Self-healing and shape-memory polymers based on cellulose acetate matrix.

The creation of self-healing polymers with superior strength and stretchability from biodegradable materials is attracting increasing attention. In this study, we synthesized new biomass-derived cellulose acetate (CA) derivatives by ring-opening graft polymerization of δ-valerolactone followed by the introduction of ureidopyrimidinone (Upy) groups in the polymer side chains. Due to the semicrystalline aliphatic characteristics of the side chain poly(δ-valerolactone) (PVL) and quadruple hydrogen bonds formed by the Upy groups, the stretchability of the resulting polymers was significantly enhanced. Moreover, the shape memory ability and self-healing property (58.3% of self-healing efficiency) were successfully imparted to the polymer. This study demonstrates the great significance of using biomass sources to create self-healing polymers.

This paper describes the first successful demonstration of self-healing polymers with superior strength and stretchability from a biodegradable material, cellulose acetate (CA). We initially introduced the ureidopyrimidinone (Upy) groups in the side chains of CA. However, the resulting polymer was not soluble and processable. In order to solve this issue, a new strategy based on the ring-opening graft polymerization of δ-valerolactone followed by the introduction of ureidopyrimidinone (Upy) groups was adopted. Due to the semicrystalline aliphatic characteristics of the side chain poly(δ-valerolactone) (PVL), the resulting polymers were soluble and processable. In addition, the quadruple hydrogen bonds formed by the Upy groups enhanced the stretchability of the resulting polymers. Moreover, the shape memory ability and self-healing property were successfully achieved due to the presence of PVL and Upy. The developed new strategy can be applied to a variety of polymers including biomass-based polymers and materials.

A thermally driven rotaxane-catenane interconversion with a dynamic bis(hindered amino) disulfide.

We have developed a versatile and simple synthetic method to produce a [3]catenane. Heating a rotaxane with bis(hindered amino) disulfide groups at both ends spontaneously and selectively produces the [3]catenane. The successful polymerization of the obtained [3]catenane provides a platform for the synthesis of various interlocking polymers.

Sacrificial Mechanical Bond is as Effective as a Sacrificial Covalent Bond in Increasing Cross-Linked Polymer Toughness.

Sacrificial chemical bonds have been used effectively to increase the toughness of elastomers because such bonds dissociate at forces significantly below the fracture limit of the primary load-bearing bonds, thereby dissipating local stress. This approach owes much of its success to the ability to adjust the threshold force at which the sacrificial bonds fail at the desired rate, for example, by selecting either covalent or noncovalent sacrificial bonds. Here, we report experimental and computational evidence that a mechanical bond, responsible for the structural integrity of a rotaxane or a catenane, increases the elastomer's fracture strain, stress, and energy as much as a covalent bond of comparable mechanochemical dissociation kinetics. We synthesized and studied 6 polyacrylates cross-linked by either difluorenylsuccinonitrile (DFSN), which is an established sacrificial mechanochromic moiety; a [2]rotaxane, whose stopper allows its wheel to dethread on the same subsecond time scale as DFSN dissociates when either is under tensile force of 1.5-2 nN; a structurally homologous [2]rotaxane with a much bulkier stopper that is stable at force >5.5 nN; similarly stoppered [3]rotaxanes containing DFSN in their axles; and a control polymer with aliphatic nonsacrificial cross-links. Our data suggest that mechanochemical dethreading of a rotaxane without failure of any covalent bonds may be an important, hitherto unrecognized, contributor to the toughness of some rotaxane-cross-linked polymers and that sacrificial mechanical bonds provide a mechanism to control material fracture behavior independently of the mechanochemical response of the covalent networks, due to their distinct relationships between structure and mechanochemical reactivity.

A rational design strategy of radical-type mechanophores with thermal tolerance.

Radical-type mechanophores (RMs) are attractive molecules that undergo homolytic scission of their central C-C bond to afford radical species upon exposure to heat or mechanical stimuli. However, the lack of a rational design concept limits the development of RMs with pre-determined properties. Herein, we report a rational design strategy of RMs with high thermal tolerance while maintaining mechanoresponsiveness. A combined experimental and theoretical analysis revealed that the high thermal tolerance of these RMs is related to the radical-stabilization energy (RSE) as well as the Hammett and modified Swain-Lupton constants at the para-position (σp). The trend of the RSE values is in good agreement with the experimentally evaluated thermal tolerance of a series of mechanoresponsive RMs based on the bisarylcyanoacetate motif. Furthermore, the singly occupied molecular orbital (SOMO) levels clearly exhibit a negative correlation with σp within a series of RMs that are based on the same skeleton, paving the way toward the development of RMs that can be handled under ambient conditions without peroxidation.

