Unfolding and Mechanochemical Scission of Supramolecular Polymers Containing a Metal–Ligand Coordination Bond

Mechanochemical Release of Non-Covalently Bound Guests from a Polymer-Decorated Supramolecular Cage

Supramolecular coordination cages show a wide range of useful properties including, but not limited to, complex molecular machine-like operations, confined space catalysis, and rich host-guest chemistries. Here we report the uptake and release of non-covalently encapsulated, pharmaceutically-active cargo from an octahedral Pd cage bearing polymer chains on each vertex. Six poly(ethylene glycol)-decorated bipyridine ligands are used to assemble an octahedral PdII6 (TPT)4 cage. The supramolecular container encapsulates progesterone and ibuprofen within its hydrophobic nanocavity and is activated by shear force produced by ultrasonication in aqueous solution entailing complete cargo release upon rupture, as shown by NMR and GPC analyses.

Single-Molecule Activation and Quantification of Mechanically Triggered Palladium-Carbene Bond Dissociation

Metal-complexed N-heterocyclic carbene (NHC) mechanophores are latent reactants and catalysts for a range of mechanically driven chemical responses, but mechanochemical scission of the metal-NHC bond has not been experimentally characterized. Here we report the single-molecule force spectroscopy of ligand dissociation from a pincer NHC-pyridine-NHC Pd(II) complex. The force-coupled rate constant for ligand dissociation reaches 50 s^-1 at forces of approximately 930 pN. Experimental and computational observations support a dissociative, rather than associative, mechanism of ligand displacement, with rate-limiting scission of the Pd-NHC bond followed by rapid dissociation of the pyridine moiety from Pd.

Mechanochemical tools for polymer materials

This review aims to provide a field guide for the implementation of mechanochemistry in synthetic polymers by summarizing the molecules, materials, and methods that have been developed in this field.

Vitrimerization: Converting Thermoset Polymers into Vitrimers

Thermoset polymers with permanently cross-linked networks have outstanding mechanical properties and solvent resistance, but they cannot be reprocessed or recycled. On the other hand, vitrimers with covalent adaptable networks can be recycled. Here we provide a simple and practical method coined as "vitrimerization" to convert the permanent cross-linked thermosets into vitrimer polymers without depolymerization. The vitrimerized thermosets exhibit comparable mechanical properties and solvent resistance with the original ones. This method allows recycling and reusing the unrecyclable thermoset polymers with minimum loss in mechanical properties and enables closed-loop recycling of thermosets with the least environmental impact.

The CuAAC: Principles, Homogeneous and Heterogeneous Catalysts, and Novel Developments and Applications

The copper-catalyzed azide/alkyne cycloaddition reaction (CuAAC) has emerged as the most useful "click" chemistry. Polymer science has profited enormously from CuAAC by its simplicity, ease, scope, applicability and efficiency. Basic principles of the CuAAC are reviewed with a focus on homogeneous and heterogeneous catalysts, ligands, anchimeric assistance, and basic chemical principles. Recent developments of ligand design and acceleration are discussed.

Synthesis and Mechanochemical Activity of Peptide-Based Cu(I) Bis(N-heterocyclic carbene) Complexes

