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Photoresponsive materials that change in response to light have been studied for a range of applications. These materials are often metastable during irradiation, returning to their pre-irradiated state after removal of the light source. Herein, we report a polymer gel comprising poly(ethylene glycol) star polymers linked by Cu24 L24 metal-organic cages/polyhedra (MOCs) with coumarin ligands. In the presence of UV light, a photosensitizer, and a hydrogen donor, this "polyMOC" material can be reversibly switched between CuII , CuI , and Cu0 . The instability of the MOC junctions in the CuI and Cu0 states leads to network disassembly, forming CuI /Cu0 solutions, respectively, that are stable until re-oxidation to CuII and supramolecular gelation. This reversible disassembly of the polyMOC network can occur in the presence of a fixed covalent second network generated in situ by copper-catalyzed azide-alkyne cycloaddition (CuAAC), providing interpenetrating supramolecular and covalent networks.

Topological molecular connections and structures, including physical entanglements in polymer networks, knots along polymer chains, and rotaxanes in sliding ring gels, have important consequences for the physical properties of polymeric materials. Often these topologies contribute through their ability to bear mechanical stress, but experimental measures of their relative mechanical strength are rare. Here, we use sonochemical polymer mechanochemistry to assess the relative mechanical strength of a multicatenane copolymer relative to copolymers of cyclic and linear analogs. The relative mechanical strengths are obtained by comparing the limiting molecular weights (Mlim) and contour lengths (Llim) of the polymers under pulsed ultrasound of their dilute solutions. The values of Mlim and Llim, and thus the inferred mechanical strengths of the polymers, are effectively identical. The mechanical bonds of the catenanes are therefore as strong, or stronger, mechanically as the covalent bonds along the polymer backbone.

Single-molecule force spectroscopy (SMFS) of multi-mechanophore polymers has been used to provide kinetic and mechanistic insights into mechanochemical reactions. Whereas biological systems have benefitted from force clamp spectroscopy, synthetic polymers have been studied primarily with constant-velocity methods. Here, force clamp SMFS is applied to the mechanically accelerated conrotatory ring opening of benzocyclobutene, and a comparison with constant-velocity SMFS extends the range of available rate-versus-force data and corroborates the use of constant-velocity SMFS to extract force dependencies.

A catalyst that couples a photoswitch to the biaryl backbone of a chiral bis(phosphine) ligand, thus allowing photochemical manipulation of ligand geometry without perturbing the electronic structure is reported. The changes in catalyst activity and selectivity upon switching can be attributed to intramolecular mechanical forces, thus laying the foundation for a new class of catalysts whose selectivity can be varied smoothly and in situ over a useful range by controlling molecular stress experienced by the catalyst during turnover. Forces on the order of 100 pN are generated, thus leading to measurable changes in the enantioselectivities of asymmetric Heck arylations and Trost allylic alkylations. The differential coupling between applied force and competing stereochemical pathways is quantified and found to be more efficient for the Heck arylations.

Hydrogels and organogels made from polymer networks are widely used in biomedical applications and soft, active devices for which the ability to sustain large deformations is required. The strain at which polymer networks fracture is typically improved through the addition of elements that dissipate energy, but these materials require extra work to achieve a given, desired level of deformation. Here, the addition of mechanically "invisible" supramolecular crosslinks causes substantial increases in the ultimate gel properties without incurring the added energetic costs of dissipation.

The tissue adhesive 2-octyl cyanoacrylate (OCA) was encapsulated in polyurethane microshells and incorporated into bone cement to form a catalyst free, self-healing bone cement comprised of all clinically approved components. The bending strength, modulus, and fatigue lifetime were investigated in accordance with ASTM and ISO standards for the testing of PMMA bone cement. The bending strength of bone cement specimens decreased with increasing wt % capsules content for capsules without or with OCA, with specimens of

