Polymer Gels Constructed Through Metal–Ligand Coordination

Post-Assembly Polymerization of Discrete Anion-Coordinated Triple Helicate

Hierarchical self-assembly has been recently employed in the construction of anion-coordination-driven gel materials. However, the post-assembly modification strategy, which may be a highly efficient strategy to realize the functionalization of discrete 'aniono' supramolecular architectures, has not been employed yet. Herein we report the first example of anion-coordination-driven gel material cross-linked by well-defined 'aniono' triple helicate through post-assembly polymerization. The obtained gel shows self-healing property and excellent compatibility with various surfaces, including glass, rubber, leaf, PP, and metal. The viscoelastic gel constructed through the post-assembly modification strategy enriches the method to construct the anion-coordination-driven smart materials.

Rational design of viscoelastic hydrogels for periodontal ligament remodeling and repair

The periodontal ligament (PDL) is a distinctive yet critical connective tissue vital for maintaining the integrity and functionality of tooth-supporting structures. However, PDL repair poses significant challenges due to the complexity of its mechanical microenvironment encompassing hard-soft-hard tissues, with the viscoelastic properties of the PDL being of particular interest. This review delves into the significant role of viscoelastic hydrogels in PDL regeneration, underscoring their utility in simulating biomimetic three-dimensional microenvironments. We review the intricate relationship between PDL and viscoelastic mechanical properties, emphasizing the role of tissue viscoelasticity in maintaining mechanical functionality. Moreover, we summarize the techniques for characterizing PDL's viscoelastic behavior. From a chemical bonding perspective, we explore various crosslinking methods and characteristics of viscoelastic hydrogels, along with engineering strategies to construct viscoelastic cell microenvironments. We present a detailed analysis of the influence of the viscoelastic microenvironment on cellular mechanobiological behavior and fate. Furthermore, we review the applications of diverse viscoelastic hydrogels in PDL repair and address current challenges in the field of viscoelastic tissue repair. Lastly, we propose future directions for the development of innovative hydrogels that will facilitate not only PDL but also systemic ligament tissue repair. STATEMENT OF SIGNIFICANCE.

Cobalt-Adenosine Monophosphate Supramolecular Hydrogel with pH-Responsive Multi-Nanozymatic Activity

Self-assembled metal-ion cross-linked multifunctional hydrogels are gaining a lot of attention in the fields of biomedical and biocatalysis. Herein, we report a heat-triggered metallogel that was spontaneously formed by the self-assembly of adenosine 5'-monophosphate (AMP) and cobalt chloride, accompanied by a color transition depicting an octahedral to tetrahedral transition at high temperature. The hydrogel shows excellent stability in a wide pH window from 1 to 12. The metallogel is being exploited as a multienzyme mimic, exhibiting pH-responsive catalase and peroxidase activity. Whereas catalase mimicking activity was demonstrated by the hydrogel under neutral and basic conditions, it shows peroxidase mimicking activity in an acidic medium. The multifunctionality of the synthesized metallogel was further demonstrated by phenoxazinone synthase-like activities. Owing to its catalase-mimicking activity, the metallogel could effectively reduce the oxidative stress produced in cells due to excess hydrogen peroxide by degrading H2O2 to O2 and H2O under physiological conditions. The biocompatible metallogel could prevent cell apoptosis by scavenging reactive oxygen species. A green and simple synthetic strategy utilizing commonly available biomolecules makes this metallogel highly attractive for catalytic and biomedical applications.

Conductivity in Thin Films of Transition Metal Coordination Complexes

Two coordination complexes have been made by combining the dithiolene complexes [M(mnt)₂]^2- (mnt = maleonitriledithiolate; M = Ni²⁺ or Cu²⁺) as anion, with the copper(II) coordination complex [Cu(Stetra)] (Stetra = 6,6'-bis(4,5-dihydrothiazol-2-yl)-2,2'-bipyri-dine) as cation. The variation of the metal centers leads to a dramatic change in the conductivity of the materials, with the M = Cu²⁺ variant (Cu-Cu) displaying semiconductor behavior with a conductivity of approximately 2.5 × 10^-8 S cm^-1, while the M = Ni²⁺ variant (Ni-Cu) displayed no observable conductivity. Computational studies found Cu-Cu enables a minimization of reorganization energy losses and, as a result, a lower barrier to the charge transfer process, resulting in the reported higher conductivity.