With the class of shock-absorbing proteins, nature created some of the most robust materials combining both mechanical strength and elasticity. Their excellent ability to dissipate energy to prevent surrounding cells from damage is an interesting property that regularly is exploited for applications in biomimetic materials. Similar to biomaterials, where mechanical stimuli are transmitted into a (bio)chemical response, mechanophoric catalysts transform mechanical energy into a chemical reaction. Force transmission is realized commonly by polymeric handles directing the applied force to the mechanophoric bond, which in turn leads to stress-induced activation of the catalyst. Therefore, shock-absorbing proteins able to take up and store mechanical energy elastically for subsequent force transduction to the labile bond seem to be perfect candidates to fulfill this task. Here, we report on the synthesis of two different latent mechanophoric copper(I) bis(N-heterocyclic carbene) complexes bearing either two carboxyl groups or two amino groups which allow conjugation reactions with either the N- or the C-terminus of amino acids or peptides. The chosen catalysts can be activated, for instance, by applying external mechanical force via ultrasound, removing one N-heterocyclic carbene (NHC) ligand. Post-modification of the mechanophoric catalysts via peptide coupling (Gly, Val) and first reactions showed that the mechanoresponsive behavior was still present after the coupling. Subsequent polycondensation of both catalysts lead to a polyamide including the Cu(I) moiety. Mechanochemical activation by ultrasound showed conversions in the copper(I)-catalyzed alkyne-azide “click” reaction (CuAAC) up to 9.9% proving the potential application for the time and spatial controlled CuAAC.

Self-Healing Mechanism of Microcracks on Waterborne Polyurethane with Tunable Disulfide Bond Contents

Self-healing mechanism of microcracks on waterborne polyurethane (WPU) with varied disulfide bond contents was investigated thoroughly in this paper. The self-healing process could be observed by an optical microscope, which was divided into two steps. The first step was the cohesive healing movement, closely related with microphase separations and further characterized by the Fourier transform infrared peak fitting, differential scanning calorimetry, and dynamic mechanical thermal analysis. The second step of the exchange reactions of disulfide bonds could be confirmed by the in situ Raman and tensile mechanics. In addition, the cohesive healing behavior was quantitatively described by the kinetic method, and the exchange reaction active energies of disulfide bonds were also calculated to be 20.42 kJ/mol by gel permeation chromatography tests. The best self-healing performance of SHWPUs was WPU-8, and it could be healed at 75 °C in 15 min with 100% self-healing efficiency. Combining the self-healing process observation with the kinetic analysis can provide deep insights into the self-healing mechanism of microcracks.

The role of a weakly coordinating thioether group in ligation controlled molecular self-assemblies and their inter-conversions in Ni(ii) complexes of l-methionine derived ligand

A series of Ni(ii) complexes of an amino acid derived ligand (Salmet) has been synthesized, and characterized by various techniques including X-ray crystallography. Alkali ions K⁺/Na⁺ direct the conversion of monomers to multi-nuclear complexes.

Synthesis of polymer-linked copper(i) bis(N-heterocyclic carbene) complexes of linear and chain extended architecture

Novel PS-based mechanophores of linear and chain-extended architecture are synthesized obtaining bis(NHC) complexes with more than one Cu(i) center per chain and molecular weights of up to 50 000 g mol⁻¹.

Quantitative and Mechanistic Mechanochemistry in Ferrocene Dissociation

Ferrocene is classically regarded as being highly inert owing to the large dissociation energy of metal-cyclopentadienyl (Cp) bonds. We show that the Fe-Cp bond in ferrocene is the preferential site of mechanochemical scission in the pulsed ultrasonication of main-chain ferrocene-containing polybutadiene-derived polymers. Quantitative studies reveal that the Fe-Cp bond is similar in strength to the carbon-nitrogen bond of an azobisdialkylnitrile (bond dissociation energy < -0 kcal/mol), despite the significantly higher Fe-Cp bond dissociation energy (approximately 90 kcal/mol). Mechanistic studies are consistent with a predominately heterolytic mechanism of chain scission. DFT calculations provide insights into the origins of ferrocene's mechanical lability.