In the past five years, the covalent mechanochemistry of polymeric1 and small molecule2 systems has generated a great deal of interest, in terms of both material science and synthetic chemistry. Most studies to date have focused on systems with a single mechanophore per polymer chain,3–7 an approach that has enabled the discovery of new mechanically induced chemistry and tools for stress monitoring. Single molecule architectures are likely to be limited, however, for applications in stress-responsive mechanical properties or scalable stoichiometric reactivity, due to low mechanophore concentration.8 gem-Dihalocyclopropane mechanophores have demonstrated a rich array of mechanochemical activity,9–12 and their post polymerization addition to polybutadiene based polymers has yielded the highest mechanophore content13 and highest single-chain toughness14 demonstrated in a synthetic polymer to date. While simple to synthesize, olefin containing polymers suffer from inherent instability due to ambient light, heat, and oxygen, often causing uncontrolled crosslinking. Furthermore, it is difficult to create complex architectures based on the post polymerization modification of polybutadiene. In considering more robust and useful synthetic approaches, we start from the desire to engineer non-scissile mechanophores, which typically means that ring systems must be embedded along the polymer backbone.15 Backbone rings cannot easily be introduced by the traditional polymerization of vinyl monomers, and although ring-opening metathesis polymerization (ROMP) has been used,11 the synthesis of complex fused ring monomers can be cumbersome. Furthermore, the ROMP methodology produces olefin-containing backbones whose stability is limited by the factors discussed above. While a rich array of functional groups can easily be incorporated into condensation polymers, mild and controllable polycondensation procedures typically generate polymers of low molecular weight (MW). This limits both bulk material utility and mechanochemical activation by pulsed ultrasound, where high molecular weights (> 40 kDa) are typically required to experience sufficient shear forces along the polymer backbone. Here, we report that the synthesis of an ABA triblock copolymer harvests the advantages of polycondensation and radical polymerization, giving access to stable, mechanophore-rich polymers of desirable molecular weight.

The goal of this review is to introduce the biomaterials community to the emerging field of self-healing materials, and also to suggest how one could utilize and modify self-healing approaches to develop new classes of biomaterials. A brief discussion of the in vivo mechanical loading and resultant failures experienced by biomedical implants is followed by presentation of the self-healing methods for combating mechanical failure. If conventional composite materials that retard failure may be considered zeroth generation self-healing materials, then taxonomically speaking, first generation self-healing materials describe approaches that "halt" and "fill" damage, whereas second generation self-healing materials strive to "fully restore" the prefailed material structure. In spite of limited commercial use to date, primarily because the technical details have not been suitably optimized, it is likely from a practical standpoint that first generation approaches will be the first to be employed commercially, whereas second generation approaches may take longer to implement. For self-healing biomaterials the optimization of technical considerations is further compounded by the additional constraints of toxicity and biocompatibility, necessitating inclusion of separate discussions of design criteria for self-healing biomaterials.

Force reactive functional groups, or mechanophores, have emerged as the basis of a potential strategy for sensing and countering stress-induced material failure. The general utility of this strategy is limited, however, because the levels of mechanophore activation in the bulk are typically low and observed only under large, typically irreversible strains. Strategies that enhance activation are therefore quite useful. Molecular-level design principles by which to engineer enhanced mechanophore activity are reviewed, with an emphasis on quantitative structure–activity studies determined for a family of gem-dihalocyclopropane mechanophores.

