Metal Coordination Crosslinking Research Articles

PH-Responsive Reversible Granular Hydrogels Based on Metal-Binding Mussel-Inspired Peptides.

Taking advantage of their thixotropic behavior, microporosity, and modular properties, granular hydrogels formed from jammed hydrogel microparticles have emerged as an exciting class of soft, injectable materials useful for numerous applications, ranging from the production of biomedical scaffolds for tissue repair to the therapeutic delivery of drugs and cells. Recently, the annealing of hydrogel microparticles in situ to yield a porous bulk scaffold has shown numerous benefits in regenerative medicine, including tissue-repair applications. Current annealing techniques, however, mainly rely either on covalent connections, which produce static scaffolds, or transient supramolecular interactions, which produce dynamic but mechanically weak hydrogels. To address these limitations, we developed microgels functionalized with peptides inspired by the histidine-rich cross-linking domains of marine mussel byssus proteins. Functionalized microgels can reversibly aggregate in situ via metal coordination cross-linking to form microporous, self-healing, and resilient scaffolds at physiological conditions by inclusion of minimal amounts of zinc ions at basic pH. Aggregated granular hydrogels can subsequently be dissociated in the presence of a metal chelator or under acidic conditions. Based on the demonstrated cytocompatibility of these annealed granular hydrogel scaffolds, we believe that these materials could be developed toward applications in regenerative medicine and tissue engineering.

Direct and Efficient Incorporation of DOPA into Resilin-Like Proteins Enables Cross-Linking into Tunable Hydrogels.

3,4-Dihydroxyphenylalanine (DOPA), a naturally occurring yet noncanonical amino acid, endows protein polymers with diverse chemical reactivities and novel functionalities. Although many efforts have been made to incorporate DOPA into proteins, the incorporation efficiency and production titer remain low and severely hinder the exploration of these peculiar proteins for biomaterial fabrication. Here, we report an efficient biosynthetic strategy to produce large amounts of DOPA-incorporated structural proteins for the fabrication of hydrogels with tunable mechanical properties. First, synthetic genes were constructed that encode repetitive resilin-like proteins (RLPs) with varying proportions of tyrosine residues and molecular weights (Mw). Decoding of these genes into RLPs incorporated with DOPA was achieved via mis-aminoacylation of DOPA by endogenous tyrosyl-tRNA synthetase (TyrRS) in recombinant Escherichia coli cells. By developing a stoichiometry-guided two-phase culture strategy, we achieved independent control of the bacterial growth and protein synthesis phases. This enabled hyperproduction of the DOPA-incorporated RLPs at gram-per-liter levels and with a high DOPA incorporation yield of 76-85%. The purified DOPA-containing RLPs were then successfully cross-linked into bulk hydrogels via facile DOPA-Fe3+ complexations. Interestingly, these hydrogels exhibited viscoelastic and self-healing properties that are highly dependent on the catechol content and Mw of the RLPs. Finally, exploration of the molecular cross-linking mechanisms revealed that higher DOPA contents of the proteins would result in the concomitant occurrence of metal coordination and oxidative covalent cross-linking. In summary, our results suggest a useful platform to generate DOPA-functionalized protein materials and provide deeper insights into the gelation systems based on DOPA chemistry.

Tough self-healing polyurethane elastomers based on interpenetrating networks containing multiple hydrogen bond networks, flexible blocks, metal coordination and covalent cross-linking

Currently, it is a challenge to prepare elastomers with high mechanical properties and high self-healing efficiency. In the present work, by mixing a polyurethane (PU) prepolymer with a PU elastomer containing 2-ureido-4[1H]-pyrimidinone (UPy) groups, we obtained a new PU elastomer based on interpenetrating networks. In this condition, four different structures coexist in the interpenetrating polymer networks: covalent cross-linking network, metal-ion coordination network, multiple hydrogen bond network and flexible blocks. The experimental results demonstrated that the presence of metal-ion coordination network, multiple hydrogen bond network and flexible blocks is beneficial for the improvement in healing efficiency, while the coexistence of covalent cross-linking network, metal-ion coordination network and multiple hydrogen bond network benefits to improve the mechanical properties.

