Sustainable and Scalable Synthesis of Acetal-Containing Polyols as a Platform for Circular Polyurethanes.

Polyurethanes (PUs) are highly versatile polymers widely utilized across industries. However, chemical recycling of PU poses significant challenges due to the harsh conditions required and the formation of complex mixtures of oligomers upon depolymerization. Addressing this inherent lack of recyclability, we developed closed-loop recyclable PU materials by integrating cleavable acetal groups. We present a sustainable and scalable synthesis method for acetal-containing polyols (APs) through aldehyde-diol polycondensation, utilizing reusable heterogeneous catalysts. Three APs with different hydrolytic stabilities depending on the structure of acetal groups were synthesized from formaldehyde, acetaldehyde, and propionaldehyde with 1,6-hexanediol (H16). These APs were employed alongside 4,4'-methylene diisocyanate (MDI) for preparation of PU materials. The resulting PUs exhibited mechanical properties comparable to or surpassing those of conventional PUs, while demonstrating excellent recyclability under acidic conditions. Notably, hydrolysis of PU materials based on acetaldehyde-derived APs yielded remarkable monomer recovery rates, with 89 % for H16 and 84 % for 4,4'-methylenedianiline, a precursor to MDI. Furthermore, we successfully demonstrated closed-loop recycling by synthesizing APs from recovered H16, resulting in PU materials with identical properties to the original PU. This achievement highlights the potential for establishing a closed-loop recycling system for acetal-containing PUs, contributing to the advancement of a sustainable and circular economy.

RNA-inspired phosphate diester dynamic covalent networks.

Neighboring group assisted rearrangement substantially increases relaxation rates in dynamic covalent networks, allowing easier (re)processing of these materials. In this work, we introduce a dynamic covalent network with anionic phosphate diesters as the sole dynamic group, incorporating ß-hydroxy groups as a neighboring group, mimicking the self-cleaving backbone structure of RNA. The diester-based networks have slightly slower dynamics, but significantly better hydrolytic (and thermal) stability than analogous phosphate triester-based networks. Catalysis by the ß-hydroxy group is vital for fast network rearrangement to occur, while the nature of the counterion has a negligible effect on the relaxation rate. Variable temperature 31P solid-state NMR demonstrated a dissociative bond rearrangement mechanism to be operative.

Benzene Tetraamide: A Covalent Supramolecular Dual Motif in Dynamic Covalent Polymer Networks.

In dynamic polyamide networks, 1,2,4,5-benzene tetraamide (B4A) units act simultaneously as a dynamic covalent cross-linker and as supramolecular stacking motif. This results in materials with a rubbery plateau modulus that is about 20 times higher than that of a corresponding reference network in which the supramolecular interaction is suppressed. In branched polyamides with the same B4A dynamic motif, hydrogen bonding and stacking lead to strong and reversible supramolecular networks, whereas a branched polyamide with the nonstacking reference linker is a viscous liquid under the same conditions. Wide-angle X-ray scattering and variable-temperature infrared experiments confirm that covalent cross-linking and stacking cooperatively contribute to the dynamics of the network. Stress relaxation in the reference network is dominated by a single mode related to the dynamic covalent chemistry, whereas relaxation in the B4A network has additional modes assigned to the stacking dynamics.

Hepatic Spheroid Formation on Carbohydrate-Functionalized Supramolecular Hydrogels.

Two synthetic supramolecular hydrogels, formed from bis-urea amphiphiles containing lactobionic acid (LBA) and maltobionic acid (MBA) bioactive ligands, are applied as cell culture matrices in vitro. Their fibrillary and dynamic nature mimics essential features of the extracellular matrix (ECM). The carbohydrate amphiphiles self-assemble into long supramolecular fibers in water, and hydrogels are formed by physical entanglement of fibers through bundling. Gels of both amphiphiles exhibit good self-healing behavior, but remarkably different stiffnesses. They display excellent bioactive properties in hepatic cell cultures. Both carbohydrate ligands used are proposed to bind to asialoglycoprotein receptors (ASGPRs) in hepatic cells, thus inducing spheroid formation when seeding hepatic HepG2 cells on both supramolecular hydrogels. Ligand nature, ligand density, and hydrogel stiffness influence cell migration and spheroid size and number. The results illustrate the potential of self-assembled, carbohydrate-functionalized hydrogels as matrices for liver tissue engineering.