Surface-grafted polymer brushes impart chemical functionality to surfaces and interfaces, and control properties such as colloidal stability, adhesion, wettability, and friction.[1,2] The ability to introduce and modulate cross-links within surface-grafted polymer brushes is expected to influence their mechanical and chemical stability, permeability, and swelling characteristics.[3] Here we report that reversibly cross-linked polymer brushes provide a previously untapped resource for modulating the mechanical properties of surfaces. Bimetallic pincer-PdII complexes (Figure 1) provide reversible, exogenous cross-linking of end-grafted poly(4-vinylpyridine) (PVP) brushes. Atomic force microscopy (AFM) experiments show that structurally similar cross-linkers with different kinetic reactivities have different effects on the lateral mechanical properties of the brush layer. Surprisingly, not only the magnitude but also the direction of the change in lateral force upon cross-linking depends on the dissociation kinetics of the cross-linkers, which therefore provide a molecular handle by which to control the mechanical properties of surface-grafted polymer brushes. Figure 1 Schematic description of the synthesis of cross-linked brushes and characterization. On a monolayer of mercaptoundecyl bromoisobutyrate on gold (a), (4-vinylpyridine) is polymerized to generate a surface-grafted polymer brush (b). Addition of the cross-linker ... Reversibly cross-linkable polymer brushes were synthesized by atom-transfer radical polymerization (ATRP)[4] from an ω-mercaptoundecyl bromoisobutyrate initiator anchored to a gold surface (Figure 1). A low concentration[4,5] (0.020 mM) of the CuI catalyst was used to decrease the steady-state concentration of radicals and to minimize bimolecular termination reactions. Under these conditions, PVP brushes with a dry thickness of 33.5 nm (50 nm in DMSO) could be polymerized in two hours. Brush heights were inferred from cross-sectional profiles obtained by AFM imaging.[6] The PVP brush substrate was divided in half, and the PVP brush was then cross-linked by the addition of DMSO solutions that contained the bis(PdII-pincer) compounds PdMe or PdEt (Figure 1), which reversibly coordinate to PVP. The compound abbreviations reflect the alkyl substituents on the amino groups of the PdII-pincer complexes. The steric bulk of these substituents has a minimal effect on the thermodynamics of pyridine (Pyr) coordination (Keq(PdMe–Pyr) = 33 M−1, Keq(PdEt–Pyr) = 29 M−1),[7] and so the uptake of PdMe and PdEt from equimolar solutions into the identical PVP brushes should be effectively equivalent, producing samples with comparable structure (number and placement of cross-links). A high concentration of the pincer complex ( ≈9 mM) was used initially to ensure significant cross-linking ( ≈30% of Pyr is bound to PdII, if Keq is unchanged in the brush). The mechanical response of the cross-links, however, is determined by the ligand dissociation rates, koff, and the bulk of the alkyl group has a dramatic influence on those rates (koff(PdMe–Pyr) ≈ 1100 s−1, koff(PdEt–Pyr) = 17 s−1).[7] The difference in the rates thus provides a probe of “mechanochemistry” that is reminiscent of the kinetic isotope effect in reaction chemistry.[8] To study the effect of cross-linking on brush mechanics, we measured the lateral force exerted onto an AFM cantilever as the cantilever tip was dragged across the brush surface (scan size 1 μm, frequency 20 Hz). The lateral force, which is phenomenologically similar to friction, was measured as a function of the applied normal force for a range of solution conditions. Figure 2 shows the relationship between lateral force and normal force for six samples. The slope of the linear regression through each data set reports the dependence of lateral force on normal force (hereafter, the coefficient of lateral resistance, or Clat).[9] The addition of cross-linker causes a dramatic change in lateral force. When the faster PdMe cross-linker is added, both the absolute lateral force values and Clat drop to ≈ 30% of those of the noncross-linked PVP control (Figure 2a). That this change is due to PdMe–PVP coordination is verified by the addition of 100 mM dimethylaminopyridine (DMAP) inhibitor, which competes for the PdMe and restores the lateral forces to their original values. The slower cross-linker PdEt also changes the mechanics, but in this case the absolute lateral force values and Clat increase by more than twofold (Figure 2b). As with PdMe, the effect of cross-linking can be chemically reversed to close to that of the noncross-linked state by the addition of DMAP inhibitor. Figure 2 a) Lateral force as a function of applied normal load, measured between a silicon nitride cantilever and PVP brushes in DMSO (○), 10 mg PdMe + 1 mL DMSO (▲), and 10 mg PdMe + 1 mL of 0.1M DMAP (□) (Clat equals 0.14, 0.05, and 0.12 ... Although the quantitative study of the mechanical properties of thin polymer brushes poses still a significant technical challenge, lateral force measurements by AFM provide a sensitive, qualitative measure of changes in the surface mechanical properties of polymer thin films. To facilitate interpretation, the PVP brushes were fabricated and treated in parallel, and AFM conditions were kept constant for all samples. Furthermore, the two cross-linkers are structurally similar, and the thermodynamics of the cross-linking equilibria are effectively equal. Clat values are relatively independent of variations between samples (0.18 ± 0.05) for noncross-linked PVP brushes of different heights (40–130 nm) and roughnesses (2.5–6 nm rms over 1 μm2) in DMSO. The changes induced by cross-linking fall well outside of this experimental uncertainty. The magnitude of the lateral forces should depend also on the scan rate, and it is possible that the differences in the measured lateral forces might disappear altogether under certain experimental conditions. To test the effect of scan rate, we measured lateral force on our brush surfaces over a lateral scan rate range from 2 to 40 μms−1. Within this range, the rate dependencies in the measured lateral forces are small compared to the differences measured between the PdMe and PdEt samples. This demonstrates that the different mechanics are not limited to an isolated set of experimental conditions (see the Supporting Information). Cross-linking could affect the measured lateral forces by changing the presentation of “dangling polymer ends”, heterogeneity, and roughness of the surface.[11,12] The structural effects should be in principle, and are in practice, indistinguishable for the two cross-linkers. The surfaces have minimal roughness (2–5 nm over 1 μm2) that varies little with or without cross-linker (Table 1). Normal force measurements show that the decay length of the steric repulsion provided by the brush surfaces of cross-linked brushes is significantly less than that for the noncross-linked brushes (data not shown). Importantly, however, the extent of the steric repulsion is independent of cross-linker type. The divergent effects of the two cross-linkers on lateral force thus suggest that the kinetics of the interaction, in addition to structural effects of the cross-linkers in the brush, make important contributions to brush mechanics. Table 1 A summary of the PVP brush heights and roughnesses measured on two samples fabricated in parallel, each in DMSO and then with 9 mM cross-linker. Initiator density in the monolayer is ca. 5 nm−2 (complete monolayer). Kinetics contributions could originate from adhesion and adhesion hysteresis, and from molecular relaxations in the brush.[10–12] Non-zero intercepts in the Clat regression lines suggest a contribution to the lateral force from adhesion,[13] and the intercept is greater for PdEt–PVP than for PdMe–PVP. The greater adhesion in PdEt–PVP that is implied by the intercepts is also observed directly; under identical experimental conditions for both surfaces, the force at which the AFM tip dissociates from the PVP–PdEt surface is greater than that observed for the PVP–PdMe surface (see the Supporting Information). In addition, the differences in Clat could arise from cross-links that bear lateral shear stress exerted by the AFM tip, in that the kinetic stability of the cross-links influences the magnitude of their resistance. Such behavior is observed in bulk PVP–PdMe and PVP–PdEt[7,8] and the rupture of single molecules.[14] The data thus suggest that cross-links reduce the lateral forces by limiting dangling-end contacts and/or penetration of the tip into the brush surface, but they increase the lateral force through mechanical resistance which depends on cross-link dissociation kinetics. The molecular origin of friction on “soft/wet” surfaces in general is an important and open area of research.[15] The use of well-defined kinetic probes, in combination with more sophisticated AFM techniques (e.g., microrheology[16]), offers potential for further mechanistic insight, for example, through frequency scaling.[7] Polymer brush layers with controlled and stimuli-responsive properties are of significant current interest,[17] with potential applications in biomedical surface engineering and nanofabrication.[1] The modulation of cross-linking interactions is shown here to be one method by which to exert control. The cross-linking is reversed here by chemical competition, but responsiveness to other stimuli, such as temperature, could be engineered. Both the magnitude of the cross-linking effect and the importance of the cross-linking kinetics must depend on the specific context, including among other factors grafting density, polymer molecular weight, the number and distribution of cross-links, and solvent quality. Ongoing work in our laboratories is addressing these relationships.

