Circularity in mixed-plastic chemical recycling enabled by variable rates of polydiketoenamine hydrolysis.

Footwear, carpet, automotive interiors, and multilayer packaging are examples of products manufactured from several types of polymers whose inextricability poses substantial challenges for recycling at the end of life. Here, we show that chemical circularity in mixed-polymer recycling becomes possible by controlling the rates of depolymerization of polydiketoenamines (PDK) over several orders of magnitude through molecular engineering. Stepwise deconstruction of mixed-PDK composites, laminates, and assemblies is chemospecific, allowing a prescribed subset of monomers, fillers, and additives to be recovered under pristine condition at each stage of the recycling process. We provide a theoretical framework to understand PDK depolymerization via acid-catalyzed hydrolysis and experimentally validate trends predicted for the rate-limiting step. The control achieved by PDK resins in managing chemical and material entropy points to wide-ranging opportunities for pairing circular design with sustainable manufacturing.

Probing calcium solvation by XAS, MD and DFT calculations.

The solvation shell structures of Ca2+ in aqueous and organic solutions probed by calcium L-edge soft X-ray absorption spectroscopy (XAS) and DFT/MD simulations show the coordination number of Ca2+ to be negatively correlated with the electrolyte concentration and the steric hindrance of the solvent molecule. In this work, the calcium L-edge soft XAS demonstrates its sensitivity to the surrounding chemical environment. Additionally, the total electron yield (TEY) mode is surface sensitive because the electron penetration depth is limited to a few nanometers. Thus this study shows its implications for future battery studies, especially for probing the electrolyte/electrode interface for electrochemical reactions under in situ/operando conditions.

Conformational Entropy as a Means to Control the Behavior of Poly(diketoenamine) Vitrimers In and Out of Equilibrium.

Control of equilibrium and non-equilibrium thermomechanical behavior of poly(diketoenamine) vitrimers is shown by incorporating linear polymer segments varying in molecular weight (MW) and conformational degrees of freedom into the dynamic covalent network. While increasing MW of linear segments yields a lower storage modulus at the rubbery plateau after softening above the glass transition (Tg ), both Tg and the characteristic time of stress relaxation are independently governed by the conformational entropy of the embodied linear segments. Activation energies for bond exchange in the solid state are lower for networks incorporating flexible chains; the network topology freezing temperature decreases with increasing MW of flexible linear segments but increases with increasing MW of stiff segments. Vitrimer reconfigurability is therefore influenced not only by the energetics of bond exchange for a given network density, but also the entropy of polymer chains within the network.

Elucidating Non-aqueous Solvent Stability and Associated Decomposition Mechanisms for Mg Energy Storage Applications From First-Principles.

Rational design of novel electrolytes with enhanced functionality requires fundamental molecular-level understanding of structure-property relationships. Here we examine the suitability of a range of organic solvents for non-aqueous electrolytes in secondary magnesium batteries using density functional theory (DFT) calculations as well as experimental probes such as cyclic voltammetry and Raman spectroscopy. The solvents considered include ethereal solvents (e.g., glymes) sulfones (e.g., tetramethylene sulfone), and acetonitrile. Computed reduction potentials show that all solvents considered are stable against reduction by Mg metal. Additional computations were carried out to assess the stability of solvents in contact with partially reduced Mg cations (Mg2+ → Mg+) formed during cycling (e.g., deposition) by identifying reaction profiles of decomposition pathways. Most solvents, including some proposed for secondary Mg energy storage applications, exhibit decomposition pathways that are surprisingly exergonic. Interestingly, the stability of these solvents is largely dictated by magnitude of the kinetic barrier to decomposition. This insight should be valuable toward rational design of improved Mg electrolytes.

Enhanced Stability of the Carba- closo-dodecaborate Anion for High-Voltage Battery Electrolytes through Rational Design.

Future energy applications rely on our ability to tune liquid intermolecular interactions and achieve designer electrolytes with highly optimized properties. In this work, we demonstrate rational, combined experimental-computational design of a new carba- closo-dodecaborate-based salt with enhanced anodic stability for Mg energy storage applications. We first establish, through a careful examination using a range of solvents, the anodic oxidation of a parent anion, the carba- closo-dodecaborate anion at 4.6 V vs Mg0/2+ (2.0 vs Fc0/+), a value lower than that projected for this anion in organic solvent-based electrolytes and lower than weakly associating bis(trifluoromethylsulfonyl)imide and tetrafluoroborate anions. Solvents such as acetonitrile, 3-methylsulfolane, and 1,1,1,3,3,3-hexafluoroisopropanol are shown to enable the direct measurement of carba- closo-dodecaborate oxidation, where the resultant neutral radical drives passive film formation on the electrode. Second, we employ computational screening to evaluate the impact of functionalization of the parent anion on its stability and find that replacement of the carbon-vertex proton with a more electronegative fluorine or trifluoromethyl ligand increases the oxidative stability and decreases the contact-ion pair formation energy while maintaining reductive stability. This predicted expansion of the electrochemical window for fluorocarba- closo-dodecaborate is experimentally validated. Future work includes evaluation of the viability of these derivative anions as efficient and stable carriers for energy storage as a function of the ionic transport through the resulting surface films formed on candidate cathodes.

Elucidating Solvation Structures for Rational Design of Multivalent Electrolytes-A Review.

