as dynamers by Lehn, [ 25,26 ] are stimuli-responsive polymers, most notably exhibiting macroscopic responses to changes in pH. [ 27,28 ] Several imine-containing polymers have been demonstrated, including pH-responsive hydrogels [ 20 ] and a working organic light-emitting diode (OLED). [ 23 ] However, the potential of polyimines as malleable, mechanically resilient polymeric materials, as well as their processability, have remained largely unexplored. We envision that imine-linked polymers can take malleability in covalent network polymers to the next level of simplicity, affordability and practicality. Herein, we present the fi rst catalyst-free malleable polyimine which fundamentally behaves like a classic thermoset at ambient conditions yet can be reprocessed by application of either heat or water. This means that green, room temperature processing conditions are accessible for this important class of functional polymers.A crosslinked polyimine network was prepared from commercially available monomers: terephthaldehyde, diethylene triamine, and triethylene tetramine ( Figure 1 a). A polyimine fi lm was obtained by simply mixing the three above components in a 3:0.9:1.4 stoichiometry in the absence of any catalyst in a mixture of organic solvents (1:1:8, v/v/v, CH 2 Cl 2 /EtOAc/EtOH), then allowing the volatiles to evaporate slowly. Alternatively, the polymer can be obtained as a powder by using ethyl acetate as the only solvent. The polymerization reaction was confi rmed by infrared spectroscopy, which revealed that aldehyde end groups were consumed while imine linkages were formed ( Figure S2, Supporting Information). The resulting translucent polymer is hard and glassy at room temperature ( T g is 56 °C) ( Figure S1, Supporting Information) and has a modulus of near 1 GPa with stress at break of 40 MPa ( Figure S3, Supporting Information).The time and temperature dependent relaxation modulus of the polyimine fi lm was tested to characterize the heat-induced malleability. Figure 1 b depicts the results of a series of relaxation tests over a wide range of temperatures (50-127.5 °C) on a double logarithmic plot. Specifi cally, at 80 °C, the bond exchange reaction is initiated and the normalized relaxation modulus is decreased from 1 to 0.11 within 30 min, indicating an 89% release of the internal stress within the thermoset polymer. By shifting each relaxation curve horizontally with respect to a reference temperature at 80 °C, a master relaxation curve was constructed (Figure 1 c), which indicates the stress relaxation of the polyimine follows the classic time-temperature superposition (TTSP) behavior. The plot of time-temperature shift factors as a function of temperature (Figure 1 d) shows that the polyimine's stress-relaxation behavior exhibits Arrheniuslike temperature dependence. Using the extrapolation, we calculated that while it takes 30 min for the stress to be relaxed by ca. 90% at 80 °C, the same process would take ca. 480 days at room temperature. The polyimine is thus the fi rst reported

Carbon-fiber reinforced composites are prepared using catalyst-free malleable polyimine networks as binders. An energy neutral closed-loop recycling process has been developed, enabling recovery of 100% of the imine components and carbon fibers in their original form. Polyimine films made using >21% recycled content exhibit no loss of mechanical performance, therefore indicating all of the thermoset composite material can be recycled and reused for the same purpose.