We have measured the translational energy dependence of a series of chloride exchange reactions in the gas phase. The translational energy dependence varies with the changes in the potential energy surfaces across the series of reactions in a manner that is consistent with the predictions of statistical reaction rate theory. These results indicate that despite reports of non-statistical behavior in simple S N 2 reactions, the assumptions of unimolecular reaction rate theory (strong coupling, energy randomization) are valid for the complexes formed in many bimolecular ion-molecule collisions.

Nucleophilic displacement reactions (the SN2 reaction) of ions in the gas phase are a prototypical reaction system that allows a study of dynamics, mechanisms, and structure-energy relations. This article reviews aspects of the kinetics (especially the applicability of statistical reaction rate theory), the relation of structure and reactivity, and the effects of small numbers of solvent molecules on the reaction and compares the behavior of the ionic reaction in the gas phase with that in solution.

Charging forward: Ionic interactions presented in a multivalent fashion in small-molecule ionic liquids lead to functional polymer-like materials (see picture) that are consistent with the formation of a supramolecular ionic network. For example, the ionic material formed from a dication consisting of two covalently linked tetraalkyl phosphonium moieties and a porphyrin tetracarboxylate has a viscosity of 10(6) Pa s at 25 degrees C.

Titanium is one of the most commonly used materials for implantable devices in humans. Scanning electron microscopy (SEM) serves as an important tool for imaging titanium surfaces and analyzing cells and other organic matter adhering to titanium implants. However, high-vacuum SEM imaging of a nonconductive sample requires a conductive coating on the surface. A gold/palladium coating is commonly used and to date no method has been described to "clean" such gold/palladium covered surfaces for repeated experiments without etching the titanium itself. This constitutes a major problem with titanium-based implantable devices which are very expensive and thus in short supply. Our objective was to devise a protocol to regenerate titaniumsurfaces after SEM analysis. In a series of experiments, titanium samples from implantable cardiac assist devices were coated with fibronectin, seeded with cells and then coated with gold/palladium for SEM analysis. X-ray photoelectron spectroscopy spectra were obtained before and after five different cleaning protocols. Treatment with aqua regia (a 1:3 solution of concentrated nitric and hydrochloric acid), with or without ozonolysis, followed by sonication in soap solution and sonication in deionized water, allowed regenerating titanium surfaces to their original state. Atomic force microscopy confirmed that the established protocol did not alter the titanium microstructure. The protocol described herein is applicable to almost all titanium surfaces used in biomedical sciences and because of its short exposure time to aqua regia, will likely work for many titanium alloys as well.

Das Titelbild zeigt Adamantan-2,6-dion in einer wasserstoffverbruckten Kapsel aus vier identischen Untereinheiten. Die Ketonfunktionen des Gastmolekuls (CPK-Modell, rot) sind an den strukturellen Nahtstellen an den jeweiligen Enden der Kapsel in Form von vergabelten Wasserstoffbrucken beteiligt. Mehr zu Verkapselungskomplexen erfahren Sie in dem Aufsatz von Rebek, Jr. et al. auf S. 1556 ff.