Dynamic polymeric materials based on reversible B–O bonds with dative boron–nitrogen coordination

To minimize the environmental pollution caused by polymeric waste, materials based dynamic chemistry have attracted extensive attention around the world. Various dynamic covalent bonds or noncovalent interactions have been employed to design multifunctional polymers with recyclability, reprocessablility, and sustainability. Among them, polymers based on reversible boron–oxygen (B–O) bonds have been widely investigated because of their unique properties. Particularly, lots of scientists have demonstrated that the combination with boron–nitrogen (B–N) coordination can effectively accelerate the dynamicity as well as enhance the stability of B–O bonds. Therefore, numerous polymers containing dynamic B–O bonds with dative B–N coordination have been designed and synthesized in recent years. These polymers exhibit excellent versatility and great potential for diverse applications such as biosensors, battery electrolytes, and artificial skins. This review provides an overview of the comprehensive influence of dynamic B–N coordination chemistry on B–O bonds in organoboron species and highlights the developments in the area of constructing boron‐containing polymeric materials with this interesting linkage. The design guidelines, existing challenges, and future perspectives in this burgeoning field are discussed and proposed.

two categories: dynamic noncovalent bonds (DNCBs) and dynamic covalent bonds (DCBs). 12,13 In the past few years, materials based on DNCBs such as hydrogen bonds, metal-ligand coordination, van der Waals forces, π-π stacking, and host-guest interactions have been widely reported. [14][15][16][17][18][19] Generally, DNCBs can provide the generated polymers with autonomously rapid exchange and rearrangement under ambient conditions at the expense of mechanical strength. On the contrary, DCBs are usually static at room temperature but become dynamic in response to specific external stimuli such as heating, pH, light, and pressure. [20][21][22] Therefore, DCBs can offer polymers with higher stability and superior mechanical properties. DCBs including disulfide bonds, B-O bonds, Diels-Alder chemistry, imine exchanges, alkoxyamine, and oxime bonds have been successfully applied to develop recyclable functional materials, which undergo dissociation and/or association bond exchange mechanisms in polymeric networks. [23][24][25] Particularly, B-O bonds play an important role in this field due to the unique advantage of boron atoms, which can form desired complexes with p-donors, Lewis bases, and nucleophiles. [26][27][28][29] Moreover, boron-containing polymers are popular in various applications such as flameretardants, luminescent devices, underwater adhesives, electronic sensors, and battery electrolytes. [30][31][32][33][34] Dynamic B-O bonds, which mainly exist in boronic ester and boroxine species, have been extensively employed to develop new dynamic advanced materials. Typically, trigonal planar boronic esters are obtained via the binding of boronic acids and diols while sixmembered cyclic boroxines are attained solely from the trimerization of boronic acids. 35,36 Although the properties of resulting organoboron compounds are different, the dynamic exchange of B-O bonds in boronic esters and boroxines can be similarly achieved through reversible hydrolysis/dehydration, transesterification, and metathesis pathways. 29,37 However, on account of the electron-deficient feature of the sp 2 hybridized boron atom, conventional B-O bonds are vulnerable to being attacked by electron-donating groups and thus exhibit considerable sensitivity to water molecules or alcohols, resulting in the limited application scenarios. 38,39 To date, tremendous efforts have been devoted to improving the stability of dynamic polymers containing B-O bonds, the most effective strategy of which is to stabilize the central boron atom by means of dative boron-nitrogen (B-N) coordination. Previous studies have demonstrated that various nitrogen-donor ligands (e.g., pyridine, amine, ortho-amino, imine) can coordinate with the boron atom of boronic esters and boroxines, giving rise to an obvious structural transformation from trigonal planar to tetrahedral geometry at boron centers. 40,41 With the incorporation of B-N coordination interaction, a variety of DMs based on boronic ester and boroxine linkages including supramolecular gels, covalent organic frameworks, and selfhealing polymers have been widely reported. [42][43][44] The resultant materials not only possess enhanced hydrolytic stability in humid conditions but also display improved dynamic exchange behaviors of B-O bonds as well as superior mechanical performances. However, a review focusing on the dynamic polymeric materials consisting of reversible B-O bonds and dative B-N coordination has not been proposed until now.
