Flexible Alkylene Bridges as a Tool To Engineer Crystal Distyrylbenzene Structures Enabling Highly Fluorescent Monomeric Emission.

To design ultrabright fluorescent solid dyes, a crystal engineering strategy that enables monomeric emission by blocking intermolecular electronic interactions is required. We introduced propylene moieties to distyrylbenzene (DSB) as bridges between the phenyl rings either side of its C=C bonds. The bridged DSB derivatives formed compact crystals that emit colors similar to those of the same molecules in dilute solution, with high quantum yields. The introduction of flexible seven-membered rings to the DSB core produced moderate distortion and steric hindrance in the DSB π-plane. However, owing to this strategy, it was possible to control the molecular arrangement with almost no decrease in the crystal density, and intermolecular electronic interactions were suppressed. The bridged DSB crystal structure differs from other DSB derivative structures; thus, bridging affords access to novel crystalline systems. This design strategy has important implications in many fields and is more effective than the conventional photofunctional molecular crystal design strategies.

Living Anionic Addition Reaction of 1,1-Diphenylethylene Derivatives: One-Pot Synthesis of ABC-type Chain-End Sequence-Controlled Polymers.

In this study, a 1:1 addition reaction using 1,1-diphenylethylene (DPE) derivatives, referred to as the "living anionic addition reaction", was established to regulate the sequence of vinyl compounds having negligible homopolymerizability. The stoichiometric and successive addition reaction between a DPE anion and more reactive DPE derivatives proceeded quantitatively when the electrophilicity of the DPE derivatives was sufficiently enhanced by electron-withdrawing groups such as (trimethylsilyl)ethynyl and acyl groups. The relative electrophilicity of the DPE derivatives was predicted by Hammett's law and the ß-carbon chemical shifts of the carbon-carbon double bonds. AB- and ABC-type chain-end sequence-controlled polystyrenes with well-defined structures were synthesized by reacting two or three DPE derivatives with difunctional anionic living polystyrene in increasing order of their electrophilicity in a one-pot reaction.

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.

Structural reorganization and crack-healing properties of hydrogels based on dynamic diselenide linkages.

We report the dynamic behavior of diselenide-containing hydrophilic polyurethanes and hydrogels based on diselenide exchange reactions in an aqueous media. Diselenide-containing linear and cross-linked polyurethanes were synthesized via polyaddition reactions using diselenide-containing diol in combination with pyridinium diol that enhances the hydrophilicity of the polymer chains. The obtained linear polyurethanes underwent photo-induced diselenide exchange reactions with small diselenide compounds and degraded to smaller fragments, confirming the dynamicity of the obtained hydrophilic polyurethanes. The prepared hydrogels displayed characteristic large swelling behavior based on the structural reorganization through diselenide exchange either under photo-irradiation at 365 nm or even in the dark at room temperature. The diselenide-containing hydrogels also showed crack-healing behavior under the same exchanging conditions, presenting the utility of diselenide linkages as simple and useful units to offer high dynamicity to hydrogels.

The photoregulation of a mechanochemical polymer scission.

Control over mechanochemical polymer scission by another external stimulus may offer an avenue to further advance the fields of polymer chemistry, mechanochemistry, and materials science. Herein, we demonstrate that light can regulate the mechanochemical behavior of a diarylethene-conjugated Diels-Alder adduct (DAE/DA) that reversibly isomerizes from a weaker open form to a stronger closed form under photoirradiation. Pulsed ultrasonication experiments, spectroscopic analyses, and density functional theory calculations support the successful photoregulation of the reactivity of this DAE/DA mechanophore, which is incorporated at the mid-chain of a polymer, and indicate that higher force and energy are required to cleave the closed form of the DAE/DA mechanophore relative to the open form. The present photoregulation concept provides an attractive approach toward the generation of new mechanofunctional polymers.

Mechanochromic Polymers That Turn Green Upon the Dissociation of Diarylbibenzothiophenonyl: The Missing Piece toward Rainbow Mechanochromism.

Mechanochromic polymers, that is, polymers sensitive to mechanical impact, promise great potential for applications in damage sensors. In particular, radical-type mechanochromic polymers, which produce colored radical species in response to mechanical stress, may enable not only the visualization of mechanical stress, but also its quantitative evaluation by electron paramagnetic resonance analysis. Herein, a radical-type mechanochromic polymer that exhibits a color change from white to green upon dissociation of a diarylbibenzothiophenonyl moiety at the mid-point of a polystyrene chain is presented, and its mechanochromic behavior is examined. Mechanochromic materials that show a variety of colors ("rainbow colors") in response to mechanical stress were prepared by simply mixing radical-type mechanochromic polymers of primary colors.

Multicolor Mechanochromic Polymer Blends That Can Discriminate between Stretching and Grinding.