Experimental Polymer Mechanochemistry and its Interpretational Frameworks

Polymer mechanochemistry is an emerging field at the interface of chemistry, materials science, physics and engineering. It aims at understanding and exploiting unique reactivities of polymer chains confined to highly non-equilibrium stretched geometries by interactions with their surroundings. Macromolecular chains or their segments become stretched in bulk polymers under mechanical loads or when polymer solutions are sonicated or flow rapidly through abrupt contractions. An increasing amount of empirical data suggests that mechanochemical phenomena are widespread wherever polymers are used. In the past decade, empirical mechanochemistry has progressed enormously, from studying fragmentations of commodity polymers by simple backbone homolysis to demonstrations of self-strengthening and stress-reporting materials and mechanochemical cascades using purposefully designed monomers. This progress has not yet been matched by the development of conceptual frameworks within which to rationalize, systematize and generalize empirical mechanochemical observations. As a result, mechanistic and/or quantitative understanding of mechanochemical phenomena remains, with few exceptions, tentative. In this review we aim at systematizing reported macroscopic manifestations of polymer mechanochemistry, and critically assessing the interpretational framework that underlies their molecular rationalizations from a physical chemist's perspective. We propose a hierarchy of mechanochemical phenomena which may guide the development of multiscale models of mechanochemical reactivity to match the breadth and utility of the Eyring equation of chemical kinetics. We discuss the limitations of the approaches to quantifying and validating mechanochemical reactivity, with particular focus on sonicated polymer solutions, in order to identify outstanding questions that need to be solved for polymer mechanochemistry to become a rigorous, quantitative field. We conclude by proposing 7 problems whose solution may have a disproportionate impact on the development of polymer mechanochemistry.

Mechanoluminescent Imaging of Osmotic Stress-Induced Damage in a Glassy Polymer Network

A chemiluminescent mechanophore, bis(adamantyl-1,2-dioxetane), is used to investigate the covalent bond scission resulting from the sorption of chloroform by glassy poly(methyl methacrylate) (PMMA) networks. Bis(adamantyl)-1,2-dioxetane units incorporated as cross-linkers underwent mechanoluminescent scission, demonstrating that solvent ingress caused covalent bond scission. At higher cross-linking densities, the light emission took the form of hundreds of discrete bursts, widely varying in intensity, with each burst composed of 107-109 photons. Camera imaging indicated a relatively slow propagation of bursts through the material and permitted analysis of the spatial correlation between the discrete bond-breaking events. The implications of these observations for the mechanism of sorption and fracture are discussed.

Ultrasound-Mediated Self-Healing Hydrogels Based on Tunable Metal-Organic Bonding

Stimulus-responsive hydrogels make up an important class of programmable materials for a wide range of biomedical applications. Ultrasound (US) is a stimulus that offers utility because of its ability to permeate tissue and rapidly induce chemical alterations in aqueous media. Here we report on the synthesis and US-mediated disintegration of stimulus-responsive telechelic Dopa-modified polyethylene glycol-based hydrogels. Fe³⁺-[PEG-Dopa]₄ hydrogels are formed through Fe³⁺-induced cross-linking of four-arm polyethylene glycol-dopamine precursors to produce networks. The relative amounts of H-bonds, coordination bonds, and covalent bonds can be controlled by the [Fe³⁺]:[Dopa] molar ratio in precursor solutions. Networks formed from precursors with high [Fe³⁺]:[Dopa] ratios create mechanically robust networks (G' = 6880 ± 240 Pa) that are largely impervious to US-mediated disintegration at intensities of ≤43 W/cm². Conversely, lightly cross-linked networks formed through [Fe³⁺]:[Dopa] molar ratios of <0.73 are susceptible to rapid disintegration upon exposure to US. Pulsatile US exposure allows temporal control over hydrogel disintegration and programmable self-healing. Sustained US energy can also stabilize hydrogels through the formation of additional cross-links via free radical-mediated coupling of pendant catechols. Taken together, the diverse ranges of mechanical behavior, self-healing capability, and differential susceptibility to ultrasonic disintegration suggest that Fe³⁺-[PEG-Dopa]₄ hydrogels yield a class of application-specific stimulus-responsive polymers as smart materials for applications ranging from transient medical implants to matrices for smart drug delivery.