Theoretical and Experimental Investigations of Stable Arylfluorene-Based Radical-Type Mechanophores.

Invited for the cover of this issue are the groups of Hideyuki Otsuka at the Tokyo Institute of Technology and Koichiro Mikami at the Sagami Chemical Research Institute. The image depicts theoretical and experimental investigations of stable arylfluorene-based radical-type mechanophores. Read the full text of the article at 10.1002/chem.202203249.

Theoretical and Experimental Investigations of Stable Arylfluorene-Based Radical-Type Mechanophores.

Radical-type mechanophores (RMs) can undergo homolytic cleavage of their central C-C bonds upon exposure to mechanical forces, which affords radical species. Understanding the characteristics of these radical species allows bespoke mechanoresponsive materials to be designed and developed. The thermal stability of the central C-C bonds and the oxygen tolerance of the generated radical species are crucial characteristics that determine the functions and applicability of such RM-containing mechanoresponsive materials. In this paper, we report the synthesis and characterization of two series of arylfluorene-based RM derivatives, that is, 9,9'-bis(5-methyl-2-pyridyl)-9,9'-bifluorene (BPyF) and 9,9'-bis(4,6-diphenyl-2-triazyl)-9,9'-bifluorene (BTAF). BPyF and BTAF derivatives were synthesized without generating any peroxides initially, albeit that BPyF slowly converted to the corresponding peroxide in solution. DFT calculations revealed the importance of the thermodynamic stability and the values of the α-SOMO levels of the corresponding radical species for their thermal stability and oxygen tolerance. Furthermore, the mechanochromism of BTAF was demonstrated by ball-milling a BTAF-centered polymer, which was synthesized by atom-transfer radical polymerization (ATRP).

Swelling-induced Mechanochromism in Multinetwork Polymers.

We report a novel and versatile approach to achieving swelling-induced mechanochemistry using a multinetwork (MN) strategy that enables polymer networks to repeatedly swell with monomers and solvents. The isotropic expansion of the first network (FN) provides sufficient force to drive the mechanochemical scission of a radical-based mechanophore, difluorenylsuccinonitrile (DFSN). Although prompt recombination generally occurs in such highly mobile environments, the resulting pink radicals are kinetically stabilized in the gels, probably due to limited diffusion in the extended polymer chains. Moreover, the DFSN embedded in the isotropically strained chain exhibits increased thermal reactivity, which can be reasonably explained by an entropic contribution of the FN to the dissociation. The utility of the MN polymers is demonstrated not only in terms of swelling-force-induced network modification, but also in the context of tunable reactivity of the dissociative unit through proper design of the hierarchical network architecture.

Mechanochromic elastomers with different thermo- and mechano-responsive radical-type mechanophores.

To design tough soft materials, the introduction of sacrificial bonds into their skeleton is a useful method. The introduction of radical-type mechanophores (RMs), which generate coloured radicals in response to mechanical stimuli, as sacrificial bonds into the cross-linking points of elastomers is expected to be a powerful tool to elucidate the fracture mechanisms as well as the toughening of materials, given that the radicals generated from the RMs are coloured and can be quantitatively evaluated using electron paramagnetic resonance (EPR) measurements. In this study, to investigate the effect of the dynamic nature, i.e., the reactivity, of RMs introduced at the cross-linking points of polymer networks on their macroscopic mechanical properties, polymer networks cross-linked by two different RMs, a symmetric radical-type mechanophore (DFSN) and a non-symmetric radical-type mechanophore (CF/ABF), were synthesized and characterized. Compared to the polymer network cross-linked by DFSN, the network with CF/ABF exhibited higher thermal and mechanical responses, in other words much more sensitive to heat and mechanical force, resulting in better stress relaxation and energy-dissipation properties. These results demonstrate that the reactivity of the radical mechanophore at the cross-linking point is an important factor for designing polymer networks.

Mechanochromic cyclodextrins.

Mechanochromic cyclodextrins (MCDs) that can generate blue radical species, which are exceptionally stable toward atmospheric oxygen and can thus be quantitatively characterized via electron paramagnetic resonance (EPR) spectroscopy, were synthesized. MCDs have a defined structure that consists of a diarylbibenzofuranone skeleton mechanophore sandwiched between two CDs. Grinding tests and EPR measurements of the MCDs revealed their high mechanoresponsiveness, reflecting the inherent rigidity of the CDs and the formation of a supramolecular structure in the bulk.

Mechanochemical Reactions of Bis(9-methylphenyl-9-fluorenyl) Peroxides and Their Applications in Cross-Linked Polymers.