Recyclable and high-performance metal coordination bonds cross-linked polybenzimidazole aerogels for flame retardancy, thermal insulation and dyes adsorption applications

Aerogels made from high-performance polymers have attracted great interests in recent years. However, traditional chemical crosslinking process is complex, and makes it difficult to adjust the skeleton structure of aerogels and achieve polymer recovery. Herein, polybenzimidazole aerogels with good mechanical property, uniform porous structure, and specific surface area as high as 660.23 m2/g are facilely prepared through metal coordination. The excellent thermal stability, high flame resistance, and low thermal conductivity make the aerogels good candidates for flame-retarding and high-temperature thermal insulating materials. Notably, the reversible metal coordination crosslinking permits the recycling of aerogels, which has not been realized in previous high-performance polymer aerogels yet. Moreover, the aerogels can repeatedly adsorb dyes from wastewater because of their large specific surface area, good chemical resistance, and abundant polar imidazole groups. This work is expected to provide a simple approach to recyclable, high-performance, and multifunctional polymer aerogels.

Investigation on metal coordination crosslinking nitrile butadiene rubber induced by the synergistic effect between tetramethylthiuram disulfide and cupric sulfate

ABSTRACT In this work, we described a simple method to manufacture a novel nitrile butadiene rubber (NBR) filled with cupric sulfate (CuSO4) and tetramethylthiuram disulfide (TMTD) via simple mixing and heat pressing. Cure characteristics analysis, the motion capability of polymer segments, physical properties, micromorphology, swelling properties and dynamic characteristics were explored comprehensively. The results demonstrated that the three-dimensional network was cross-linked by the metal coordination interactions between the nitrile group and copper ion. The cure curve, tensile strength and crosslinking density exhibited a considerable increasing with the addition of TMTD in NBR/CuSO4 compounds, which was due to the synergistic effect between Cu2+ and Me2NCSn• (n = 1–2) radicals generated by the pyrolysis of TMTD, resulting in the formation of the hyperactive compounds [Cu(SnCNMe2)]•2+ and promoting the coordination crosslinking between -CN and Cu2+ significantly. The NBR/CuSO4/TMTD vulcanizates induced by the synergistic effect would play a crucial role in practical application for the excellent physical properties.

Converting histidine-induced 3D protein arrays in crystals into their 3D analogues in solution by metal coordination cross-linking

Histidine (His) residues represent versatile motifs for designing protein-protein interactions because the protonation state of the imidazole group of His is the only moiety in protein to be significantly pH dependent under physiological conditions. Here we show that, by the designed His motifs nearby the C4 axes, ferritin nanocages arrange in crystals with a simple cubic stacking pattern. The X-ray crystal structures obtained at pH 4.0, 7.0, and 9.0 in conjunction with thermostability analyses reveal the strength of the π–π interactions between two adjacent protein nanocages can be fine-tuned by pH. By using the crystal structural information as a guide, we constructed 3D protein frameworks in solution by a combination of the relatively weak His–His interaction and Ni2+-participated metal coordination with Glu residues from two adjacent protein nanocages. These findings open up a new way of organizing protein building blocks into 3D protein crystalline frameworks.

Coordination crosslinking of helical substituted oligoamide nanorods with Cu(II)

ABSTRACT Substituted oligoamides are short sequences of unnatural amino acids. Oligoamides made entirely of β3 amino acids yield helical monomers that, if N-acylated, assemble into nanorod structures via a supramolecular assembly motif. In this work, coordination crosslinking was used to create complex nanomaterials from oligoamides WKLWEL (KE) and WELWEL (EE) (the letters denote the analogous α-amino acids). Upon Cu(II) addition, atomic force microscopy and small angle neutron scattering revealed morphologic changes specific to KE but absent in EE. Vibration spectroscopy measurements revealed that Cu(II) can coordinate to the amine moieties of the side chains, without direct effect on the backbone amides. While coordination in excess solvent lead to regular nanostructures, fast drying of the sample yielded oligoamide templated crystallization of CuCl2. The metal coordination crosslinking of supramolecular assemblies as reported here is the first realization of a metallosupramolecular framework structure.