Effect of Mechanical Stress on Spiropyran-Merocyanine Reaction Kinetics in a Thermoplastic Polymer

Mechanical force alters the potential energy surface of a mechanophore reaction by modifying the activation energy for conversion. The effects of force on the rate constants and activation energies are not well characterized for mechanophores in bulk polymers. In this work, spiropyran-linked polyurethanes are synthesized and the kinetics of the spiropyran-merocyanine transition in the bulk polymer measured under different values of a macroscopic tensile stress. Above a critical threshold stress, the forward rate constant (spiropyran to merocyanine transition) increases, while the reverse rate constant (merocyanine to spiropyran transition) decreases with applied stress. A tensile stress of 50 MPa enhances the forward rate constant by 110% and lowers the forward activation energy by 1.8 kJ/mol compared to the unstressed condition. Also, this same amount of stress reduces the reverse rate constant by 65% and increases the reverse activation energy by 2.5 kJ/mol.

Relative Mechanical Strengths of Weak Bonds in Sonochemical Polymer Mechanochemistry

The mechanical strength of scissile chemical bonds plays a role in material failure and in the mechanical activation of latent reactivity, but quantitative measures of mechanical strength are rare. Here, we report the relative mechanical strength of polymers bearing three putatively "weak" scissile bonds: the carbon-nitrogen bond of an azobisdialkylnitrile (<30 kcal mol(-1)), the carbon-sulfur bond of a thioether (71-74 kcal mol(-1)), and the carbon-oxygen bond of a benzylphenyl ether (52-54 kcal mol(-1)). The mechanical strengths are assessed in the context of chain scission triggered by pulsed sonication of polymer solutions, by using two complementary techniques: (i) the competition within a single polymer chain between the bond scission of interest and the nonscissile mechanochemical ring opening of gem-dichlorocyclopropane mechanophores and (ii) the molecular weights at long (4 h) sonication times of multimechanophore polymers. The two methods produce a consistent story: in contrast to their thermodynamic strengths, the relative mechanical strengths of the three weak bonds are azobisdialkylnitrile (weakest) < thioether < benzylphenyl ether. The greater mechanical strength of the benzylphenyl ether relative to the thermodynamically stronger carbon-sulfur bond is ascribed to poor mechanochemical coupling, at least in part as a result of the rehybridization that accompanies carbon-oxygen bond scission.

Mechanochemical Reactions Reporting and Repairing Bond Scission in Polymers

The past 10 years have seen a resurgence of interest in the field of polymer mechanochemistry. Whilst the destructive effects of mechanical force on polymer chains have been known for decades, it was only recently that researchers tapped into these forces to realize more useful chemical transformations. The current review discusses the strategic incorporation of weak covalent bonds in polymers to create materials with stress-sensing and damage-repairing properties. Firstly, the development of mechanochromism and mechanoluminescence as stress reporters is considered. The second half focuses on the net formation of covalent bonds as a response to mechanical force, via mechanocatalysis and mechanically unmasked chemical reactivity, and concludes with perspectives for the field.

Mechanochemical Activation of Covalent Bonds in Polymers with Full and Repeatable Macroscopic Shape Recovery

Covalent mechanochemistry within bulk polymers typically occurs with irreversible deformation of the parent material. Here we show that embedding mechanophores into an elastomeric poly(dimethylsiloxane) (PDMS) network allows for covalent bond activation under macroscopically reversible deformations. Using the colorimetric mechanophore spiropyran, we show that bond activation can be repeated over multiple cycles of tensile elongation with full shape recovery. Further, localized compression can be used to pattern strain-induced chemistry. The platform enables the reversibility of a secondary strain-induced color change to be characterized. We also observe mechanical acceleration of a flex-activated retro-Diels-Alder reaction, allowing a chemical signal to be released in response to a fully reversible deformation.