Photo-Crosslinked Coumarin-Containing Bis-Urea Amphiphile Hydrogels.

The design of photo-responsive supramolecular hydrogels based on coumarin dimerization and de-dimerization is described. The photo-responsive coumarin unit is chemically incorporated into an oligo(ethylene glycol) (OEG) bis-urea amphiphile that is capable of co-assembling with non-functionalized OEG amphiphile, to form supramolecular fibers. UV light with two different wavelengths (365 nm and 254 nm) is employed to induce a photo-reversible dimerization and de-dimerization process of coumarin units, respectively. The co-assembled solutions could be photo-crosslinked to induce a sol-to-gel transition through dimerization of coumarin with 365 nm UV light, and de-dimerization occurs with 254 nm UV light, to provide a weaker gel. In this system, the mechanical strength of supramolecular hydrogels can be tuned using the irradiation time, providing precise control of gelation in a supramolecular hydrogelator.

Polymer Mechanochromism from Force-Tuned Excited-State Intramolecular Proton Transfer.

Real-time monitoring of strain/stress in polymers is a big challenge to date. Herein, we for the first time report an ESIPT (excited-state intramolecular proton transfer)-based mechanochromic mechanophore (MM). The synthesis of target MM PhMz-4OH [(2-hydroxyphenyl)benzimidazole with four aliphatic hydroxyls] is quite facile. PhMz-4OH possesses characteristic dual emissions, and its ESIPT activity is greatly affected by steric hindrance. Then, PhMz-4OH was covalently linked into polyurethane chains (PhMz-4OH@PU). Upon stretching, the PhMz-4OH@PU films showed fluorescence color change and spectral variation with the increase in enol emission and blueshift of keto emission due to the force-induced torsion of the dihedral angle between the proton donor and the proton acceptor. The PhMz-4OH@PU films with high mechanophore concentrations (>0.36 mol %) might undergo a two-stage force-responsive process, including torsion of the dihedral angle via force-induced disaggregation and direct chain-transduced force-induced torsion of the dihedral angle. The intensity ratio of enol emission to keto emission (IE/IK) shows a quantitative correlation with elongation, and real-time strain sensing is achieved. PhMz-4OH is a successful type II MM (without covalent bond scission) and displays high sensitivity and excellent reversibility to stress. Two control structures PhMz-NH2 and PhMz-2OH were also embedded into PU but no spectral or color changes were detected, further confirming that mechanochromism of PhMz-4OH@PU films arises from the chain-transduced force. Density function theory (DFT) calculation was performed to study the force-tuned ESIPT process theoretically and rationalize the experimental results. This study might lay the foundation for real-time stress/strain sensing in practical applications.

Dynamic Polyamide Networks via Amide-Imide Exchange.

The diamide-imide equilibrium was successfully exploited for the synthesis of dynamic covalent polymer networks in which a dissociative bond exchange mechanism leads to high processibility at temperatures above ≈110 °C. Dynamic covalent networks bridge the gap between thermosets and thermoplastic polymers. At the operating temperature, when the network is fixed, dynamic covalent networks are elastic solids, while at high temperatures, chemical exchange reactions turn the network into a processible viscoelastic material. Upon heating a dissociative network, the viscosity may also decrease due to a shift of the chemical equilibrium; in such materials, the balance between processibility and excessive flow is important. In this study, a network is prepared that upon heating to above ≈110 °C dissociates to a significant extent due to a shift in the amide-imide equilibrium of a bisimide, pyromellitic diimide, in combination with poly(tetrahydrofuran) diamines. At room temperature, the resulting materials are elastic rubbers with a tensile modulus of 2-10 MPa, and they become predominantly viscous above a temperature of approximately 110 °C, which is dependent on the stoichiometry of the components. The diamide-imide equilibrium was studied in model reactions with NMR, and variable temperature infrared (IR) spectroscopy was used to investigate the temperature dependence of the equilibrium in the network. The temperature-dependent mechanical properties of the networks were found to be fully reversible, and the material could be reprocessed several times without loss of properties such as modulus or strain at break. The high processibility of these networks at elevated temperatures creates opportunities in additive manufacturing applications such as selective laser sintering.

