Selective bond cleavage strategies for chemical recycling of thermosets and their composites

The surface-induced dissociation (SID) of six model peptides containing either methionine sulfoxide or aspartic acid (GAILM(O)GAILR, GAILM(O)GAILK, GAILM(O)GAILA, GAILDGAILR, GAILDGAILK, and GAILDGAILA) have been studied using a specially configured Fourier transform ion-cyclotron resonance mass spectrometer (FT-ICR MS). In particular, we have investigated the energetics and dynamics associated with (i) preferential cleavage of the methionine sulfoxide side chain via the loss of CH3SOH (64 Da), and (ii) preferential cleavage of the amide bond C-terminal to aspartic acid. The role of proton mobility in these selective bond cleavage reactions was examined by changing the C-terminal residue of the peptide from arginine (nonmobile proton conditions) to lysine (partially mobile proton conditions) to alanine (mobile proton conditions). Time- and energy-resolved fragmentation efficiency curves (TFECs) reveal that selective cleavages due to the methionine sulfoxide and aspartic acid residues are characterized by slow fragmentation kinetics. RRKM modeling of the experimental data suggests that the slow kinetics is associated with large negative entropy effects and these may be due to the presence of rearrangements prior to fragmentation. It was found that the Arrhenius pre-exponential factor (A) for peptide fragmentations occurring via selective bond cleavages are 1-2 orders of magnitude lower than nonselective peptide fragmentation reactions, while the dissociation threshold (E0) is relatively invariant. This means that selective bond cleavage is kinetically disfavored compared to nonselective amide bond cleavage. It was also found that the energetics and dynamics for the preferential loss of CH3SOH from peptide ions containing methionine sulfoxide are very similar to selective C-terminal amide bond cleavage at the aspartic acid residue. These results suggest that while preferential cleavage can compete with amide bond cleavage energetically, dynamically, these processes are much slower compared to amide bond cleavage, explaining why these selective bond cleavages are not observed if fragmentation is performed under mobile proton conditions. This study further affirms that fragmentation of peptide ions in the gas phase are predominantly governed by entropic effects.

Plastic recycling strategies to combat rapidly increasing waste buildup are of utmost environmental importance. Chemical recycling to monomers has emerged as a powerful strategy that enables infinite recyclability through depolymerization. However, methods for chemical recycling to monomers typically rely on bulk heating of polymers, which leads to unselective depolymerization in complex polymer mixtures and the formation of degradation byproducts. Here, we report a selective chemical recycling strategy facilitated by photothermal carbon quantum dots under visible light irradiation. Upon photoexcitation, we found that carbon quantum dots generate thermal gradients that induce depolymerization of various polymer classes, including commodity and postconsumer waste plastics, in a solvent-free system. This method also provides selective depolymerization in a mixture of polymers, not possible by bulk heating alone, enabled by localized photothermal heat gradients and the subsequent spatial control imparted over radical generation. Photothermal conversion by metal-free nanomaterials facilitates chemical recycling to monomers, an important approach in addressing the plastic waste crisis. More broadly, photothermal catalysis enables challenging C-C bond cleavages with the generality of heating but without indiscriminate side reactions typical of bulk thermolysis processes.

Engineering carbon vacancies in porous carbon nitride to enhance photocatalytic selective CC bond cleavage in lignin models.