The exploration of mechanochemical reactions has brought new opportunities for the design of functional materials. We synthesized the novel organic peroxide mechanophore bis(9-methylphenyl-9-fluorenyl) peroxide (BMPF) and examined its mechanochromic properties. The mechanism behind its mechanofluorescence was clarified and harnessed in polymer networks that can release the small fluorescent molecule 9-fluorenone upon exposure to a mechanical stimulus. Additionally, polymer networks cross-linked with BMPF units are able to tolerate temperatures up to 110 °C without any change in optical properties or mechanical strength. As mechanophores based on organic peroxide have rarely been documented so far, these fascinating results suggest excellent potential for applications of BMPF in stress-responsive materials. The mechanochemical protocol demonstrated here may provide guiding principles to expand the field of mechanochromic peroxides.

Non-symmetric mechanophores prepared from radical-type symmetric mechanophores: bespoke mechanofunctional polymers.

A non-symmetric radical-type mechanophore (CF/ABF) was synthesized by molecular crossing between two radical-type mechanophores. The thermal stability and mechanoresponsiveness of CF/ABF were found to be tunable by altering the properties of the parent RMs. The CF/ABF-centred polymers showed mixed mechanochromism derived from the simultaneous generation of two radical species.

Segmented Polyurethane Elastomers with Mechanochromic and Self-Strengthening Functions.

Mechanochromic elastomers that exhibit force-induced cross-linking reactions in the bulk state are introduced. The synthesis of segmented polyurethanes (SPUs) that contain difluorenylsuccinonitrile (DFSN) moieties in the main chain and methacryloyl groups in the side chains was carried out. DFSN was selected as the mechanophore because it dissociates under mechanical stimuli to form pink cyanofluorene (CF) radicals, which can also initiate the radical polymerization of methacrylate monomers. The obtained elastomers generated CF radicals and changed color by compression or extension; they also became insoluble due to the mechanically induced cross-linking reactions. Additionally, an SPU containing diphenylmethane units also exhibited highly sensitive mechanofluorescence. To the best of our knowledge, this is the first report to demonstrate damage detection ability and changes in the mechanical properties of bulk elastomers induced by simple compression or extension.

Crystallization-induced mechanofluorescence for visualization of polymer crystallization.

The growth of lamellar crystals has been studied in particular for spherulites in polymeric materials. Even though such spherulitic structures and their growth are of crucial importance for the mechanical and optical properties of the resulting polymeric materials, several issues regarding the residual stress remain unresolved in the wider context of crystal growth. To gain further insight into micro-mechanical forces during the crystallization process of lamellar crystals in polymeric materials, herein, we introduce tetraarylsuccinonitrile (TASN), which generates relatively stable radicals with yellow fluorescence upon homolytic cleavage at the central C-C bond in response to mechanical stress, into crystalline polymers. The obtained crystalline polymers with TASN at the center of the polymer chain allow not only to visualize the stress arising from micro-mechanical forces during polymer crystallization via fluorescence microscopy but also to evaluate the micro-mechanical forces upon growing polymer lamellar crystals by electron paramagnetic resonance, which is able to detect the radicals generated during polymer crystallization.

Polystyrene Functionalized with Diarylacetonitrile for the Visualization of Mechanoradicals and Improved Thermal Stability.

The direct scission of polymer main chains leads to a decrease in the performance of the polymeric materials. Polystyrene-functionalized with diarylacetonitrile (DAAN) was prepared through a postpolymerization modification with 4-methoxymandelonitrile to generate mechanofluorescent polymers that enable the visualization of the scission of the polymer main chain. The polymeric mechanoradicals obtained from the homolytic cleavage of the polymer main chain in response to mechanical stress were observed using fluorescence and electron paramagnetic resonance spectroscopy. Moreover, a thermogravimetric analysis showed that the thermal stability of the polymers was greatly improved relative to the parent polystyrene, that is, the introduction of the DAAN moiety via postpolymerization modification endowed the original polymers with multiple functions in one step; specifically, the ability to visualize polymer main-chain scission and improved thermal stability.

Visualization of the Necking Initiation and Propagation Processes during Uniaxial Tensile Deformation of Crystalline Polymer Films via the Generation of Fluorescent Radicals.

To visualize and simultaneously quantify the necking behavior of crystalline polymer films during uniaxial stretching, tetraarylsuccinonitrile (TASN) moieties were introduced into polymers at the center of the main chain. TASN can produce a relatively stable radical that emits yellow fluorescence in response to mechanical stress. During the uniaxial elongation test of the TASN-centered crystalline polymers, the yellow fluorescence derived from the dissociated TASN radicals was used for microscale observations that showed the orientation of the polymer chains in the stretching direction. Furthermore, by comparing the radical generation in linear and star-shaped TASN-centered crystalline polymers during their tensile deformation, we found that the TASN dissociation ratio is higher in the star-shaped polymer, which has more chains connected to the lamellar crystal. Thus, the microforces generated in the amorphous region during uniaxial stretching were probed via the use of TASN, which allowed a direct visualization of the necking initiation and propagation processes as well as a quantification via electron paramagnetic resonance spectroscopy.