Efficient Exchange in a Bioinspired Dynamic Covalent Polymer Network via a Cyclic Phosphate Triester Intermediate.

Bond exchange via neighboring group-assisted reactions in dynamic covalent networks results in efficient mechanical relaxation. In Nature, the high reactivity of RNA toward nucleophilic substitution is largely attributed to the formation of a cyclic phosphate ester intermediate via neighboring group participation. We took inspiration from RNA to develop a dynamic covalent network based on ß-hydroxyl-mediated transesterifications of hydroxyethyl phosphate triesters. A simple one-step synthetic strategy provided a network containing phosphate triesters with a pendant hydroxyethyl group. 31P solid-state NMR demonstrated that a cyclic phosphate triester is an intermediate in transesterification, leading to dissociative network rearrangement. Significant viscous flow at 60-100 °C makes the material suitable for fast processing via extrusion and compression molding.

Elucidating dynamic behavior of synthetic supramolecular polymers in water by hydrogen/deuterium exchange mass spectrometry.

A comprehensive understanding of the structure, self-assembly mechanism, and dynamics of one-dimensional supramolecular polymers in water is essential for their application as biomaterials. Although a plethora of techniques are available to study the first two properties, there is a paucity in possibilities to study dynamic exchange of monomers between supramolecular polymers in solution. We recently introduced hydrogen/deuterium exchange mass spectrometry (HDX-MS) to characterize the dynamic nature of synthetic supramolecular polymers with only a minimal perturbation of the chemical structure. To further expand the application of this powerful technique some essential experimental aspects have been reaffirmed and the technique has been applied to a diverse library of assemblies. HDX-MS is widely applicable if there are exchangeable hydrogen atoms protected from direct contact with the solvent and if the monomer concentration is sufficiently high to ensure the presence of supramolecular polymers during dilution. In addition, we demonstrate that the kinetic behavior as probed by HDX-MS is influenced by the internal order within the supramolecular polymers and by the self-assembly mechanism.

Correction: Mechanochemical tools for polymer materials.

Correction for 'Mechanochemical tools for polymer materials' by Yinjun Chen et al., Chem. Soc. Rev., 2021, 50, 4100-4140, DOI: 10.1039/D0CS00940G.

Role of Acetate Anions in the Catalytic Formation of Isocyanurates from Aromatic Isocyanates.

The formation of isocyanurates via cyclotrimerization of aromatic isocyanates is widely used to enhance the physical properties of a variety of polyurethanes. The most commonly used catalysts in industries are carboxylates for which the exact catalytically active species have remained controversial. We investigated how acetate and other carboxylates react with aromatic isocyanates in a stepwise manner and identified that the carboxylates are only precatalysts in the reaction. The reaction of carboxylates with an excess of aromatic isocyanates leads to irreversible formation of corresponding deprotonated amide species that are strongly nucleophilic and basic. As a result, they are active catalysts during the nucleophilic anionic trimerization, but can also deprotonate urethane and urea species present, which in turn catalyze the isocyanurate formation. The current study also shows how quantum chemical calculations can be used to direct spectroscopic identification of reactive intermediates formed during the active catalytic cycle with predictive accuracy.

Mechanochemical tools for polymer materials.

Mechanochemistry provides a unique approach to investigate macroscopic deformation, failure and healing of polymer materials. The development of mechanophores - molecular units that respond to mechanical force - has been instrumental in the success of this endeavor. This review aims to provide a critical evaluation of the large variety of mechanophores reported in literature, and to assess the molecular and macroscopic factors that determine their activation. Applications in materials science are highlighted, and challenges in polymer mechanochemistry are discussed.