Herein, we initiate the review with the comprehensive introduction to the effect of dative B-N coordination on the thermodynamic and kinetic properties of dynamic B-O bonds. Furthermore, we highlight the development of multifunctional DMs containing dynamic boronic ester and boroxine linkages with dative B-N coordination bonds, respectively, from the perspectives of design concepts, mechanical properties, reprocessability, and self-healing performance. Finally, we critically analyze the existing challenges and present an outlook on the opportunities in this emerging field.

| EFFECT OF DATIVE B-N COORDINATION ON THE PROPERTY OF B-O BONDS
Generally, the Lewis acidic organoboron compounds containing sp 2 hybridized boron atoms can form adducts with Lewis basic groups (e.g., nitrogen-donor ligands), which will lead to the structural transition of boron species from trigonal planar to tetrahedral configuration. In 1958, Lennarz and coworkers reported the pioneering preparation of the B-N coordinated compound between triphenylboroxine and pyridine ( Figure 1A). 48 The pure product with a stoichiometry of 1:1 was successfully obtained and characterized by an infrared spectrum, after which a series of B-N coordinated complexes have been intensively synthesized and investigated.
It is well-known that the stability and thermodynamics of boronic esters/boroxines mainly depend on the neighboring substituents. 49,50 Similarly, many research have proved that the strength of dative B-N coordination bonds can be readily tuned by modulating the electronic and steric effects of the interacting components. In the review of boron coordination compounds by Contreras et al., the heterocycles with B-N coordination bonds were demonstrated to be more stable because the basic nitrogen provided a stereogenic atom to the molecular configuration. 41 Moreover, Beckmann et al. 45 used density functional theory (DFT) calculations to explore the reasons for the 1:1 stoichiometry of amines to boroxines, and the more susceptible ring cleavage of boroxines than amine adducts ( Figure 1B). The calculated results of ring strain in boroxine rings indicated that boroxine-ligand compounds exhibit improved stability because of the lower strain of adducts. Moreover, the "energy cost" of deviation for the ideal angle was studied by relaxed potential energy scans. The determined energy decreased from 14.7 to 8.7 kJ/mol with the amine coordination, illustrating that the adduct between boroxine and amine is more reluctant to cleavage. On the basis of this research, Iovine and coworkers further proved that the formation of a 1:1 adduct with amine coordination in boroxines is highly favorable. Applying DFT at the B3LYP//6-311 + G* level including Poisson-Boltzmann implicit solvent, they observed that when the substitution in the paraposition of phenylboroxine-amine adduct ring is πelectron-withdrawing group (such as -CHO and -C(O) CH 3 ), the 1:1 adducts become unstable. 46,47 On the contrary, the electron-donating groups in the paraposition can stabilize the trimer adducts ( Figure 1C-E). Furthermore, they investigated the effect of polarity of solvents on the formation of boroxine-amine adducts through nuclear magnetic resonance (NMR) measurements and DFT calculations. As presented in Figure 1F, the B-N adduct formation is demonstrated to be enthalpically stabilized by increasing the polarity of the selected solvent from chloroform to acetone.
