Direct Experimental Evidence for Alkoxyl Radicals Reacting as Hydrogen Atom Donors toward Pyridines.

Nanosecond transient absorption spectroscopy was used to generate ethoxyl radicals and demonstrate that they react with 2,6-lutidine and 4-phenylpyridine to give the corresponding N-hydropyridinyl radicals-products of a novel hydrogen atom transfer from the alkoxyl radical to the nitrogen atom of the substituted pyridines. Nanosecond kinetics show that both reactions are rapid (k ∼ 107 M-1 s-1) in acetonitrile at room temperature. Rate constants measured for reaction of the ethoxyl vs. d5-ethoxyl radical with 2,6-lutidine and 4-phenylpyridine show that both reactions exhibit primary H/D kinetic isotope effects for the hydrogen (deuterium) atom transfer reactions.

Spontaneous Patterning during Frontal Polymerization.

Complex patterns integral to the structure and function of biological materials arise spontaneously during morphogenesis. In contrast, functional patterns in synthetic materials are typically created through multistep manufacturing processes, limiting accessibility to spatially varying materials systems. Here, we harness rapid reaction-thermal transport during frontal polymerization to drive the emergence of spatially varying patterns during the synthesis of engineering polymers. Tuning of the reaction kinetics and thermal transport enables internal feedback control over thermal gradients to spontaneously pattern morphological, chemical, optical, and mechanical properties of structural materials. We achieve patterned regions with two orders of magnitude change in modulus in poly(cyclooctadiene) and 20 °C change in glass transition temperature in poly(dicyclopentadiene). Our results suggest a facile route to patterned structural materials with complex microstructures without the need for masks, molds, or printers utilized in conventional manufacturing. Moreover, we envision that more sophisticated control of reaction-transport driven fronts may enable spontaneous growth of structures and patterns in synthetic materials, inaccessible by traditional manufacturing approaches.

Next-Generation Ultrafiltration Membranes Enabled by Block Polymers.

Reliable and equitable access to safe drinking water is a major and growing challenge worldwide. Membrane separations represent one of the most promising strategies for the energy-efficient purification of potential water sources. In particular, porous membranes are used for the ultrafiltration (UF) of water to remove contaminants with nanometric sizes. However, despite exhibiting excellent water permeability and solution processability, existing UF membranes contain a broad distribution of pore sizes that limit their size selectivity. To maximize the potential utility of UF membranes and allow for precise separations, improvements in the size selectivity of these systems must be achieved. Block polymers represent a potentially transformative solution, as these materials self-assemble into well-defined domains of uniform size. Several different strategies have been reported for integrating block polymers into UF membranes, and each strategy has its own set of materials and processing considerations to ensure that uniform and continuous pores are generated. This Review aims to summarize and critically analyze the chemistries, processing techniques, and properties required for the most common methods for producing porous membranes from block polymers, with a particular focus on the fundamental mechanisms underlying block polymer self-assembly and pore formation. Critical structure-property-performance metrics will be analyzed for block polymer UF membranes to understand how these membranes compare to commercial UF membranes and to identify key research areas for continued improvements. This Review is intended to inform readers of the capabilities and current challenges of block polymer UF membranes, while stimulating critical thought on strategies to advance these technologies.

Mechanism and Selectivity of Aryltrimethylgermane Cation Radical Fragmentations.

Aryltrimethylgermane cation radicals were generated by nanosecond transient absorption spectroscopy. Transient kinetics experiments show that the aryltrimethylgermane cation radicals react with added nucleophiles in reactions that are first-order in both the cation radicals and the nucleophiles. Preparative photo-oxidation experiments demonstrate that the intermediate cation radicals react with nucleophiles, resulting in aryl-Ge or Me-Ge nucleophile-assisted fragmentations. The aryltrimethylgermane cation radicals were found to react more slowly than analogous stannane cation radicals; however, loss of the thermodynamically disfavored aryl radicals remains competitive with methyl radical loss.

Cyclic Poly(phthalaldehyde): Thermoforming a Bulk Transient Material.

Cyclic poly(phthalaldehyde) (cPPA) is a metastable and stimuli responsive polymer that undergoes rapid solid state depolymerization and has been utilized as a packaging and encapsulating material for transient applications. However, the early onset thermal depolymerization of cPPA severely hinders the fabrication and processing of plastic parts. Herein, the thermally triggered depolymerization of cPPA was investigated and tailored to enable thermal processing and molding of cPPA at moderate temperatures below the thermal depolymerization temperature. Stabilization of cPPA at elevated temperature was accomplished by removal of the latent Lewis acid catalyst BF3 and by addition of radical inhibitors and a Lewis base. Addition of a plasticizer to the stabilized cPPA enabled the fabrication of a monolithic solid polymer via hot press molding. Importantly, it is shown that the thermally processed cPPA retains its stimuli responsive depolymerization capability and will enable future work in the fabrication of bulk plastic parts that depolymerize and disintegrate on demand.

Multiple Intermolecular Exciplexes in Highly Polar Solvents.

Exciplexes of 2,6,9,10-tetracyanoanthracene (TCA) with alkylbenzenes were investigated in solvents ranging from cyclohexane to acetonitrile. Plots of the reduced emission maxima or the average emission frequency (hνav) versus redox potential differences (Eredox) were linear with a slope of ∼1 in all solvents, which is consistent with the highly ionic character of the exciplexes. The exciplex spectra were analyzed in terms of the energy gap between the exciplex minimum and the AD pair (ΔG), the energy difference between ΔG and Eredox (δEx), and the total reorganization energy (Σλ). A plot of (Eredox - hνav), equivalent to (Σλ - δEx), versus a solvent polarity function showed a linear dependency for the low-to-moderate polarity solvents, whereas highly polar solvents deviated significantly. δEx showed a smooth linear dependency for all solvents. Thus, the deviation of the polar solvents is due to a larger-than-expected Σλ. Additionally, the full width at half-maximum (fwhm) of the emission spectra in polar solvents deviates significantly from the extrapolated trend in less-polar solvents. The deviations of Σλ and fwhm in highly polar solvents can plausibly be explained by composite emissions from two exciplex structures, with the donor overlapping with the inner or outer ring of TCA.

Accurate oxidation potentials of 40 benzene and biphenyl derivatives with heteroatom substituents.

The redox equilibrium method was used to determine accurate oxidation potentials in acetonitrile for 40 heteroatom-substituted compounds. These include methoxy-substituted benzenes and biphenyls, aromatic amines, and substituted acetanilides. The redox equilibrium method allowed oxidation potentials to be determined with high precision (≤ ±6 mV). Whereas most of the relative oxidation potentials follow well-established chemical trends, interestingly, the oxidation potentials of substituted N-methylacetanilides were found to be higher than those of the corresponding acetanilides. Density functional theory calculations provided insight into the origin of these surprising results in terms of the preferred conformations of the amides versus their cation radicals.