New chemical reagents and methods have been employed for mass spectrometry (MS)-based proteomics investigations. Many chemical reagents are synthesized to be covalently attached to biomolecules, especially peptides and proteins. The properties of the resulting peptide conjugates are characterized by various tandem mass spectrometric techniques (e.g., collision-induced dissociation (CID), electron capture dissociation (ECD), electron transfer dissociation (ETD), infrared multiphoton dissociation (IRMPD), and free radical initiated peptide sequencing (FRIPS)). In Chapter 2, the effect of high electron affinity tags in ECD and ETD is investigated using their peptide conjugates. The initial intramolecular electron transfer from the high-lying Rydberg states to the covalently attached high electron affinity tag occurs in competition with the Coulomb stabilized π* orbitals of the amide bonds in the model peptides. This leads to the inhibition of the normal sequence of ECD and ETD processes, yielding no backbone fragmentations. In Chapter 3, selective disulfide bond cleavages are observed by the FRIPS method. A newly prepared TEMPO-based FRIPS reagent is labeled to model peptides containing disulfide bonds and subject to CID to monitor free radical induced cleavages. Highly selective C–S and S–S bond cleavages are observed and their reaction mechanisms are proposed. In Chapter 4, novel Caltech isobaric tags (CITs) for protein quantification are developed and validated using various model samples. A newly discovered low-energy gas-phase fragmentation pathway, a nucleophilic substitution of the N3 in the 1,2,3-triazole ring generated by copper-catalyzed azide-alkyne cycloaddition (CuAAC) inspired us to create CITs. This selective cleavage is applied to the formation of the reporter ions to quantify protein expression level in cells. Chapter 5 describes clickable cross-linkers (CXLs) developed for elucidation of three-dimensional protein structures and protein-protein interactions (PPIs). In CXLs, cross-linking reactions are separated from the conjugation of affinity tags, avoiding steric hindrance. Cross-linked peptides are enriched from the complex mixture of yeast lysate and cross-linked ubiquitin digests using avidin affinity chromatography, showing high sensitivity of the CXL-based analysis. The low-energy pathway used for CIT reagents is also adopted to produce the reporter ion, filtering MS/MS scans of cross-linked peptides from those of unmodified peptides.

Selective C–C and C–O bond cleavage strategies for the thermochemical upgrading of (hemi)cellulosic biomass

Plastic wastes have contaminated both land and ocean environments throughout the world and hence, it is urgent to explore feasible yet cost‐effective recycling and reuse strategies for plastic wastes in order to solve this menacing problem. The so‐called “back‐to‐oligomer” chemical recycling process may be an efficient and affordable strategy to solve this global issue. However, structural control of the produced oligomers has never been achieved using this strategy until now. Here, it is reported that polymeric blends with different structures and molecular weights can be successfully tailored to homo‐/co‐ oligomers with a predesigned molecular weight (MW) and an acceptable MW distribution, just by changing the feed ratio or adding a predetermined amount of alcohol. Moreover, the molecular weight of the produced oligomers can be further increased by in situ addition of the corresponding monomers into the reaction system. This recycling strategy provides an efficient method for the preparation of high‐value polymers from plastic wastes without prior sorting of mixed plastic wastes. The newly developed process through polymer chemistry and environmental engineering not only helps with the selective recycling of polyester blends, but it can also enable the recycling of polymeric mixtures composed of polyester and polycarbonate.

As one of the most widely used thermosets due to its excellent performances, epoxy resin (EP) is widely used in various fields and often employed as a component of composite actuator devices, strengthening their mechanical properties. However, the expanding production of EP inevitably leads to the accumulation of waste end-of-life equipment and the corresponding increasingly serious environmental problems. This review summarizes the recycling strategies of EP, divided into two perspectives: recycling from wastes and sources. Chemical recycling is expected to be the future of waste EP treatment, and we discuss the chemical recycling methods of existing waste EP based on different mechanisms, including the selective cleavage of ester bonds, C–N bonds, and C–O bonds. On the other hand, epoxy vitrimer networks based on various dynamic covalent linkages are also outlined, which can respond to multiple external stimuli and provide materials with recyclability from the origin. Therefore, the use of epoxy vitrimer actuators can prevent waste generation throughout the whole lifecycle. We present some issues of concern in both waste-based and source-based recycling strategies and emphasize the significance of scaling-up. Finally, we summarized the current situation and present some future perspectives with the aim of making practical contributions to environmental issues.

Selective and nonselective cleavages in ion trap low-energy collision-induced dissociation (CID) experiments of the fragments generated from in-source decay (ISD) with matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS) of intact proteins are described in both positive and negative ion modes. The MALDI-ISD spectra of the proteins demonstrate common, discontinuous, abundant c- and z'-ions originating from cleavage at the N-Cα bond of Xxx-Asp/Asn and Gly-Xxx residues in both positive- and negative-ion modes. The positive ion CID of the c- and z'-ions resulted in product ions originating from selective cleavage at Asp-Xxx, Glu-Xxx and Cys-Xxx residues. Nonselective cleavage product ions rationalized by the mechanism of a "mobile proton" are also observed in positive ion CID spectra. Negative ion CID of the ISD fragments results in complex product ions accompanied by the loss of neutrals from b-, c-, and y-ions. The most characteristic feature of negative ion CID is selective cleavage of the peptide bonds of acidic residues, Xxx-Asp/Glu/Cys. A definite influence of α-helix on the CID product ions was not obtained. However, the results from positive ion and negative ion CID of the MALDI-ISD fragments that may have long α-helical domains suggest that acidic residues in helix-free regions tend to degrade more than those in helical regions.

