rings, can lead to tougher, more robust networks. An alternative class of MIPs is the polycatenane network (PCN), in which the covalent crosslinks are replaced with interlocked rings that introduce the unusual catenane's mobility elements (elongation, rotation, and twisting) as connections between polymer chains.

Relative effects of polymer composition and sample preparation on glass dynamics.

Modern design of common adhesives, composites and polymeric parts makes use of polymer glasses that are stiff enough to maintain their shape under a high stress while still having a ductile behavior after the yield point. Typically, material compositions are tuned with co-monomers, polymer blends, plasticizers, or other additives to arrive at a tradeoff between the elastic modulus and toughness. In contrast, strong changes to the mechanics of a glass are possible by changing only the molecular packing during vitrification or even deep in the glassy state.

Designing Stress-Adaptive Dense Suspensions Using Dynamic Covalent Chemistry.

The non-Newtonian behaviors of dense suspensions are central to their use in technological and industrial applications and arise from a network of particle-particle contacts that dynamically adapt to imposed shear. Reported herein are studies aimed at exploring how dynamic covalent chemistry between particles and the polymeric solvent can be used to tailor such stress-adaptive contact networks, leading to their unusual rheological behaviors. Specifically, a room temperature dynamic thia-Michael bond is employed to rationally tune the equilibrium constant ( ) of the polymeric solvent to the particle interface.

Modeling Brittle Fractures in Epoxy Nanocomposites Using Extended Finite Element and Cohesive Zone Surface Methods.

Linear elastic fracture modeling coupled with empirical material tensile data result in good quantitative agreement with the experimental determination of mode I fracture for both brittle and toughened epoxy nanocomposites. The nanocomposites are comprised of diglycidyl ether of bisphenol A cured with Jeffamine D-230 and some were filled with core-shell rubber nanoparticles of varying concentrations. The quasi-static single-edge notched bending (SENB) test is modeled using both the surface-based cohesive zone (CZS) and extended finite element methods (XFEM) implemented in the Abaqus software.

Supramolecular Salts for Additive Manufacturing of Polyimides.

Recent advances in vat photopolymerization (VP) additive manufacturing of fully aromatic polyimides employed photoreactive high-molecular-weight precursors dissolved at modest loadings (<20 wt %) in organic solvent. These earlier efforts revealed high isotropic shrinkage, approaching 52% on a linear basis while converting to the desired polyimide. To increase the polyimide precursor concentration and decrease shrinkage during VP processing of high-performance polyimides, photoreactive fully aromatic polyimide and thermoplastic polyetherimide (PEI) supramolecular salt precursors now serve as versatile alternatives.

Hebbian Learning on Small Data Enables Experimental Discovery of High Polyimides.

We report a study combining computational design and experimental evaluation of polyimides with high glass transition temperatures: between 220 °C and 500 °C. The computational approach is based on the recently introduced competitive learning algorithm, supervised self-organizing maps (SUSI), which we recast as an ensemble method, e-SUSI. We use e-SUSI to solve both unsupervised and supervised/semisupervised learning tasks capturing structure-property relationships of high- polyimides historically studied at Almaden Research Center.

The Quest for the Ideal Base: Rational Design of a Nickel Precatalyst Enables Mild, Homogeneous C-N Cross-Coupling.

Palladium-catalyzed amination reactions using soluble organic bases have provided a solution to the many issues associated with heterogeneous reaction conditions. Still, homogeneous C-N cross-coupling approaches cannot yet employ bases as weak and economical as trialkylamines. Furthermore, organic base-mediated methods have not been developed for Ni(0/II) catalysis, despite some advantages of such systems over those employing Pd-based catalysts.

Nickel-Mediated Cross-Coupling of Boronic Acids and Phthalimides for the Synthesis of -Substituted Benzamides.

The decarbonylative coupling of phthalimides with aryl boronic acids provides ready access to a broad range of -substituted benzamides. This nickel-mediated methodology extends reactivity from previously described air-sensitive diorganozinc reagents of limited availability to easily handled and widely commercially available boronic acids. The decarbonylative coupling is tolerant of a broad range of functional groups and demonstrates little sensitivity to steric factors on either of the coupling partners.

Pd-Catalyzed C-N Coupling Reactions Facilitated by Organic Bases: Mechanistic Investigation Leads to Enhanced Reactivity in the Arylation of Weakly Binding Amines.

The ability to use soluble organic amine bases in Pd-catalyzed C-N cross-coupling reactions has provided a long-awaited solution to the many issues associated with employing traditional, heterogeneous reaction conditions. However, little is known about the precise function of these bases in the catalytic cycle and about the effect of variations in base structure on catalyst reactivity. We used F NMR to analyze the kinetic behavior of C-N coupling reactions facilitated by different organic bases.

Breaking the Base Barrier: An Electron-Deficient Palladium Catalyst Enables the Use of a Common Soluble Base in C-N Coupling.

Due to the low intrinsic acidity of amines, palladium-catalyzed C-N cross-coupling has been plagued continuously by the necessity to employ strong, inorganic, or insoluble bases. To surmount the many practical obstacles associated with these reagents, we utilized a commercially available dialkyl triarylmonophosphine-supported palladium catalyst that facilitates a broad range of C-N coupling reactions in the presence of weak, soluble bases. The mild and general reaction conditions show extraordinary tolerance for even highly base-sensitive functional groups.

Rhodium-Catalyzed Interconversion of Quinolinyl Ketones with Boronic Acids via C-C Bond Activation.

A rhodium-catalyzed cross-coupling of aryl and aliphatic quinolinyl ketones with boronic acids has been developed. Proceeding via quinoline-directed carbon-carbon σ bond activation, the transformation demonstrates tolerance of a range of functional groups on both the ketone and aryl boronic acid substrates, providing good to excellent yields of the new ketones, particularly those containing electron-withdrawing substituents. Catalyst reactivity is dependent on quinolinyl ketone substrates, with alkyl ketones requiring Rh(PPh3)3Cl instead of the more reactive [Rh(C2H4)2Cl]2.