Keiichi Imato

Mechanophores with a Reversible Radical System and Freezing-Induced Mechanochemistry in Polymer Solutions and Gels†

Visualization and quantitative evaluation of covalent bond scission in polymeric materials are highly important for understanding failure, fatigue, and deterioration mechanisms and improving the lifetime, durability, toughness, and reliability of the materials. The diarylbibenzofuranone-based mechanophore radical system enabled, through electron paramagnetic resonance spectroscopy, in situ quantitative evaluation of scission of the mechanophores and estimation of mechanical energy induced along polymer chains by external forces. The coagulation of polymer solutions by freezing probably generated force but did not cleave the mechanophores. On the other hand, cross-linking led to efficient propagation of the force of more than 80 kJ mol(-1) to some mechanophores, resulting their cleavage and generation of colored stable radicals. This mechanoprobe concept has the potential to elucidate other debated issues in the polymer field as well.

Network Reorganization of Dynamic Covalent Polymer Gels with Exchangeable Diarylbibenzofuranone at Ambient Temperature

Reversible bonds and interactions have been utilized to build stimuli-responsive and reorganizable polymer networks that show recyclability, plasticity, and self-healing. In addition, reorganization of polymer gels at ambient temperature, such as room or body temperature, is expected to lead to several biomedical applications. Although these stimuli-responsive properties originate from the reorganization of the polymer networks, not such microscopic structural changes but instead only macroscopic properties have been the focus of previous work. In the present work, the reorganization of gel networks with diarylbibenzofuranone (DABBF)-based dynamic covalent linkages in response to the ambient temperature was systematically investigated from the perspective of both macroscopic and microscopic changes. The gels continued to swell in suitable solvents above room temperature but attained equilibrium swelling in nonsolvents or below room temperature because of the equilibrium of DABBF linkages, as supported by electron paramagnetic resonance measurements. Small-angle X-ray scattering measurements revealed the mesh sizes of the gels to be expanded and the network structures reorganized under control at ambient temperature.

Dynamic covalent diarylbibenzofuranone-modified nanocellulose: mechanochromic behaviour and application in self-healing polymer composites

The surface of cellulose nanocrystals (CNCs) was modified with a scissile but reversibly recombinable dynamic covalent mechanophore, and the activation of the mechanophore on the CNC surface in bulk was investigated. The recombination behaviour of the activated surface-modified mechanophore exhibited high sensitivity to mechanical stress because of the limited molecular mobility. The modified CNCs could be used to effectively reinforce a self-healable polymer containing similar dynamic covalent linkages through the formation of reversible covalent bonds between the CNC surfaces and the polymer matrix, while the nanocomposite retained the ability to heal. The results of the present study appear to be broadly useful for designing composite materials with fascinating functional properties such as damage self-reporting and self-healing.

Repeatable mechanochemical activation of dynamic covalent bonds in thermoplastic elastomers

Repeated mechanical scission and recombination of dynamic covalent bonds incorporated in segmented polyurethane elastomers are demonstrated by utilizing a diarylbibenzofuranone-based mechanophore and by the design of the segmented polymer structures. The repeated mechanochemical reactions can accompany clear colouration and simultaneous fading.

Diarylbibenzofuranone-Based Dynamic Covalent Polymer Gels Prepared via Radical Polymerization and Subsequent Polymer Reaction

Diarylbibenzofuranone (DABBF) is a dynamic covalent bonding unit, which is in equilibrium with the corresponding radicals at room temperature, and polymers with DABBF linkages show notable properties such as self-healing. The preparation routes have been strictly limited, however, and no polymer with the linkages has been synthesized via radical polymerization because of the strong antioxidant activity of DABBF. Here we present a new method to prepare DABBF-containing polymers via radical polymerization of the precursor, arylbenzofuranone (ABF), and subsequent polymer reaction, dimerization of ABF units in the linear polymers. Polymer gels cross-linked by DABBF linkages were obtained against the relatively strong antioxidant activity of ABF and showed dynamic network reorganization at room temperature.

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.Control over mechanochemical polymer scission by external stimuli may offer an avenue to further advance the fields of polymer chemistry, mechanochemistry, and materials science. Here the authors show light regulating the mechanochemical behavior of a diarylethene-conjugated Diels–Alder adduct.