Analogous to the aforementioned complexes which are composed of external B-N coordination, boron compounds can also form internal dative B-N bonds with N-donor species. In 1981, Wulff et al. synthesized the first case of boronic esters containing intramolecular B-N coordination bonds via the esterification of N(CH 3 ) 2 substituted boronic anhydrides with corresponding diols. 51 By utilizing 1 H NMR, 13 C NMR, 11 B NMR spectra, they not only proved the presence of such B-N bond in obtained compounds, but also investigated the fast thermal dynamic breaking-reformation procedure of this internal B-N coordination bond in solution ( Figure 2A). 38,52,56 After that, more and more works focusing on the development of novel boron complexes with specific internal B-N interaction have been successively reported. In 2006, Anslyn and coworkers conducted a systematic structural study of the internal B-N interaction in o-(N,N-dialkylaminomethyl) arylboronate by X-ray crystallography, 11 B NMR spectroscopy, and computational analysis. They revealed that the internal B-N coordination bonds prefer to form in an aprotic solvent and demonstrated that the enhanced binding properties of the prepared boronic esters/ boroxines are due to the stabilization effect of internal B-N bonds ( Figure 2B). 53 The groups of Shinkai and Wang studied the intramolecular acid-base interaction model systems and employed the electron transfer in molecules as fluorescent sensors, respectively ( Figure 2C,D). 54,55 Particularly, it is worth noting that a  Figure 3A). 58 In the research about the stability of boronic esters by Brown and coworkers, the transesterification between dioxaborolane and diethanolamine with different substituents was evaluated by 11 B and 1 H NMR spectra. 49 The transesterification ratio sharply decreased as the diethanolamine was substituted by Nbutyldiethanolamine and 2,6-pyridinedimethanol (diols 23 and 24 in Figure 3B), suggesting that the boronic ester chelated with diethanolamine (diol 21 in Figure 3B) is the most stable complex. In 2020, Jing's group synthesized a series of five-membered cyclic boronic esters with internal B-N dative bonds. 39 They found that the bond lengths of internal B-N bonds in the resultant compounds (NCBCs) are shorter than that of the external B-N bonds, indicating the higher stability of NCBCs linkages. Besides, the hydrolytic stability of NCBCs was studied via hydrolysis experiments monitored by 1 H NMR. The calculated hydrolysis percentages and equilibrium constant (K eq ) illustrated that the hydrolytic stability of NCBC is higher than other conventional boronic ester linkages ( Figure 3C). Although boronic esters with internal B-N coordination exhibit enhanced stability, dynamic exchanges can still occur between molecules under heated conditions without catalysts. The kinetics of the dynamic transesterification reactions in nitrogen-coordinating cyclic boronic ester (NCB) linkages were studied by liquid chromatography and variable temperature 1 H NMR ( Figure 3D). The calculated activation energy (E a ) of the exchange reaction is 62.9 kJ/mol, which is higher than the value of the reaction between conventional cyclic boronic esters. The kinetic studies revealed that the introduction of internal B-N coordination will cause the reduction of the exchange rate between boronic ester linkages. Based on the theory of ring strain, our group further developed sixmembered cyclic boronic esters with internal interaction (BN-6). 59 The kinetics and stability investigations indicated that BN-6 containing heat-responsive internal B-N coordination possesses higher hydrolysis stability at ambient conditions while demonstrating more active dynamic exchanges under heating ( Figure 3E hindrance of neighboring substituents as well as changing the polarity of solvents. Attributing to the structural transformation from trigonal planar to tetrahedral geometry at boron atom centers, the hydrolytic stability of resultant boronic esters/boroxines shows significant improvement. Therefore, the utilization of novel B-O bonds with this unique dative B-N interaction will certainly benefit to construction of more advanced DMs.

| DMs BASED ON BORONIC ESTERS WITH B-N COORDINATION
The introduction of B-N coordination would reinforce the intermolecular interaction and promote the selforganization of boronic esters. Utilizing the thermodynamically stable but kinetically labile features, Severin et al. have been devoted to the synthesis of crystalline and soft supramolecular polymers by the connection of boronic esters through dative B-N bonds. [60][61][62] In 2011, they first reported the two-dimensional networks based on dative B-N interactions, which were achieved via combining triboronate esters with dipyridyl linkers of different lengths ( Figure 4A,B). 63 On this basis, Severin and coworkers synthesized a series of supramolecular polymers based on dative B-N bonds, of which the binding strengths could be increased due to complementary electronic effects ( Figure 4C). 64 Furthermore, they successfully observed the unprecedented formation of chains of macrocycles in crystalline polymeric networks by using conformationally restricted tripyridyl ligands and linear diboronate esters as starting materials, which promoted the development of materials based on B-N adducts into a new level ( Figure 4D). 65 Taking advantage of the self-assembled structures of dynamic boronic esters and B-N bonds, Iwasawa and coworkers developed a brand-new concept for designing stimuli-responsive gels by utilizing the guest-included macrocyclic boronic esters as gelator and appropriate diamine as cross-linker, which could realize the release and recovery of the guest molecule under various external stimuli (such as heat, acid, and alkali) ( Figure 5A,B). 42 In 2015, Dai et al. used intermolecular B-N interaction as a building bridge and constructed supramolecular nanoassemblies with pHswitchable fluorescent emission and stimuli-responsive properties ( Figure 5C). 66 The introduction of B-N coordination would reinforce the intermolecular interaction and promote the self-organization of molecules. Furthermore, it also endowed the supramolecular assemblies with responsive properties to pH and glucose, making the fluorescent material have potential application in biomedical imaging and sensing ( Figure 5D).