Cross-Linked Luminescent Polymers Based on β-Diketone-Modified Polysiloxanes and Organoeuropiumsiloxanes

Nowadays, luminescent materials attract wide attention due to their valuable characteristics and broad area of potential application. Luminescent silicone-based polymers possess unique properties, such as flexibility, hydrophobicity, thermal and chemical stabilities, etc., which allow them to be utilized in various fields, such as optoelectronics, solid-state lasers, luminescent solar concentrators, sensors, and others. In the present work, a metal-ligand interaction approach was applied to obtain new cross-linked luminescent polymers based on multiligand polysiloxanes with grafted β-diketone fragments and organoeuropiumsiloxanes containing various organic substituents. Organoeuropiumsiloxanes were utilized as a source of Eu³⁺ ions due to their compatibility with the silicon matrix. All synthesized polymers were fully characterized and their physicochemical, mechanical, self-healing, optical, and thermal properties were studied.

Chain-End Effects on Supramolecular Poly(ethylene glycol) Polymers

In this work we present a fundamental analysis based on small-angle scattering, linear rheology and differential scanning calorimetry (DSC) experiments of the role of different hydrogen bonding (H-bonding) types on the structure and dynamics of chain-end modified poly(ethylene glycol) (PEG) in bulk. As such bifunctional PEG with a molar mass below the entanglement mass Me is symmetrically end-functionalized with three different hydrogen bonding (H-bonding) groups: thymine-1-acetic acid (thy), diamino-triazine (dat) and 2-ureido-4[1H]-pyrimidinone (upy). A linear block copolymer structure and a Newtonian-like dynamics is observed for PEG-thy/dat while results for PEG-upy structure and dynamics reveal a sphere and a network-like behavior, respectively. These observations are concomitant with an increase of the Flory-Huggins interaction parameter from PEG-thy/dat to PEG-upy that is used to quantify the difference between the H-bonding types. The upy association into spherical clusters is established by the Percus-Yevick approximation that models the inter-particle structure factor for PEG-upy. Moreover, the viscosity study reveals for PEG-upy a shear thickening behavior interpreted in terms of the free path model and related to the time for PEG-upy to dissociate from the upy clusters, seen as virtual crosslinks of the formed network. Moreover, a second relaxation time of different nature is also obtained from the complex shear modulus measurements of PEG-upy by the inverse of the angular frequency where G' and G'' crosses from the network-like to glass-like transition relaxation time, which is related to the segmental friction of PEG-upy polymeric network strands. In fact, not only do PEG-thy/dat and PEG-upy have different viscoelastic properties, but the relaxation times found for PEG-upy are much slower than the ones for PEG-thy/dat. However, the activation energy related to the association dynamics is very similar for both PEG-thy/dat and PEG-upy. Concerning the segmental dynamics, the glass transition temperature obtained from both rheological and calorimetric analysis is similar and increases for PEG-upy while for PEG-thy/dat is almost independent of association behavior. Our results show how supramolecular PEG properties vary by modifying the H-bonding association type and changing the molecular Flory-Huggins interaction parameter, which can be further explored for possible applications.

Nanopore gates via reversible crosslinking of polymer brushes: a theoretical study

Polymer-brush-modified nanopores are synthetic structures inspired by the gated transport exhibited by their biological counterparts. This work theoretically analyzes how the reversible crosslinking of a polymer network by soluble species can be used to control transport through nanochannels and pores. The study was performed with a molecular theory that allows inhomogeneities in the three spatial dimensions and explicitly takes into account the size, shape and conformations of all molecular species, considers the intermolecular interactions between the polymers and the soluble crosslinkers and includes the presence of a translocating particle inside the pore. It is shown than increasing the concentration of the soluble crosslinkers in bulk solution leads to a gradual increase of its number within the pore until a critical bulk concentration is reached. At the critical concentration, the number of crosslinkers inside the pore increases abruptly. For long chains, this sudden transition triggers the collapse of the polymer brush to the center of the nanopore. The resulting structure increases the free-energy barrier that a translocating particle has to surmount to go across the pore and modifies the route of translocation from the axis of the pore to its walls. On the other hand, for short polymer chains the crosslinkers trigger the collapse of the brush to the pore walls, which reduces the translocation barrier.

A supramolecular polymer with ultra-stretchable, notch-insensitive, rapid self-healing and adhesive properties

Supramolecular elastomers, possessing excellent mechanical, reusable adhesivity, and rapid self-healing properties, are essential for use in various applications.