Synthesis of Superheat-Resistant Polyimides with Enhanced Dielectric Constant by Introduction of Cu(ΙΙ)-Coordination.

To achieve polyimide-metal complexes with enhanced properties, 5-amine-2-(5-aminopyridin-2-yl)-1-methyl-benzimidazole (PyMePABZ) that contains stiff 2-(2′-pyridyl)benzimidazole (PyBI) was synthesized and exploited to construct the Cu(ΙΙ)-crosslinked polyimides (Cu-PIs). These Cu-PIs exhibited higher dielectric, thermal, and mechanical properties with an increase in Cu2+ content. Among them, their dielectric constants (εrS) were up to 43% superior to that of the neat PI, glass transition temperatures (Tgs) were all over 400 °C, and 5% weight loss temperature (T5%) maintained beyond 500 °C. These data indicate that the metal coordination crosslinking provided a useful guide to develop high performance PIs which possess potential application as useful high temperature capacitors.

Zn(II)-modified imidazole containing polyimide/ZIF-8 mixed matrix membranes for gas separations

Finely tailoring the interfacial interaction to minimize the defective structure in the hybrid membranes is a key to yield a mixed matrix membrane with enhanced gas separation performance. Here, a highly selective mixed matrix membrane based on imidazole containing polyimides and ZIF-8 fillers is reported. The ZIF-8 with imidazole linker offers a more compatible interface with imidazole containing polyimides. At a MOF loading of 20 wt. %, the membranes have a 2.2-fold increase in the gas permeabilities over unfilled polymers with exceptional H2/CH4 and CO2/CH4 gas selectivities of 224 and 58, respectively. The H2 and CO2 gas permeabilities are 78.5 and 20.3 Barrer, respectively. The Zn2+ post-modification enables the formation of metal coordination crosslinking with enhanced polymer/ZIF-8 interaction, as indicated by increased glass transition temperature, improved thermal stability and insoluble properties in organic solvents. With the proper control of Zn2+ treatment conditions, gas permeabilities of H2 and CO2 can be increased to 110.1 and 27.4 Barrer, respectively, with the constant H2/CH4 and CO2/CH4 gas selectivity. For membranes treated at higher Zn2+ concentration, an enhancement in H2/CH4 gas selectivity was observed with the selectivity increasing from 223.9 to 318.3 and H2 permeability around 72.3 Barrer. The separation performance of H2/CH4 for all Zn2+ modified membranes exceeds the 2008 Robeson upper bound. This facile approach to tune polymer/MOF interaction via metal ion modification promotes the rational design of high-performance gas separation membranes.

A Metal Coordination Crosslinking Novel High Performance Thermoset

N-substituted crosslinking polybenzimidazole pyridine sulfone, as novel high performance functional polymers, was prepared by the coordination of N-substituted polybenzimidazole pyridine sulfone (Py-N-PBIS) ligand with varying content of metallic ion (Co2+, Ni2+, Zn2+). The structures of the polymers were characterized by means of FT-IR and 1H NMR spectroscopy, and the results showed a good agreement with the proposed structures. TGA measurements exhibited that the crosslinking polymers possessed good thermal stability with high thermal decomposition temperatures (thermally stable up to 405-510 °C). Additionally, the thermal stability of the coordination polymers was improved constantly with the increasing of the content of Co2+, Ni2+ or Zn2+. The metal coordination crosslinking N-substituted polybenzimidazole pyridine sulfone could be considered as a novel high performance thermoset.

Metal-coordination crosslinked N-polyindoles as recyclable high-performance thermosets and nondestructive detection for their tensile strength and glass transition temperature.