Self-healing materials based on dynamic boronic ester covalent bonds, which can exhibit excellent mechanical properties and reprocessability, have been widely studied. However, the humidity-sensitive boronic ester bonds will lead to the collapse of polymeric networks in a humid environment. The self-healing processes of B-Obased materials are generally achieved with external stimuli (such as water, heating, and pH). In recent years, boronic esters with dative B-N interactions have attracted more and more attention in the field of selfhealing polymers because of the enhanced hydrolytic stability and dynamic B-N bonds. Yoshie and coworkers realized a nonswellable self-healing polymer that exhibits high self-healing ability under seawater through utilizing the sterically hindered tetrahedral boronate ester and catechol-Ca 2+ coordination bonds as crosslinking linkages ( Figure 6A). 67 The dative B-N interactions were formed between the Lewis acidic boron atoms in boronic esters and the Lewis base trimethylamine (TEA), which could endow the coordinated bulk network with efficient water stability and self-healing ability underwater ( Figure 6B-D). Furthermore, the authors synthesized dynamic covalent crosslinking polymers P-PDBAs containing the same boronic ester linkages with B-N interactions ( Figure 6E). 68 The linear copolymers bearing catechol units were crosslinked by p-dibenzeneboronic acid (PDBA), which could form coordination bonds with TEA in the polymeric networks. The structure of boron centers was confirmed by solid-state 11 B NMR spectroscopy ( Figure 6F). The as-prepared material could maintain its mechanical properties (7.3 MJ/m 3 ) under highly humid conditions ( Figure 6G). In addition, the self-healing ability of P-PDBAs was achievable under ambient conditions, and the polymers demonstrated moisture-or heat-responsive reprocessability due to the existence of reversible B-O/B-N bonds ( Figure 6H,I).
Except for the contribution to self-healing ability, dative B-N coordination has been proven to be valuable in improving the mechanical performance of polymeric materials. As shown in Figure 7A, Ding et al. reported a new strategy for synthesizing self-healing supramolecular polyurethane elastomers (SPUEs) with unprecedented mechanical properties based on dynamic boronic esters with B-N interactions. 69 Boron-nitrogen bonds between boronic esters and tertiary amines on the backbone of polyurethane were investigated by DFT calculation, which indicated that the length of B-O near B-N coordination is longer than those in normal boronic ester molecules ( Figure 7B). Therefore, the authors inferred that the dissociation and reformation of boronic ester bonds can be expedited by the coordination interactions, resulting in the improvement of healing efficiency could reach 84.3% after 36 h recovery ( Figure 7D). Experiments illustrated that water could improve the self-healing capacity of polymers effectively due to that the dynamic behavior of the B-N coordinated boronic esters were accelerated by H 2 O at the damaged surfaces ( Figure 7E). By utilizing the dynamic B-N interactions in boronic esters, this work afforded a novel strategy for synthesizing robust room-temperature selfhealing elastomers.