Stretchy and strong polyurethane–urea supramolecular (PUUS) hydrogels with various stimulus-responsive behaviours: the effect of chain-extenders

This paper provides an easy synthetic strategy combined with straightforward tailoring of the physical properties and functionalities, such that optimal performance can be targeted for various applications of strong and tough supramolecular hydrogels.

Gel-based soft actuators driven by light

Gels that display light-induced motile, life-like actions are reviewed and their potential applications as light-driven soft actuators are also discussed.

Physical Networks from Multifunctional Telechelic Star Polymers: A Rheological Study by Experiments and Simulations

The equilibrium mechanical properties of a cross-linked gel of telechelic star polymers are studied by rheology and Brownian dynamics simulations. The Brownian dynamics model consists of cores to which Rouse arms are attached. Forces between the cores are obtained from a potential of mean force model developed by Likos and co-workers. Both experimentally and in the simulations, networks were created by attaching sticker groups to the ends of the arms of the polymers, which were next allowed to form bonds among them in a one to one fashion. Simulations were sped up by solving the Rouse dynamics exactly. Moreover, the Rouse model was extended to allow for different frictions on different beads. In order to describe the rheology of the non-cross-linked polymers, it had to be assumed that bead frictions increase with increasing bead number along the arms. This friction model could be transferred to describe the rheology of the network without any adjustments other than an overall increase of the frictions due to the formation of bonds. The slowing down at intermediate times of the network rheology compared to that of the non-cross-linked polymers is well described by the model. The percentage of stickers involved in forming inter-star bonds in the system was determined to be 25%, both from simulations and from an application of the Green-Tobolsky relation to the experimental plateau value of the shear relaxation modulus. Simulations with increasing cross-link percentages revealed that on approaching the gel transition the shear relaxation modulus develops an algebraic tail, which gets frozen at a percentage of maximum cross-linking of about 11%.

Responsive Hydrogels from Associative Block Copolymers: Physical Gelling through Polyion Complexation

The present review article highlights a specific class of responsive polymer-based hydrogels which are formed through association of oppositely charged polyion segments. The underpinning temporary three-dimensional network is constituted of hydrophilic chains (either ionic or neutral) physically crosslinked by ion pair formation arising from intermolecular polyionic complexation of oppositely charged repeating units (polyacid/polybase ionic interactions). Two types of hydrogels are presented: (i) hydrogels formed by triblock copolymers bearing oppositely charged blocks (block copolyampholytes), forming self-assembled networks; and (ii) hydrogels formed by co-assembly of oppositely charged polyelectrolyte segments belonging to different macromolecules (either block copolymers or homopolyelectrolytes). Due to the weak nature of the involved polyions, these hydrogels respond to pH and are sensitive to the presence of salts. Discussing and evaluating their solution, rheological and structural properties in dependence on pH and ionic strength, it comes out that the hydrogel properties are tunable towards potential applications.

Control over the assembly and rheology of supramolecular networks via multi-responsive double hydrophilic copolymers

An orthogonal control over network formation and dynamics is achieved in metallo-supramolecular micellar gels via multi-responsive double hydrophilic copolymers.

Orthogonal Control of the Dynamics of Supramolecular Gels from Heterotelechelic Associating Polymers

One of the first examples of supramolecular gels presenting independent dual dynamics is built through a combination of hydrophobic and metal-ligand interactions. The associating building block consists in a water-soluble linear polymer terminated by a short hydrophobic sticker at one end, and a coordinating moiety at the other end. The distinct supramolecular nature of these noninterfering binding motifs allows the dynamics of the hydrogels to be finely tuned in an orthogonal fashion by the application of specific stimuli. Precisely, the solvent-induced plasticization of the hydrophobic associations and the acid-promoted dissociation of the metal-ligand complexes are used to control the network dynamics. By opposition to classically encountered binary gel-sol responses, we demonstrate that the stimuli-induced transition in material properties can be gradual, provided that the material structure is well designed and strong enough.

Amphiphilic Metallopolymers for Photoswitchable Supramolecular Hydrogels

A series of amphiphilic metallopolymers is described that features zinc(II) bis-terpyridine coordination nodes as well as a backbone with hydrophobic azoaryl moieties and hydrophilic phenylene-ethynylene units decorated with PEG brushes. Using such metallopolymers at very low concentration, stable, photo-responsive and self-healing hydrogels are obtained. UV irradiation of the gel allows modulation of the degree of hydrophobic π-π interactions between photoisomerizable azoaryl units and a polarity switch that overall induces a fast gel-to-sol transition. Finally, the material phase can be readily and fully restored to the thermodynamically stable state either thermally or photochemically by using visible light. The presented strategy can be further generalized towards modular supramolecular metallopolymers for injectable gels in drug delivery and bio-engineering applications.