Mechanochromic polymers, which react to mechanical force by changing color, are expected to find applications in smart materials such as damage sensors. Although numerous types of mechanochromic polymers have been reported so far, developing mechanochromic polymers that can recognize different mechanical stimuli remains a formidable challenge. Materials that not only change their color in response to a mechanical stimulus but also detect its nature should be of great importance for practical applications. In this paper, we report our preliminary findings on multicolor mechanochromic polymer blends that can discriminate between two different mechanical stimuli, i.e., stretching and grinding, by simply blending two mechanochromic polymers with different architectures. The rational design and blending of two mechanochromic polymers with radical-type mechanochromophores embedded separately in positions adjacent to soft or hard domains made it possible to achieve multicolor mechanochromism in response to different stimuli. Electron paramagnetic resonance and solid-state UV-vis measurements supported the mechanism proposed for this discrimination.

Freezing-Induced Mechanoluminescence of Polymer Gels.

Mechanochromism can be triggered by different mechanical stimuli, such as tension, compression, shearing, and sonication. Freezing a polymer gel also induces mechanical stress on the polymer network. Herein, freezing-induced mechanoluminescence is demonstrated for the first time by introduction of a tetraarylsuccinonitrile moiety as a light-emitting mechanochromophore at the cross-linking points of a polymer network, in which the mechanical stress induces not only a color change but also light emission. The detailed mechanism and characteristics of this freezing-induced mechanoluminescence were quantitatively evaluated by electron paramagnetic resonance spectroscopy.

Thermally Stable Radical-Type Mechanochromic Polymers Based on Difluorenylsuccinonitrile.

Mechanochromism, a color change induced by mechanical force, has attracted much attention in materials science, as it can be used to create stress- and damage-detecting sensors. In particular, radical-type mechanochromic molecules (mechanochromophores), which produce colored radicals upon exposure to mechanical force, enable the qualitative visualization of mechanical stress and the quantitative evaluation of the generated radical species by electron paramagnetic resonance spectroscopy. However, the sensitivity of radical-type mechanochromophores to thermal stimuli limits their range of applications. Herein, we report the radical-type mechanochromophore difluorenylsuccinonitrile (DFSN), which can be used to synthesize mechanochromic polymers via living radical polymerization techniques, as its central carbon-carbon bond exhibits high thermal stability. The obtained DFSN-centered polymers show mechanochromism and desirably high thermal resistance.

Tetraarylsuccinonitriles as mechanochromophores to generate highly stable luminescent carbon-centered radicals.

This communication reports on the design and synthesis of mechanochromophores with a dynamic covalent system composed of a tetraarylsuccinonitrile skeleton that generate a metastable organic luminescent carbon radical. The mechanically generated radical species showed pink color and yellow light emission under UV irradiation. Unusually high stability of the luminescent carbon-centered radicals was also observed in a polymer system.

Thermally Adjustable Dynamic Disulfide Linkages Mediated by Highly Air-Stable 2,2,6,6-Tetramethylpiperidine-1-sulfanyl (TEMPS) Radicals.

Intrinsically exchangeable dynamic covalent bonds that can be triggered by readily usable stimuli offer easy incorporation of their dynamic properties in various molecular systems, but the library of such bonds is still being developed. Herein, we report the dynamic covalent chemistry of 2,2,6,6-tetramethylpiperidine-1-sulfanyl (TEMPS) dimers derived from thermally reversible homolytic dissociation of disulfide linkages. High air stability of TEMPS was observed even at 100 °C, affording facile employment of thermal dissociation-association equilibria and adjustable bond exchange properties under atmospheric conditions. We also established an efficient synthetic route for a modifiable derivative of the dimer that enabled incorporation of dynamic properties into linear and network polymer structures. The obtained polymers showed controllable molecular weights, temperature-dependent swelling properties, healing ability, and recyclability, reflecting the thermally tunable dynamics of the dimer.

Precise Synthesis of Macromolecular Architectures by Novel Iterative Methodology Combining Living Anionic Polymerization with Specially Designed Linking Chemistry.

This article reviews the development of a novel all-around iterative methodology combining living anionic polymerization with specially designed linking chemistry for macromolecular architecture syntheses. The methodology is designed in such a way that the same reaction site is always regenerated after the polymer chain is introduced in each reaction sequence, and this "polymer chain introduction and regeneration of the same reaction site" sequence is repeatable. Accordingly, the polymer chain can be successively and, in principle, limitlessly introduced to construct macromolecular architectures. With this iterative methodology, a variety of synthetically difficult macromolecular architectures, i.e., multicomponent µ-star polymers, high generation dendrimer-like hyperbranched polymers, exactly defined graft polymers, and multiblock polymers having more than three blocks, were successfully synthesized.

Thermally Healable and Reprocessable Bis(hindered amino)disulfide-Cross-Linked Polymethacrylate Networks.