Boron compounds with sp 2 hybridized boron are sensitive to water, and the applications of boron-based polymers are limited by this vulnerability. Dative B-N bonds have been proven to stabilize boronic ester linkages against hydrolysis, as the vacant orbital of boron atoms will be fully occupied upon coordination and thus producing sp 3 hybridization. Jing and coworkers were the first to report boronic ester-based vitrimers with enhanced water resistance through internal B-N interactions. Based on B-N coordination and dynamic covalent chemistry, they synthesized a series of internal NCB linkages. 39 Through stability and kinetics studies on small molecules, they proved that NCB linkages are dynamic and possess enhanced hydrolytic stability (see details in Section 2). Furthermore, they synthesized NCB linkages-based PU and PUU vitrimers through NCBCs reacting with trifunctional isocyanate ( Figure 8A). The prepared vitrimers exhibit excellent mechanical strengths (about 40 MPa) and water resistance ( Figure 8B). Due to the enhanced hydrolytic stability of internal B-N coordinated NCB linkages, the mechanical property of PUU 1.1 vitrimer showed no significant reduction after being exposed to 40% RH for a month ( Figure 8C). The materials can retain their structural integrity even immersed in boiling water. According to stress relaxation experiments, the obtained high E a values of vitrimers demonstrated that the heat resistance has also been improved by the stable NCB linkages ( Figure 8D,E). Based on the dynamic B-N coordination and boronic ester linkages, the reprocessing and recycling procedure of vitrimers can be implemented from solid-phase (150°C, 1 MPa for 30 min) and liquid-phase (TEA in DMF) aspects ( Figure 8F,G). This work provides a new design strategy for constructing reprocessable thermosets based on dynamic B-N coordinated boronic esters. After that, this group reported a recyclable vitrimer (PS-VM) based on polystyrene and the internal nitrogen coordinating boronic ester linkages ( Figure 9A). 70 The solvent resistance, thermal stability, and creep resistance of the material were improved by this stable crosslinking, and the vitrimer can be reprocessed for multiple cycles upon heating without a catalyst ( Figure 9B-D). Incorporating glass fiber could effectively reinforce the mechanical property of this vitrimer, and the composite also could be recycled via the dynamic exchange between boronic ester linkages ( Figure 9E). Moreover, using a similar strategy, they synthesized reprocessable and recyclable polyolefin/ polystyrene vitrimers (COC-PSVM) with low dielectric loss (D r ) based on NCB linkages, which exhibited enhanced water resistance, thermal stability, mechanical properties, and recyclability ( Figure 9F-H). 71 The low D r of the material endows it with the application for highfrequency copper-clad laminates ( Figure 9I).
Inspired by this novel five-number cyclic boronic ester linkages with internal B-N coordination, our group reported a more stable boronic ester (BN-6) with changeable ring strain. 59 We proposed that the thermalresponsive B-N bonds can make the 6-membered ring transfer to 10-membered ring under heating, resulting in the boronic ester molecules being more stable at room temperature but more dynamic at elevated temperature ( Figure 10A). The investigations of small boronic ester molecules illustrated that the stability at ambient conditions and kinetics upon heating of BN-6 were improved. Meanwhile, the model crosslinking polymers based on dynamic BN-6 linkages further verified that the six-membered boronic ester with intramolecular B-N coordination possesses enhanced hydrolysis stability and dynamicity ( Figure 10B,C). The mechanical strength of vitrimers containing BN-6 were higher than those based on BN-5 because of the strong rigidity of six-membered rings ( Figure 10D). Utilizing this stable but dynamic linkage, we prepared an adhesive PTBN6 through a convenient method and this adhesive material could form multiple interactions (B-O, hydrogen, and coordination bonds) with substrates ( Figure 10E). The lapshear measurement indicated that the adhesive exhibited excellent adhesion strength (higher than 4 MPa). Based on the reversible interactions between adhesive and substrates, PTBN6 demonstrated superior rebonding ability ( Figure 10F). Additionally, PTBN6 could lift a weight of 15 kg underwater ( Figure 10G). After being immersed in water and other harsh conditions (acidic or alkaline solutions) for a month, the adhesion strength showed no obvious decrease ( Figure 10H). This novel boronic ester-based adhesive is a breakthrough in the field of boronic ester-based adhesive materials.

| DMs BASED ON BOROXINES WITH B-N COORDINATION
Boroxine is the dehydration product of organoboronic acids, which mainly exists in the form of sixmembered triorganoboroxine rings (RBO) 3 . The diversification of substitution on boroxine and its easy synthesis route provides abundant opportunities for boroxine linkages to construct polymeric or networked materials. 33,[72][73][74] To avoid the hydrolysis back reaction of boroxine linkages, the B-N interaction has been used to stabilize the acidic boron sites and improve the hydrolytic stability of boroxines. In 2009, Sumerlin and his coworkers reported a dynamic covalent macromolecular polymer assembled by three-armed boroxine linkages with B-N coordination ( Figure 11A). 75 1 H NMR and UV-vis experiments proved that the dative bonds formed between boroxines and piperidine ligands. The improved hydrolytic stability of boroxine linkages also has been verified via NMR and GPC analysis ( Figure 11B,C). Besides, Guan et al. constructed a recyclable thermoset based on boroxines with B-N interaction. Upon the addition of 4-undecylpyridine as a plasticizer, B-N dative bonds formed between boroxine and pyridine ( Figure 11D). 76 The material also demonstrated stability under humid conditions and exhibited high malleability, strong mechanical property, and recyclability ( Figure 11E,F).