Supramolecular polymer networks: hydrogels and bulk materials

Supramolecular polymer networks are materials crosslinked by reversible supramolecular interactions, such as hydrogen bonding or electrostatic interactions. Supramolecular materials show very interesting and useful properties resulting from their dynamic nature, such as self-healing, stimuli-responsiveness and adaptability. Here we will discuss recent progress in polymer-based supramolecular networks for the formation of hydrogels and bulk materials.

Sol–gel reversible metallo-supramolecular hydrogels based on a thermoresponsive double hydrophilic block copolymer

A new thermoresponsive double hydrophilic block copolymer bearing a terpyridine moiety formed a hydrogel with a sol–gel thermoreversible transition in the presence of Fe²⁺ ions.

Precise Control over the Rheological Behavior of Associating Stimuli-Responsive Block Copolymer Gels

"Smart" materials have considerably evolved over the last few years for specific applications. They rely on intelligent macromolecules or (supra-)molecular motifs to adapt their structure and properties in response to external triggers. Here, a supramolecular stimuli-responsive polymer gel is constructed from heterotelechelic double hydrophilic block copolymers that incorporate thermo-responsive sequences. These macromolecular building units are synthesized via a three-step controlled radical copolymerization and then hierarchically assembled to yield coordination micellar hydrogels. The dynamic mechanical properties of this particular class of materials are studied in shear flow and finely tuned via temperature changes. Notably, rheological experiments show that structurally reinforcing the micellar network nodes leads to precise tuning of the viscoelastic response and yield behavior of the material. Hence, they constitute promising candidates for specific applications, such as mechano-sensors.

Kinetically stable metal ligand charge transfer complexes as crosslinks in nanogels/hydrogels: Physical properties and cytotoxicity

A terpyridine end-functionalized 8-arm poly(ethylene glycol) was prepared using the reaction of a 4'-aminopentanoxy substituted terpyridine with a p-nitrophenyl chloroformate activated PEG-(OH)8. Supramolecular complexation of the polymer terpyridine moieties by Fe(2+) ions was investigated using NMR, UV-Vis and dynamic light scattering experiments. At low concentrations addition of Fe(2+) ions to an aqueous solution of the polymer conjugate afforded nanogels with a single size distribution around 250 nm. At concentrations above 3 wt%, and at a 1:2 metal to ligand molar ratio, hydrogels were formed with increasing mechanical properties at increasing polymer concentrations. Using bovine chondrocytes, the biocompatibility and potential cytotoxicity of the polymer conjugate, nanogels and hydrogels were studied. The polymer conjugate with free ligands was toxic to the cells likely due to depletion of essential metal ions. When the terpyridine groups were complexed with Fe(2+) ions, both nanogel suspensions and hydrogels showed no cytotoxicity in direct contact with chondrocytes. Indirect contact of gels with chondrocytes using transwells revealed the absence of toxic components by leaching. A Live-Dead assay on chondrocytes encapsulated in the hydrogels indicated that the hydrogels are cytocompatible, revealing the potential use of these materials for biomedical and pharmaceutical applications.

STATEMENT OF SIGNIFICANCE

The binding between transition metal ions and ligands with multiple binding sites can be almost as strong as covalent bonds. This metal-ligand charge transfer (MLCT) complexation was used to crosslink water soluble polymers into hydrogels. This approach to novel materials may find applications in the biomedical and pharmaceutical fields. Transition metal ions are essential trace elements present in tissue but up to now no cytotoxicity data of free ligands are available. Data presented show that free ligands are toxic to cells likely by depletion of trace metal ions, whereas kinetically stable complexes are not cytotoxic even when embedded in hydrogels. These results provide fundamental issues to be considered in the design of hydrogels crosslinked through metal ligand complexation.

Controlling the melt rheology of linear entangled metallo-supramolecular polymers

We study in the melt the linear viscoelastic properties of supramolecular assemblies obtained by adding different amounts of nickel ions to linear entangled poly(ethylene oxide) (PEO) building blocks end-functionalized by a terpyridine group. We first show that the elasticity of these supramolecular assemblies is mainly governed by the entanglement dynamics of the building blocks, while the supramolecular interactions delay or suppress their relaxation. By adjusting the amount of metal ions, the relaxation time as well as the level of the low-frequency plateau of these supramolecular assemblies can be controlled. In particular, the addition of metal ions above the 1:2 metal ion/terpyridine stoichiometric ratio allows secondary supramolecular interactions to appear, which are able to link the linear supramolecular assemblies and thus, lead to the reversible gelation of the system. By comparing the rheological behavior of different linear PEO samples, bearing or not functionalized chain-ends, we show that these extra supramolecular bonds are partially due to the association between the excess of metal ions and the oxygen atoms of the PEO chains. We also investigate the possible role played by the terpyridine groups in the formation of these secondary supramolecular interactions.