Pyranine Based Ion-Paired Complex as a Mechanophore in Polyurethanes.

A new mechanophore for polyurethane thermoplastic elastomers based on ion-paired complexes is developed. 8-(2-hydroxyethoxy)pyrene-1,3,6-trisulfonate (HEPTS) is incorporated into polyurethanes as an end-capper and aggregates in apolar media. Aggregation of the ionic HEPTS end groups in solution depends on concentration solvent polarity. The addition of dimethylformamide to a tetrahydrofuran solution of the polymer results in the dissociation of the aggregates and a significant shift in fluorescence emission from yellow to blue. The same shift in fluorescence emission is induced by stretching the solid polymer at strains larger than 1 and stresses above 7.5 MPa, with a clear increase above 12.5 MPa. Strain induced dissociation of HEPTS aggregates not attached to the polymer chain leads to fluorescence changes that are much less reproducible.

Nanoporous Films with Photoswitchable Absorption Kinetics Based on Polymerizable Columnar Discotic Liquid Crystals.

A photoresponsive nanoporous polymer film has been produced from the templated self-assembly of a columnar liquid crystal containing azo units. A liquid crystalline complex of polymerizable azobenzoic acid and a tris-benzimidazolyl benzene template molecule was cross-linked via thiol-ene radical copolymerization with dodecanedithiol. Subsequent removal of the template yielded nanoporous polymer films with pores of approximately 1 nm in diameter. Both trans-cis and cis-trans photoisomerizations of azobenzoic acid took place in the porous films. At room temperature, the cis isomer was sufficiently long-lived to establish a difference in dye absorption kinetics of the two isomers. The cationic dye rhodamine 6G was bound to both isomers, but the rate of binding to films enriched in the cis isomer was 8 times faster.

Intramolecularly Catalyzed Dynamic Polyester Networks Using Neighboring Carboxylic and Sulfonic Acid Groups.

Dynamic covalent bonds in a polymer network lead to plasticity, reshapability, and potential recyclability at elevated temperatures in combination with solvent-resistance and better dimensional stability at lower temperatures. Here we report a simple one-step procedure for the catalyst-free preparation and intramolecularly catalyzed stress-relaxation of dynamic polyester networks. The procedure is based on the coupling of branched OH-end functional polyesters (functionality ≥ 3) by pyromellitic dianhydride (PMDA) or 2,5-bis(methoxy-carbonyl) benzenesulfonic acid resulting in ester linkages with, respectively, a COOH or a SO3H group in a position ortho to the ester bond. This approach leads to an efficient external catalyst-free dynamic polyester network, in which the topology rearrangements occur via a dissociative mechanism involving anhydrides. The SO3H-containing network is particularly interesting, as it shows the fastest stress relaxation and does not suffer from unwanted additional transesterification reactions, as was observed in the COOH-containing network.

Phosphate Triester Dynamic Covalent Networks.

Dynamic covalent networks are a class of polymeric materials that combine the merits of classical thermosets and thermoplastics, in terms of mechanical properties and reprocessability, in one material. Various dynamic covalent chemistries have thus been implemented in polymeric materials with recent interests shifting toward chemistries that would allow rearrangements in network topology without the aid of external catalysts. Here we introduce transesterification in phosphate triesters as a new dynamic covalent chemistry in polymeric networks. A simple one-step synthetic strategy has been utilized to synthesize polytetrahydrofuran networks with phosphate triester cross-links. The materials showed finite viscous flow at elevated temperatures via transesterification at the cross-links without externally added catalyst. This approach provides an easy method for cross-linking OH-end-functionalized polyethers and has the potential for general use with other OH-functionalized polymers.

Suppressing depolarization by tail substitution in an organic supramolecular ferroelectric.