Fundamental molecular-level understanding of functional properties of liquid solutions provides an important basis for designing optimized electrolytes for numerous applications. In particular, exhaustive knowledge of solvation structure, stability, and transport properties is critical for developing stable electrolytes for fast-charging and high-energy-density next-generation energy storage systems. Accordingly, there is growing interest in the rational design of electrolytes for beyond lithium-ion systems by tuning the molecular-level interactions of solvate species present in the electrolytes. Here we present a review of the solvation structure of multivalent electrolytes and its impact on the electrochemical performance of these batteries. A direct correlation between solvate species present in the solution and macroscopic properties of electrolytes is sparse for multivalent electrolytes and contradictory results have been reported in the literature. This review aims to illustrate the current understanding, compare results, and highlight future needs and directions to enable the deep understanding needed for the rational design of improved multivalent electrolytes.

Stacking and Electrostatic Interactions Drive the Stereoselectivity of Silylium-Ion Asymmetric Counteranion-Directed Catalysis.

Computational analysis shows that the enantioselectivity of asymmetric Lewis-acid organocatalysis of the Diels-Alder cycloaddition of cyclopentadiene to cinnamates arises from stacking interactions that favor the addition of the diene to the more hindered face of the dienophile, while electrostatic interactions control the diastereoselectivity by selectively stabilizing the endo transition state. These results not only explain the stereoselectivity of these silylium-ion-ACDC reactions but should also guide the development of more effective ion-pairing asymmetric organocatalysts.

Noncovalent Interactions in Organocatalysis and the Prospect of Computational Catalyst Design.

Noncovalent interactions are ubiquitous in organic systems, and can play decisive roles in the outcome of asymmetric organocatalytic reactions. Their prevalence, combined with the often subtle line separating favorable dispersion interactions from unfavorable steric interactions, often complicates the identification of the particular noncovalent interactions responsible for stereoselectivity. Ultimately, the stereoselectivity of most organocatalytic reactions hinges on the balance of both favorable and unfavorable noncovalent interactions in the stereocontrolling transition state (TS). In this Account, we provide an overview of our attempts to understand the role of noncovalent interactions in organocatalyzed reactions and to develop new computational tools for organocatalyst design. Following a brief discussion of noncovalent interactions involving aromatic rings and the associated challenges capturing these effects computationally, we summarize two examples of chiral phosphoric acid catalyzed reactions in which noncovalent interactions play pivotal, although somewhat unexpected, roles. In the first, List's catalytic asymmetric Fischer indole reaction, we show that both π-stacking and CH/π interactions of the substrate with the 3,3'-aryl groups of the catalyst impact the stability of the stereocontrolling TS. However, these noncovalent interactions oppose each other, with π-stacking interactions stabilizing the TS leading to one enantiomer and CH/π interactions preferentially stabilizing the competing TS. Ultimately, the CH/π interactions dominate and, when combined with hydrogen bonding interactions, lead to preferential formation of the observed product. In the second example, a series of phosphoric acid catalyzed asymmetric ring openings of meso-epoxides, we show that noncovalent interactions of the substrates with the 3,3'-aryl groups of the catalyst play only an indirect role in stereoselectivity. Instead, the stereoselectivity of these reactions are driven by the electrostatic stabilization of a fleeting partial positive charge in the SN2-like transition state by the chiral electrostatic environment of the phosphoric acid catalyst. Next, we describe our studies of bipyridine N-oxide and N,N'-dioxide catalyzed alkylation reactions. Based on several examples, we demonstrate that there are many potential arrangements of ligands around a hexacoordinate silicon in the stereocontrolling TS, and one must consider all of these in order to identify the lowest-lying TS structures. We also present a model in which electrostatic interactions between a formyl CH group and a chlorine in these TSs underlie the enantioselectivity of these reactions. Finally, we discuss our efforts to develop computational tools for the screening of potential organocatalyst designs, starting in the context of bipyridine N,N'-dioxide catalyzed alkylation reactions. Our new computational tool kit (AARON) has been used to design highly effective catalysts for the asymmetric propargylation of benzaldehyde, and is currently being used to screen catalysts for other reactions. We conclude with our views on the potential roles of computational chemistry in the future of organocatalyst design.

Biomimetic Total Syntheses of (-)-Leucoridines†A and C through the Dimerization of (-)-Dihydrovalparicine.

Concise biomimetic syntheses of the Strychnos-Strychnos-type bis-indole alkaloids (-)-leucoridine A (1) and C (2) were accomplished through the biomimetic dimerization of (-)-dihydrovalparicine (3). En route to 3, the known alkaloids (+)-geissoschizoline (8) and (-)-dehydrogeissoschizoline (10) were also prepared. DFT calculations were employed to elucidate the mechanism, which favors a stepwise aza-Michael/spirocyclization sequence over the alternate hetero-Diels-Alder cycloaddition reaction.

Enantioselectivity in Catalytic Asymmetric Fischer Indolizations Hinges on the Competition of π-Stacking and CH/π Interactions.

Computational analyses of the first catalytic asymmetric Fischer indolization (J. Am. Chem. Soc. 2011, 133, 18534) reveal that enantioselectivity arises from differences in hydrogen bonding and CH/π interactions between the substrate and catalyst in the operative transition states. This selectivity occurs despite strong π-stacking interactions that reduce the enantioselectivity.