Photocatalytic selective oxidation of glycerol to formic acid and formaldehyde over surface cobalt-doped titanium dioxide.

In recent years, several six-membered ring phosph(on)ates and phosphonamides have been reported as potent prodrugs against liver diseases such as hepatitis B and C and also as antitumor agents. Apparently, the success for their biological activity depends on the selective cleavage of the C4-O3 bond within the respective P-heterocyclic ring. Empirical observations have suggested that the group attached to the C4 position (aryl or pyridyl group) is responsible for the selective cleavage. In this regard, we show in the present work that the configuration at the P-atom, the conformation of the P-heterocyclic ring, and particularly, the anomeric effect are involved in the spontaneous and selective cleavage of the C4-O3 bond in cyclic phosph(on)ates. We arrived at this assumption based on the conformational and configurational study of simple model phosphates and phosphonates, where it was observed that the spontaneous conversion of unstable six-membered ring phosphates to their most stable six-membered ring phosphate (4d, 6d and 7d to 5d), by a selective C4-O3 bond cleavage, depends on both: the stereochemistry of the aryl group at C4 and the electronic nature of the substituent attached to the P-atom. Thus, we postulated that the anomeric effect weakens the C4-O3 bond within the 1,3,2-dioxaphosphorinane ring, favoring thus their selective cleavage and spontaneous conversion, similarly to the proposed mechanistic mode of action of six-membered ring P-heterocyclic prodrugs.

Recycling strategies for polyhydroxyalkanoate-based waste materials: An overview

Vitrimer is a new type of material that combine the advantages of thermoplastic and thermoset materials. The rapid dynamic exchange reactions at high temperature allow the topology of cross-linked networks to change and rearrange while keeping material structures and properties intact. The concept of vitrimer has emerged to provide a viable strategy for the recycling of high-performance polymer materials, and lots of research works have been carried out for the development of various types of vitrimers. In addition, the recycling strategies for vitrimers are particularly important to determine the performance and potential applications of the recovered materials. Therefore, it is an innovative and valuable perspective to discuss vitrimer materials according to their different recycling strategies. In this review, we start with a brief overview of vitrimers, and then, focus on recycling strategies for vitrimers. Specifically, we highlight the advantages and disadvantages of the two different recycling strategies: physical and chemical recycling methods, and then explore the feasibility of upcycling vitrimers using 3D printing technology. Finally, the impact of recycling strategies on vitrimer materials and the prospects for maximizing the use of vitrimer materials are discussed.

Hydrothermal synthesis of bismuth cluster-modified covalent triazine framework composite photocatalysts with effective performance in selective C[sbnd]C bond cleavage of lignin

The selective carbon-carbon bond cleavage of 1,2-diols in the presence of an iron-porphyrin complex, molecular oxygen, and 1-benzyl-3-carbamoyl-1,4-dihydropyridine is reported. The C-C bonds of aryl-substituted ethane-1,2-diols were cleaved exclusively to aldehydes or ketones as the oxidation products at room temperature. The reaction rates were influenced by the steric hindrance of the substituents both in the catalysts and diols, but no differences in the reactivities were observed between the two stereo isomers (meso and dl) of diols. A kinetic analysis of this bond cleavage reaction is consistent with the reaction mechanism consisting of the initial binding of diol on the active catalyst forming an intermediate complex and its subsequent breakdown in the rate-determining step of the catalytic cycle. The initial binding step is favorable for electron-deficient diols and is influenced by steric hindrance, whereas the rate-determining bond cleavage step is accelerated by electron-rich diols and unaffected by the steric effect. The mechanism of this diol cleavage reaction is discussed on the basis of these observations.

Mapping Disulfide Bonds in Insulin with the Route 66 Method: Selective Cleavage of S [sbnd]C Bonds Using Alkali and Alkaline Earth Metal Enolate Complexes