Photo-modulated activation of organic bases enabling microencapsulation and on-demand reactivity.

A method is developed for facile encapsulation of reactive organic bases with potential application for autonomous damage detection and self-healing polymers. Highly reactive chemicals such as bases and acids are challenging to encapsulate by traditional oil-water emulsion techniques due to unfavorable physical and chemical interactions. In this work, reactivity of the bases is temporarily masked with photo-removable protecting groups, and the resulting inactive payloads are encapsulated via an in situ emulsion-templated interfacial polymerization method. The encapsulated payloads are then activated to restore the organic bases via photo irradiation, either before or after being released from the core-shell carriers. The efficacy of the photo-activated capsules is demonstrated by a damage-triggered, pH-induced color change in polymeric coatings and by recovery of adhesive strength of a damaged interface. Given the wide range of potential photo-deprotection chemistries, this encapsulation scheme provides a simple but powerful method for storage and targeted delivery of a broad variety of reactive chemicals, promoting design of diverse autonomous functionalities in polymeric materials.

Odd-even effect on the thermal conductivity of liquid crystalline epoxy resins.

Rapid developments in high-performance computing and high-power electronics are driving needs for highly thermal conductive polymers and their composites for encapsulants and interface materials. However, polymers typically have low thermal conductivities of ∼0.2 W/(m K). We studied the thermal conductivity of a series of epoxy resins cured by one diamine hardener and seven diepoxide monomers with different precise ethylene linker lengths (x = 2-8). We found pronounced odd-even effects of the ethylene linker length on the liquid crystalline order, mass density, and thermal conductivity. Epoxy resins with even x have liquid crystalline structure with the highest density of 1.44 g/cm3 and highest thermal conductivity of 1.0 W/(m K). Epoxy resins with odd x are amorphous with the lowest density of 1.10 g/cm3 and lowest thermal conductivity of 0.17 W/(m K). These findings indicate that controlling precise linker length in dense networks is a powerful route to molecular design of thermally conductive polymers.

Impact of dynamic covalent chemistry and precise linker length on crystallization kinetics and morphology in ethylene vitrimers.

Vitrimers, dynamic polymer networks with topology conserving exchange reactions, have emerged as a promising platform for sustainable and reprocessable materials. While prior work has documented how dynamic bonds impact stress relaxation and viscosity, their role on crystallization has not been systematically explored. Precise ethylene vitrimers with 8, 10, or 12 methylene units between boronic ester junctions were investigated to understand the impact of bond exchange on crystallization kinetics and morphology. Compared to linear polyethylene which has been heavily investigated for decades, a long induction period for crystallization is seen in the vitrimers ultimately taking weeks in the densest networks. An increase in melting temperatures (Tm) of 25-30 K is observed with isothermal crystallization over 30 days. Both C10 and C12 networks initially form hexagonal crystals, while the C10 network transforms to orthorhombic over the 30 day window as observed with wide angle X-ray scattering (WAXS) and optical microscopy (OM). After 150 days of isothermal crystallization, the three linker lengths led to double diamond (C8), orthorhombic (C10), and hexagonal (C12) crystals indicating the importance of precision on final morphology. Control experiments on a precise, permanent network implicate dynamic bonds as the cause of long-time rearrangements of the crystals, which is critical to understand for applications of semi-crystalline vitrimers. The dynamic bonds also allow the networks to dissolve in water and alcohol-based solvents to monomers, followed by repolymerization while preserving the mechanical properties and melting temperatures.

Critical Role of Ion Exchange Conditions on the Properties of Network Ionic Polymers.

Ionic polymers are important in a wide range of applications and can exhibit widely different properties depending on the ionic species. In the case of single ion conducting polymers, where one charge is attached to the backbone or as a side group, ion exchange is performed to control the mobile species. While the conditions are often specified, the final ion content is not always quantified, and there are no clear criteria for what concentration of salt is needed in the exchange. A series of ammonium network ionic polymers with different precise carbon spacers (C4-C7) between ionic junctions were synthesized as model systems to understand how the ion exchange conditions impact the resultant polymer properties. The initial networks with free bromide anions were exchanged with 1.5, 3, or 10 equiv of lithium bis(trifluoromethane)sulfonimide (LiTFSI) salt in solution. For networks with seven carbons between cross-links, increasing the LiTFSI concentration led to an increase in ion exchange efficiency from 83.6 to 97.6 mol %. At the highest conversion, the C7 network showed a 4 °C decrease in glass transition temperature (Tg), a 50 °C increase in degradation temperature, 12-fold lower water uptake from air, and a greater than 10-fold increase in conductivity at 90 °C. These results illustrate that properties such as Tg are less sensitive to residual ion impurities, whereas the conductivity is highly dependent on the final exchange conversion.

Effect of Network Architecture and Linker Polarity on Ion Aggregation and Conductivity in Precise Polymerized Ionic Liquids.

Four polymerized ionic liquids (PILs) were systematically designed to study the effect of polymer architecture and linker polarity on ion aggregation and transport. Specifically, linear and network PILs with the same ammonium cations (Am) and bis(trifluoromethane)sulfonimide (TFSI) anions were prepared by step-growth polymerization, and polarity was tuned by incorporating two precise linkers, either polar tetra(ethylene oxide) (4EO) linker or nonpolar undecyl (C11) linker. The glass transition temperature (Tg) substantially increased with the nonpolar C11 linker or upon cross-linking to form a network. The low wave-vector (q) ion aggregation peak from wide-angle X-ray scattering (WAXS) was not observable in the linear 4EO PIL, while it was most pronounced in the network C11 PIL. The network C11 PIL exhibited the strongest decoupling, where the ionic conductivity at Tg is greater than 1 order of magnitude higher than the other PILs. This systematic comparison suggests that network structure and nonpolar linkers can promote both ion aggregation and ionic conductivity close to Tg.

Carbenylative Amination and Alkylation of Vinyl Iodides via Palladium Alkylidene Intermediates.

Most palladium-catalyzed reactions involving insertion of alkylidenes with α-hydrogens undergo ß-hydride elimination from alkylpalladium(II) intermediates to form alkenes. Vinyl iodides were shown to generate η(3)-allylpalladium intermediates that resist ß-hydride elimination, preserving the sp(3) center adjacent to the carbene moiety. Acyclic stereocontrol (syn/anti) for carbenylative amination and alkylation reactions was low, suggesting a lack of control in the migratory insertion step. Highly hindered carbene precursors inexplicably led to formation of Z-alkenes with high levels of stereocontrol.

Potent Synergy between Spirocyclic Pyrrolidinoindolinones and Fluconazole against Candida albicans.

A spiroindolinone, (1S,3R,3aR,6aS)-1-benzyl-6'-chloro-5-(4-fluorophenyl)-7'-methylspiro[1,2,3a,6a-tetrahydropyrrolo[3,4-c]pyrrole-3,3'-1H-indole]-2',4,6-trione, was previously reported to enhance the antifungal effect of fluconazole against Candida albicans. A diastereomer of this compound was synthesized, along with various analogues. Many of the compounds were shown to enhance the antifungal effect of fluconazole against C. albicans, some with exquisite potency. One spirocyclic piperazine derivative, which we have named synazo-1, was found to enhance the effect of fluconazole with an EC50 value of 300 pM against a susceptible strain of C. albicans and going as low as 2 nM against some resistant strains. Synazo-1 exhibits true synergy with fluconazole, with an FIC index below 0.5 in the strains tested. Synazo-1 exhibited low toxicity in mammalian cells relative to the concentrations required for antifungal synergy.