A facile approach to polymethacrylate networks that contain thermally exchangeable bis(2,2,6,6-tetramethylpiperidin-1-yl)disulfide (BiTEMPS) cross-linkers is reported, and the easily inducible healability and reprocessability of the obtained networks are discussed. The free radical polymerization of BiTEMPS cross-linkers and hexyl methacrylate (HMA) monomers afforded insoluble and colorless networks of poly(hexyl methacrylate) (PHMA) films, whose structures were characterized after de-cross-linking via thermal BiTEMPS exchange reactions with added low-molecular-weight BiTEMPS. Swelling experiments and stress-relaxation measurements at elevated temperatures revealed the contribution of BiTEMPS as a polymer chain exchanger both in the gels and in the bulk. The BiTEMPS-cross-linked PHMA networks showed damage healability and repeatable reprocessability in the bulk by simple hot pressing at 120 °C under mild pressure (∼70 kPa). These results should grant facile access to various vinyl polymer networks with on-demand malleability.

Biomimetic Synthesis of Antireflective Silica/Polymer Composite Coatings Comprising Vesicular Nanostructures.

Antireflective (AR) silica/polymer composite coatings on glass and poly(methyl methacrylate) (PMMA) substrates were prepared by silica mineralization of layer-by-layer (LbL) assembled films composed of polystyrene-block-poly(l-lysine)/poly(l-glutamic acid) (PS-b-PLL/PGA) complex vesicles without any post-treatments. PS-b-PLL AB and A2B block copolymers with appropriate block ratio can self-assemble to form vesicles, which can be deposited onto substrates without dissociation. Silica deposition specifically onto the complex vesicles in the multilayer films through amine-catalyzed polycondensation results in the continuous, intact composite coatings comprising vesicular nanostructures, which provided an additional parameter for tuning their optical properties. The film thickness and porosity are mainly dictated by the bilayer number and the degree of deformation/fission of vesicles upon complexation and mineralization, depending on polymer composition. The coated PMMA substrate with maximum transmission over 98% can be achieved at the optimized wavelength region. The AR composite films were mechanically stable to withstand both the wipe and adhesion tests due to the preparation of continuous, intact films. This study demonstrated that the concept of preparing composite films comprising vesicular nanostructures through the combination of LbL assembly and biomineralization is feasible.

Enhancing Mechanochemical Activation in the Bulk State by Designing Polymer Architectures.

Mechanoresponsive polymers can have attractive functions; however, the relationship between polymer architecture and mechanoresponsiveness in the bulk state is still poorly understood. Here, we designed well-defined linear and star polymers with a mechanophore at the center of each architecture, and investigated the effect of molecular weight and branched structures on mechanoresponsiveness in the solid state. Diarylbibenzofuranone, which can undergo homolytic cleavage of the central C-C bond by mechanical force to form blue-colored radicals, was used as a mechanophore because the cleaved radicals could be evaluated quantitatively using electron paramagnetic resonance measurements. We confirmed that longer polymer chains induce mechanochemical activation more effectively and found that, in the bulk state, the star polymers have higher sensitivity to mechanical stress compared with a linear polymer having similar molecular weight arm segment.

Cylindrical Nanostructure of Rigid-Rod POSS-Containing Polymethacrylate from a Star-Branched Block Copolymer.

A nanostructure consisting of rectangular polyhedral oligomeric silsesquioxane (POSS) nanodomains packed into a hexagonal lattice was observed in POSS-containing A2B star-branched polymers. The A2B star-branched polymers, which comprised polystyrene (A) and bulky POSS-containing poly(methacrylate) (PMAPOSS) (B) units, were synthesized by anionic polymerization and addition reaction. The self-assembled structures of the A2B star-branched polymers were studied via transmission electron microscopy (TEM) and small- and wide-angle X-ray scattering (SAXS and WAXS, respectively). It was found that a rectangular nanostructure was packed into a hexagonal arrangement of nanodomains for a PMAPOSS volume fraction of 40 vol. % in the star-branched polymer. In addition, the cylinder-like nanostructure of bulky POSS observed, which is not observed in the case of PS-b-PMAPOSS diblock copolymers, was formed owing to the curvature effect, which is the result of the branched architecture of the copolymer.

Self-assembled lamellar nanostructures of wholly aromatic rod-rod-type block molecules.

Wholly aromatic rod-rod type di- and triblock molecules, oligo(ether sulfone)-b-oligo(ether ketone)s (OES-OEK), were synthesized to study a solid-state self-assembled nanostructure. The OES and OEK segments in the block molecules form segregated crystalline domains. The energy-filtering transmission electron microscopy images revealed that the di- and triblock OES-OEK co-oligomers formed lamellar nanostructures with a periodicity of approximately 9 and 13 nm, respectively. [structure: see text].