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.     

Crystal‐to‐Gel Transformation Stimulated by a Solid‐State E→Z Photoisomerization

The molecule (E)‐(5‐(3‐anthracen‐9‐yl‐allylidene)‐2,2‐dimethyl‐[1,3] dioxane‐4,6‐dione) (E‐ AYAD ) undergoes E→Z photoisomerization. In the solid state, this photoisomerization process can initiate a physical transformation of the crystal that is accompanied by a large volume expansion (ca. 10 times), loss of crystallinity, and growth of large pores. This physical change requires approximately 10 % conversion of the E isomer to the Z isomer and results in a gel‐like solid with decreased stiffness that still retains its mechanical integrity. The induced porosity allows the expanding gel to engulf superparamagnetic nanoparticles from the surrounding liquid. The trapped superparamagnetic nanoparticles impart a magnetic susceptibility to the gel, allowing it to be moved by a magnetic field. The photoinduced phase transition, starting with a compact crystalline solid instead of a dilute solution, provides a new route for in situ production of functional porous materials.     

Real‐Time In‐Situ Monitoring of a Tunable Pentapeptide Gel–Crystal Transition

Supramolecular gels often become destabilized by the transition of the gelator into a more stable crystalline phase, but often the long timescale and sporadic localization of the crystalline phase preclude a persistent observation of this process. We present a pentapeptide gel–crystal phase transition amenable for continuous visualization and quantification by common microscopic methods, allowing the extraction of kinetics and visualization of the dynamics of the transition. Using optical microscopy and microrheology, we show that the transition is a sporadic event in which gel dissolution is associated with microcrystalline growth that follows a sigmoidal rate profile. The two phases are based on β‐sheets of similar yet distinct configuration. We also demonstrate that the transition kinetics and crystal morphology can be modulated by extrinsic factors, including temperature, solvent composition, and mechanical perturbation. This work introduces an accessible model system and methodology for studying phase transitions in supramolecular gels.     

Hyper‐Crosslinkers Lead to Temperature‐ and pH‐Responsive Polymeric Nanogels with Unusual Volume Change

Hydrogels consisting of carboxylic acid groups and N‐isopropylacrylamide as pendants on their polymeric network usually exhibit volume expansion upon deprotonation or volume contraction when being heated. Demonstrated here is an anti‐intuitive case of a hydrogel containing multiple carboxylic acid groups at each crosslinking point in the polymeric network, which shrinks upon increasing pH from 1 to 7 at 37 °C or expands upon heating from 25 to 37 °C at pH 1. The unexpected volume change originates from the high percentage of the crosslinker in the polymers, as detected by solid‐state ¹³C NMR spectroscopy. In addition, the volume changes are thermally reversible. As the first example of the use of functional hyper‐crosslinkers to control the pH and thermal responses of nanogels, this work illustrates a new way to design soft materials with unusual behaviors.