Despite being very well established in the field of electro-optics, ferroelectric liquid crystals so far lacked interest from a ferroelectric device perspective due to a typically high operating temperature, a modest remnant polarization and/or poor polarization retention. Here, we experimentally demonstrate how simple structural modification of a prototypical ferroelectric liquid-crystal benzene-1,3,5-trisamide (BTA) - introduction of branched-tail substituents - results in materials with a wide operating temperature range and a data retention time of more than 10 years in thin-film solution-processed capacitor devices at room temperature. The observed differences between linear- and branched-tail compounds are analyzed using density functional theory (DFT) and molecular dynamics (MD) simulations. We conclude that morphological factors like improved packing quality and reduced disorder, rather than electrostatic interactions or intra/inter-columnar steric hindrance, underlay the superior properties of the branched-tailed BTAs. Synergistic effects upon blending of compounds with branched and linear side-chains can be used to further improve the materials' characteristics.

Mimicking Active Biopolymer Networks with a Synthetic Hydrogel.

Stiffening due to internal stress generation is of paramount importance in living systems and is the foundation for many biomechanical processes. For example, cells stiffen their surrounding matrix by pulling on collagen and fibrin fibers. At the subcellular level, molecular motors prompt fluidization and actively stiffen the cytoskeleton by sliding polar actin filaments in opposite directions. Here, we demonstrate that chemical cross-linking of a fibrous matrix of synthetic semiflexible polymers with thermoresponsive poly( N-isopropylacrylamide) (PNIPAM) produces internal stress by induction of a coil-to-globule transition upon crossing the lower critical solution temperature of PNIPAM, resulting in a macroscopic stiffening response that spans more than 3 orders of magnitude in modulus. The forces generated through collapsing PNIPAM are sufficient to drive a fluid material into a stiff gel within a few seconds. Moreover, rigidified networks dramatically stiffen in response to applied shear stress featuring power law rheology with exponents that match those of reconstituted collagen and actomyosin networks prestressed by molecular motors. This concept holds potential for the rational design of synthetic materials that are fluid at room temperature and rapidly rigidify at body temperature to form hydrogels mechanically and structurally akin to cells and tissues.

Carbon Nanotube Reinforced Supramolecular Hydrogels for Bioapplications.

Nanocomposite hydrogels based on carbon nanotubes (CNTs) are known to possess remarkable stiffness, electrical, and thermal conductivity. However, they often make use of CNTs as fillers in covalently cross-linked hydrogel networks or involve direct cross-linking between CNTs and polymer chains, limiting processability properties. Herein, nanocomposite hydrogels are developed, in which CNTs are fillers in a physically cross-linked hydrogel. Supramolecular nanocomposites are prepared at various CNT concentrations, ranging from 0.5 to 6 wt%. Incorporation of 3 wt% of CNTs leads to an increase of the material's toughness by over 80%, and it enhances electrical conductivity by 358%, compared to CNT-free hydrogel. Meanwhile, the nanocomposite hydrogels maintain thixotropy and processability, typical of the parent hydrogel. The study also demonstrates that these materials display remarkable cytocompatibility and support cell growth and proliferation, while preserving their functional activities. These supramolecular nanocomposite hydrogels are therefore promising candidates for biomedical applications, in which both toughness and electrical conductivity are important parameters.

Strain-Stiffening in Dynamic Supramolecular Fiber Networks.

The cytoskeleton is a highly adaptive network of filamentous proteins capable of stiffening under stress even as it dynamically assembles and disassembles with time constants of minutes. Synthetic materials that combine reversibility and strain-stiffening properties remain elusive. Here, strain-stiffening hydrogels that have dynamic fibrous polymers as their main structural components are reported. The fibers form via self-assembly of bolaamphiphiles (BA) in water and have a well-defined cross-section of 9 to 10 molecules. Fiber length recovery after sonication, H/D exchange experiments, and rheology confirm the dynamic nature of the fibers. Cross-linking of the fibers yields strain-stiffening, self-healing hydrogels that closely mimic the mechanics of biological networks, with mechanical properties that can be modulated by chemical modification of the components. Comparison of the supramolecular networks with covalently fixated networks shows that the noncovalent nature of the fibers limits the maximum stress that fibers can bear and, hence, limits the range of stiffening.