Metal coordination crosslinking between stiff N-polyindole chains was constructed, and the crosslinked films exhibited high tensile strength, high heat resistance and excellent polar solvent resistance. The noncovalent crosslinking can be further removed via external pyrophosphate, which endows the crosslinked polymer with a recyclable behavior. The tensile strength and glass transition temperature of the polymers can be nondestructively detected by taking advantage of the fluorescence quenching effect of metal coordination to the adjacent bipyridine structure.

Mussel-Inspired Self-Healing Double-Cross-Linked Hydrogels by Controlled Combination of Metal Coordination and Covalent Cross-Linking.

Mussel-inspired hydrogels held together by reversible catecholato-metal coordination bonds have recently drawn great attention owing to their attractive self-healing, viscoelastic and adhesive properties together with their pH-responsive nature. A major challenge in these systems is to orchestrate the degree of catechol oxidation that occurs under alkaline conditions in air and has a great impact on the aforementioned properties because it introduces irreversible covalent cross-links to the system, which stiffens the hydrogels but consume catechols needed for self-healing. Herein, we present a catechol-based hydrogel design that allows for the degree of oxidative covalent cross-linking to be controlled. Double cross-linked hydrogels with tunable stiffness are constructed by adding the oxidizable catechol analogue, tannic acid, to an oxidation-resistant hydrogel construct held together by coordination of the dihydroxy functionality of 1-(2'-carboxyethyl)-2-methyl-3-hydroxy-4-pyridinone to trivalent metal ions. By varying the amount of tannic acid, the hydrogel stiffness can be customized to a given application while retaining the self-healing capabilities of the hydrogel's coordination chemical component.

Doubly-crosslinked, emulsion-templated hydrogels through reversible metal coordination

Double crosslinking can be used to significantly improve the mechanical properties of gels. Here, doubly-crosslinked hydrogel polyHIPEs (DC-PHs), which exhibited rapid water absorption, enhanced mechanical properties, and shape memory behaviour, were generated by forming metal coordination crosslinks within covalently crosslinked hydrogel polyHIPEs (HG-PHs), hydrogels formed by templating within oil-in-water high internal phase emulsions (HIPEs). The HG-PHs were based on acrylamide (AAm) and sodium acrylate (NaA), which provided −COO− groups, and were covalently crosslinked with N,N'-methylenebisacrylamide. The metal coordination crosslinking between the −COO− groups and Fe3+ was generated by adding FeCl3 to the HG-PHs. The interconnected macroporous structures and the rapid water absorption of the HG-PHs were preserved in the DC-PHs, while the mechanical properties, in both the swollen and dry states, were significantly enhanced. The metal coordination crosslinking was easily removed through the light-induced reduction of the Fe3+ in the presence of citric acid. The reversibility of the metal coordination crosslinking endowed the DC-PHs with a multiple-cycle shape memory behaviour.

Facile synthesis and properties of the chemo-reversible and highly tunable metallogels based on polydicyclopentadiene

We report our study on the facile synthesis of reversible metallogels (metal coordinated polymer network) with tunable crosslinking density and metal ions, and the properties of the metallogels. The metallogels were prepared via non-covalent metal coordination crosslinking of functionalized soluble polydicyclopentadiene (sPDCPD). sPDCPD was first synthesized by ring-opening metathesis polymerization (ROMP) and functionalized using 3,6-di-2- pyridinyl-1,2,4,5-tetrazine (DPTZ) via the inverse electron-demand Diels-Alder (iEDDA) reaction. The structures of the synthesized macromolecular ligands DPTZ-sPDCPD were characterized by size-exclusion chromatography (SEC), Fourier transfer infrared spectroscopy (FITR) and nuclear magnetic resonance (NMR). Subsequently, DPTZ-sPDCPD was crosslinked by adding metal ions, e.g., Zinc or Nickel salts. Using the series of metallogels prepared, we investigated the effects of crosslinking densities, metal ion types, solvent types and polymer concentrations on the rheological properties of the prepared metallogels. We also demonstrated the excellent reversibility of the metallogel via repeated reclamation of the macromolecular ligands and metal coordination.

Siderophores and mussel foot proteins: the role of catechol, cations, and metal coordination in surface adhesion.