The current research in the field of dynamic covalent chemistry includes the study of dynamic covalent reactions, catalysts, and their applications. Unlike noncovalent interactions utilized in supramolecular chemistry, the formation/breakage of covalent bonding has slower kinetics and usually requires the aid of a catalyst. Catalytic systems that enable efficient thermodynamic equilibrium are thus essential. In this Account, we describe the development of efficient catalysts for alkyne metathesis, and discuss the application of dynamic covalent reactions (mainly imine, olefin, and alkyne metathesis) in the development of organic functional materials. Alkyne metathesis is an emerging dynamic covalent reaction that offers robust and linear acetylene linkages. By introducing a podand motif into the catalyst ligand design, we have developed a series of highly active and robust alkyne metathesis catalysts, which, for the first time, enabled the one-step covalent assembly of ethynylene-linked functional molecular cages. Imine chemistry and olefin metathesis are among the most well-established reversible reactions, and have also been our main synthetic tools. Various shape-persistent macrocycles and covalent organic polyhedrons have been efficiently constructed in one-step through dynamic imine chemistry and olefin metathesis. The geometrical features and solubilizing groups of the building blocks as well as the reaction kinetics have significant effect on the outcome of a covalent assembly process. More recently, we explored the orthogonality of imine and olefin metatheses, and successfully synthesized heterosequenced macrocycles and molecular cages through one-pot orthogonal dynamic covalent chemistry. In addition to discrete molecular architectures, functional polymeric materials can also be accessed through dynamic covalent reactions. Defect-free solution-processable conjugated polyaryleneethynylenes and polydiacetylenes have been prepared through alkyne metathesis polymerization. We prepared imine- or ethynylene-linked porous polymer networks, which exhibit permanent porosity with high specific surface areas. Our most recent contribution is the discovery of a recyclable polyimine material whose self-healing can be activated simply by heating or water treatment. The facile access to complex functional organic molecules through dynamic covalent chemistry has allowed us to explore their exciting applications in gas adsorption/separation, host-guest chemistry, and nanocomposite fabrication. It is clear that there are significant opportunities for improved dynamic covalent systems and their more widespread applications in materials science.

Malleable thermosets are crosslinked polymers containing dynamic covalent bonds, which can be reversibly cleaved and reformed. They have attracted considerable attention in recent years due to their combined advantages of thermosets and thermoplastics. They have excellent mechanical properties and thermal and chemical stabilities like traditional thermosets yet are reprocessable and recyclable like thermoplastics. Although the chemical composition plays an important role in determining the mechanical and thermal properties of materials, the application of dynamic covalent chemistry is the key to achieving the unique properties of malleable thermosets. The mechanism of reversible bond cleavage and reformation, bond activation energies and kinetics, and the conditions triggering such reversibility define the malleable properties of the materials, how and why they can be reprocessed, and when the materials fail. In this review, we introduce fundamental concepts and principles of malleable thermosets, dynamic covalent chemistry, and the characteristic materials properties, including reprocessability, rehealability, and possible recyclability. We categorize the recent literature examples based on the underlying chemistry to demonstrate how dynamic covalent chemistry is exploited in malleable thermosets and how their malleable properties can be achieved and altered; we also discuss intriguing future opportunities based on such exploitation.Highly crosslinked covalent network polymers, commonly called thermosets, generally provide outstanding mechanical properties, chemical and heat resistance, and dimensional stability. Thermosets have found extensive variety of applications ranging from kettle handles and surface coatings to auto bodies. However, since thermosets are cured through the formation of irreversible chemical bonds, 1,2 they cannot be reprocessed or recycled upon failure. 3 Furthermore, any shape change that occurs due to reversible bond-exchange reactions (e.g., disulfide crosslinks) has been known as ''creep'' and has been considered a drawback of polymeric materials. 4 Therefore, it is by design that thermoset networks are irreversible and essentially unrecyclable.Thermoplastics, which consist of linear polymer chains with no crosslinks between them, represent reprocessable and fully recyclable polymeric materials. The thermoprocessing involves weakening of intermolecular forces between polymer chains, and no chemical bonding takes place. As a result, thermoplastics can be reshaped by heating many times without negatively affecting physical properties of the materials. However, they usually exhibit inferior chemical resistance and hightemperature mechanical properties compared with thermosets.Recently, covalent adaptable network (CAN) polymers have been developed, which combine excellent mechanical properties of thermosets with reprocessability of thermoplastics. [5][6][7] This new class of polymer networks incorporates dynamic

Thin solid membranes are formed by a new strategy, whereby an in situ derived self-healing polymer matrix that penetrates the void space of an inorganic solid is created. The concept is applied as a separator in an all-solid-state battery with an FeS2 -based cathode and achieves tremendous performance for over 200 cycles. Processing in dry conditions represents a paradigm shift for incorporating high active-material mass loadings into mixed-matrix membranes.