Self-Strengthening of Cross-Linked Elastomers via the Use of Dynamic Covalent Macrocyclic Mechanophores.

The creation of polymeric materials that self-strengthen in response to a mechanical force is an important objective in the field of polymer chemistry. Here, the mechanochemical strengthening of cross-linked elastomers using macrocyclic mechanophores that contain a dynamic covalent disulfide bond is reported. Cross-linked poly(hexyl methacrylate) (CPHMA) polymers with macrocyclic mechanophores inserted at the cross-linking points were synthesized via free radical polymerization. Tensile and swelling tests showed that the addition of the macrocyclic mechanophores to the CPHMA polymers successfully impart them with self-strengthening functionality following compression, without the need for any additives such as monomers and modifiers, or any other stimuli.

A Diarylacetonitrile as a Molecular Probe for the Detection of Polymeric Mechanoradicals in the Bulk State through a Radical Chain-Transfer Mechanism.

Since the beginning of polymer science, understanding the influence of mechanical stress on polymer chains has been a fundamental and important research topic. The detection of mechanoradicals generated by homolytic cleavage of the polymer chains in solution has been studied in many cases. However, the detection of mechanoradicals in the bulk is still limited owing to their high reactivity. Herein, we propose an innovative strategy to detect mechanoradicals visually and quantitatively using a chain-transfer agent that generates relatively stable fluorescent radicals as a molecular probe. Mechanoradicals generated by ball milling of polystyrene samples were successfully detected by using a diarylacetonitrile compound as a fluorescent molecular probe through this radical chain-transfer mechanism. This probe enables the visualization and quantitative evaluation of mechanoradicals generated by polymer-chain scission.

Mechanochromic Polymers That Recognize the Duration of the Mechanical Stimulation via Multiple Mechanochromism.

Mechanochromic polymers can be used as stress- and damage-detecting sensors in polymeric materials, given that mechanical stimuli can be visualized by color changes. Although many types of mechanochromic polymers have been reported so far, there are only few examples on their further functionalization based on multiple color changes (multicolor mechanochromism). Herein, preliminary results are reported on the use of multicolor mechanochromism to detect the duration of the mechanical stimulation by simply mixing white powders of two mechanochromic polystyrene samples that contain a different radical-type mechanochromophore at the midpoint of each polymer chain and thus exhibit different colors in response to mechanical stimuli. The mechanosensitivity can be tuned via the polymer length and shape, and a combination of these two types of mechanochromic polymers allows detecting the duration with multicolor mechanochromism, i.e., a color change from white to blue upon short exposure to grinding and a color change from white to gray upon longer exposure to grinding. Electron paramagnetic resonance and solid-state UV-vis measurements support the mechanism proposed for this multiple mechanochromism.

Characterization of N-phenylmaleimide-terminated poly(ethylene glycol)s and their application to a tetra-arm poly(ethylene glycol) gel.

Tetra-arm poly(ethylene glycol) (TetraPEG) gels are tough materials whose toughness originates from their uniform network structure. They can be formed by combining the termini of tetra-arm polymers via chemical reactions with high conversion efficiency, such as the Michael addition, condensations using an active ester group, and alkyne-azide cycloadditions. Herein, we report the synthesis of a tetra-PEG gel using a tetra-arm polymer with N-phenylmaleimide moieties at the polymer ends (tetra-N-aryl MA PEG) as a scaffold. Tetra-N-aryl MA PEG can be obtained via a simple maleimidation using the modification agent p-maleimidophenyl isocyanate (PMPI), which directly transforms the hydroxy groups at the polymer ends into reactive N-aryl maleimide groups in a one-pot reaction. The thus-obtained tetra-N-aryl MA PEG was fully characterized using high-performance liquid chromatography (HPLC), matrix-assisted laser desorption ionization time of flight mass spectrometry, and proton nuclear magnetic resonance spectroscopy. HPLC analysis not only demonstrated the high purity of tetra-N-aryl MA PEG and the full conversion of the hydroxy groups, but also provided an effective characterization method for N-aryl maleimide-based PEG using a simple protocol, which enables us quantitative analysis of functionalized polymers with different N-aryl maleimide numbers. Furthermore, we fabricated a TetraPEG gel via Michael addition of the obtained tetra-N-aryl MA and thiol-terminated TetraPEGs. Thus, this report presents the application of tetra-N-aryl MA PEG as an effective precursor to obtain a uniform network structure and a method for its characterization; these results should provide support for the development of functional molecules, soft materials, and further functional materials based on the uniform-network-structure concept.