Metal coordination, hydrogen bonding, redox reactions, and covalent crosslinking are seemingly disparate chemical and physicochemical processes that are all accomplished in natural materials by the catechol functional group. This review focuses on the reactivity of catechols in tris-2,3-dihydroxybenzoyl-containing microbial siderophores and synthetic analogs, as well as Dopa-(3,4-dihydroxyphenylalanine)-containing mussel foot proteins that adhere to surfaces in aqueous conditions. Mussel foot proteins with a high content of Dopa and cationic amino acids, Lys and Arg, adhere strongly to mica, an aluminosilicate mineral, in aqueous conditions. The siderophore cyclic trichrysobactin, tris-(2,3-dihydroxybenzoyl-D-Lys-L-Ser) and related synthetic analogs in which the tri-Ser macrolactone is replaced by Tren, tris-(2-aminoethyl)amine, also adheres strongly to mica. Variation in the nature of the catechol and cationic groups in synthetic analogs reveals a synergism between the cationic amino acid and the catechol, required for strong aqueous adhesion. Autoxidation and iron(III)-catalyzed oxidation of 2,3-dihydroxy and 3,4-dihydroxy catechols are also considered. These siderophore analogs provide a platform to understand catechol interactions and reactivity on surfaces, which may ultimately improve the design of synthetic materials that address diverse challenges in medicine, materials science, as well as other disciplines, in which surface adhesion in aqueous conditions is important.

Tough, rapid-recovery composite hydrogels fabricated via synergistic core-shell microgel covalent bonding and Fe3+ coordination cross-linking.

We developed tough, rapid-recovery composite hydrogels that are fabricated via core-shell microgel covalent bonding and Fe3+ dynamic metal coordination cross-linking. First, core-shell microgels are used as cross-linking agents and initiators to prepare homogeneous hydrogel networks with rapid recovery in the absence of an organic cross-linking agent. The toughness and recoverability of the composite hydrogels can be improved by adding the dynamic reversibility of ionic cross-linking. Owing to the synergistic effect of microgel covalent bonding, Fe3+ coordination cross-linking, and H-bond cross-linking, the multi-cross-linked composite hydrogels exhibit excellent toughness and a fast recovery rate. These characteristics demonstrate that the dynamic reversibility of the ionic cross-linking can significantly improve the toughness and recoverability of the hydrogels. In addition, the core-shell microgels play a key role in toughening the hydrogels and accelerating their recovery by transferring stress to grafted polymer chains and homogenizing the hydrogel network.

Metal-coordination: Using one of nature's tricks to control soft material mechanics.

Growing evidence supports a critical role of dynamic metal-coordination crosslinking in soft biological material properties such as self-healing and underwater adhesion1. Using bio-inspired metal-coordinating polymers, initial efforts to mimic these properties have shown promise2. Here we demonstrate how bio-inspired aqueous polymer network mechanics can be easily controlled via metal-coordination crosslink dynamics; metal ion-based crosslink stability control allows aqueous polymer network relaxation times to be finely tuned over several orders of magnitude. In addition to further biological material insights, our demonstration of this compositional scaling mechanism should provide inspiration for new polymer material property-control designs.

アクリロニトリル・ブタジエンゴムの配位架橋とその物性

A novel vulcanization method, vulcanization through metal coordination crosslinking, is developed. In this method copper sulfate (CuSO4) particles are simply incorporated into acrylonitrile-butadiene rubber (NBR), and no other vulcanizing agent nor additional additives are involved. It is totally different from traditional covalent or non-covalent vulcanization approaches of rubber. The crosslinking of CuSO4 particles filled NBR was induced by in situ coordination between nitrogen atoms of nitrile groups (-CN) and copper ions (Cu2+) from CuSO4. Several factors including heating temperature and time, the contents of CuSO4 and nitrile groups were investigated in this paper. The generation of a core (CuSO4 solid particle) -shell (adherent NBR) structure leads to the excellent mechanical properties of the coordination vulcanizates.