Mechanical Activation of Mechanophore Enhanced by Strong Hydrogen Bonding Interactions

A mechanically active spiropyran (SP) mechanophore is incorporated into the backbone of prepolymer which is further end-capped with ureidopyrimidinone (UPy) or urethane. Strong mechanochromic reaction of SP arises in the bulk films of UPy containing materials whereas much weaker activation occurs in urethane containing counterparts, coincident with their stress-strain responses. The difference in the magnitudes of supramolecular interactions leads to different degrees of chain orientation and strain induced crystallization (SIC) in the bulk and consequently distinct capabilities to transfer the load to the mechanophores. This study may aid the design of novel mechanoresponsive materials whose mechanoresponsiveness can be tailored by tuning supramolecular interactions.

Joining forces: integrating the mechanical and optical single molecule toolkits

Combining single molecule force measurements with fluorescence detection opens up exciting new possibilities for the characterization of mechanoresponsive molecules in Biology and Materials Science.

Mechanochemistry of Topological Complex Polymer Systems

Although existing since the concept of macromolecules, polymer mechanochemistry is a burgeoning field which attracts great scientific interest in its ability to bias conventional reaction pathways and its potential to fabricate mechanoresponsive materials. We review here the effect of topology on the mechanical degradation of polymer chains and the activation of mechanophores in polymer backbones. The chapter focuses on both experimental and theoretical work carried out in the past 70 years. After a general introduction (Sect. 1), where the concept, the history, and the application of polymer mechanochemistry are briefly described, flow fields to study polymer mechanochemistry are discussed (Sect. 2), results of mechanochemistry study are presented for linear polymers (Sect. 3), cyclic polymers (Sect. 4), graft polymers (Sect. 5), star-shaped polymers (Sect. 6), hyperbranched polymers and dendrimers (Sect. 7), and systems with dynamic topology (Sect. 8). Here we focus on (1) experimental results involving the topological effect on the coil-to-stretch transition and the fracture of the polymer chains, (2) the underlying mechanisms and the key factor that determines the mechanical stability of the macromolecules, (3) theoretical models that relate to the experimental observations, and (4) rational design of mechanophores in complex topology to achieve multiple activations according to the existing results observed in chain degradation.

Enhancing covalent mechanochemistry in bulk polymers using electrospun ABA triblock copolymers

The mechanochemical activation of covalent bonds in bulk polymers is often characterized by low conversions. Here we report that the activation of gem-dibromocyclopropane (gDBC) mechanophores embedded in a poly(1,4-butadiene) (PB) is enhanced when a central gDBC-PB block is flanked by two polystyrene (PS) end blocks in an ABA-type triblock architecture. Electrospinning the PS-(gDBC)PB-PS leads to even greater activation in aligned fiber mats under tension.

Influence of cross-linkers on the cohesive and adhesive self-healing ability of polysulfide-based thermosets

Synthetic systems with intrinsic self-repairing or self-healing abilities have emerged during the past decade. In this work, the influence of the cross-linker and chain rigidity on the healing ability of thermoset rubbers containing disulfide bonds have been investigated. The produced materials exhibit adhesive and cohesive self-healing properties. The recovery of these two functionalities upon the thermally triggered healing events has shown to be highly dependent on the network cross-link density and chain rigidity. As a result, depending on the rubber thermoset intrinsic physical properties, the thermal mending leading to full cohesive recovery can be achieved in 20-300 min at a modest healing temperature of 65 °C. The adhesive strength ranges from 0.2 to 0.5 MPa and is fully recovered even after multiple failure events.

Performance of Mechanochemically Activated Catalysts Is Enhanced by Suppression of the Thermal Effects of Ultrasound

In this work, we demonstrate that the performance of mechanochemically activated transesterification and alkene metathesis catalysts is significantly enhanced when the thermal effects of ultrasound are suppressed. Suppression of these effects is realized by performing the reaction under methane instead of argon. Not only do these results provide further confirmation of the true mechanochemical nature of the ultrasonic activation of the catalysts, but it also strongly recommends the use of methane as standard saturation gas when studying the mechanochemical effects of ultrasound.