Revealing the supramolecular nature of side-chain terpyridine-functionalized polymer networks

Nowadays, finely controlling the mechanical properties of polymeric materials is possible by incorporating supramolecular motifs into their architecture. In this context, the synthesis of a side-chain terpyridine-functionalized poly(2-(dimethylamino)ethyl methacrylate) is reported via reversible addition-fragmentation chain transfer polymerization. By addition of transition metal ions, concentrated aqueous solutions of this polymer turn into metallo-supramolecular hydrogels whose dynamic mechanical properties are investigated by rotational rheometry. Hence, the possibility for the material to relax mechanical constrains via dissociation of transient cross-links is brought into light. In addition, the complex phenomena occurring under large oscillatory shear are interpreted in the context of transient networks.

Stimuli-responsive hydrogels prepared by simultaneous “click chemistry” and metal–ligand coordination

A novel strategy to synthesize stimuli-responsive metal-coordinated polymeric gels (MCPGs) via one-pot CuAAC and non-covalent metal coordination chemistry was demonstrated.

Hydrogels by supramolecular crosslinking of terpyridine end group functionalized 8-arm poly(ethylene glycol)

Metallo supramolecular assemblies of an 8-arm poly(ethylene glycol) partially substituted with terpyridyl end-groups and the transition metal ions Ni(2+), Fe(2+), Co(2+) and Zn(2+) were studied for their nano-particle formation at dilute conditions and gelation at higher concentrations. The large differences in dissociation rate constants of the metal ligand complexes largely determine the assembly behavior. Thermodynamically stable complexes are generated with Ni(2+) and Fe(2+) chlorides, which lead to distinct particle sizes of ∼200 nm in dilute conditions. The Co(2+) and Zn(2+) chlorides provide multiple size distributions revealing that mono and bis-complexes are present at equilibrium. Upon complexation, terpyridyl groups move to the outer sphere giving aggregates with a charged surface. At polymer concentrations above 5 wt%, crosslinking upon addition of transition metal ions provides hydrogels. Elastic hydrogels were obtained with Ni(2+), Fe(2+) and Co(2+) having storage moduli in excess of 20 kPa, whereas Zn(2+) gels are relatively viscous. Only Zn(2+) gels show a thermoreversible sol to gel transition at a temperature of 25 °C independent of polymer concentration.

Dynamic self-assembly of coordination polymers in aqueous solution

The construction of supramolecular polymers has been intensively pursued because the nanostructures formed through weak non-covalent interactions can be triggered by external stimuli leading to smart materials and sensors. Self-assemblies of coordination polymers consisting of metal ions and organic ligands in aqueous solution also provide particular contributions in this area. The main motivation for developing those coordination polymers originates from the value-added combination between metal ions and ligands. This review highlights the recent progress of the dynamic self-assembly of coordination polymers that result from the sophisticated molecular design, towards fabricating stimuli-responsive systems and bio-related materials. Dynamic structural changes and switchable physical properties triggered by various stimuli are summarized. Finally, the outlook for aqueous nanostructures originated from the dynamic self-assembly of coordination polymers is also presented.

Thermo-responsive properties of metallo-supramolecular block copolymer micellar hydrogels

Metallo-supramolecular micellar hydrogels exhibiting thermo-mechanical responsiveness are prepared through the hierarchical assembly of a heterotelechelic associating copolymer. The copolymer consists of a linear thermo-sensitive water-soluble sequence terminated by a short hydrophobic sticker at one end, the other being functionalized by a chelating ligand. As the first level of assembly, the associating copolymer is dissolved in aqueous solution to yield micellar nanostructures, bearing coordinative motifs at the end of the coronal chains. The second level of assembly is achieved when transition metal ions are added to the micellar solutions, resulting in almost instantaneous gelation. The thermo-mechanical response of those materials is investigated in detail by rotational rheometry, showing abrupt changes within the temperature boundaries corresponding to the phase transition of the polymer block located in the micellar corona.