Similar to boronic esters with B-N dative bonds, the coordination can accelerate the dynamic equilibrium between boronic acids and boroxines, resulting in excellent dynamicity and self-healing ability of DMs. Based on the nitrogen-coordinated boroxines, Sun and his group synthesized a polymer composite with enhanced toughness and self-healing ability at room temperature. 77 Through DFT calculation, they have revealed that the configurations of amine-coordinated triphenylboroxines are 1:1 and 1:2 boroxine-amine adducts. Moreover, the calculation results illustrated that the lengths of B-O bonds near B-N coordination bonds are longer than those in common triphenylboroxines, indicating that the addition of amine ligands can promote the reversible formation and dissociation of boroxines ( Figure 12A). The 3D polymer network was prepared by the dehydration of poly(propylene glycol) (PPG) terminated with aminomethyl-phenylboronic acids ( Figure 12B). The soft material has excellent selfhealing capacity at room temperature, but the mechanical property of it is quite poor (0.19 MPa of tensile strength). The incorporation of poly(acrylic acid) (PAA) could significantly increase the mechanical strength of the polymer owing to the hydrogen bonds between amine groups and PAA. The tensile strength of composites increased to 12.7 MPa and the modified materials also exhibited self-healing abilities at room temperature with the assistance of water ( Figure 12C-E). The dynamic characteristics of H bonds and nitrogencoordinated boroxines endow the composites with excellent recycle and reshape properties. Furthermore, through the fabrication of linear and T-shaped PPG polymers crosslinked by the dynamic boroxines with B-N coordination, the group reported robust supramolecular thermosets with self-healing and recycling abilities under mild conditions ( Figure 13A). 78 The mechanical property of thermosets is tailorable by the ratios of the bi-and tri-PPG polymers ( Figure 13B). Based on the reversible nitrogen-coordinated boroxines, these strong thermosets can be recycled at 60°C repeatedly, and the mechanical strength showed no obvious decrease ( Figure 13C,D). Using the dynamic characteristics, Sun's group developed a series of self-healing materials with different functionalities. In 2020, they fabricated a nonfluorinated superhydrophobic coating via the preparation of N-coordinated-boroxines based polydimethylsiloxane (PDMS) and SiO 2 nanoparticles, which showed satisfactory transparency, stability and self-healing ability at room temperature ( Figure 13E). 79 In addition, this group introduced the dynamic boroxines with dative B-N coordination into polyurethanes. 80 Utilizing the boroxines as crosslinking, the mechanically strong poly(urea-urthane) s (PUUs) were fabricated ( Figure 14A). The tensile strength and toughness reached~47 MPa and~190 MJ/m 3 , respectively ( Figure 14B). PUUs were healable and recyclable with the assistance of water/ethanol at room temperature because of the reversible hydrogen, B-N, and B-O bonds in the crosslinking networks ( Figure 14C,D). Moreover, the transparent material can be used as protecting coating with recyclability and self-healing capacity ( Figure 14E).
In iminoboronate chemistry, the dative bonds between boron and nitrogen atoms are formed along with the accelerated dehydration reaction. The Lewis acid-base interaction can improve the thermodynamic stability of iminoboronate complexes. However, Gao et al. demonstrated that the dative B-N bonds could activate the hydrolysis and exchange reactions of imines, leading to the more labile kinetics of reversible imine bonds. 81 Therefore, dynamic iminoboronates have been widely used in the field of self-healing materials recently. Raquez and his group reported an ambient humidity-sensitive self-healing polymer based on the dynamic iminoboronate compounds ( Figure 15A). 82 They studied the kinetics and stability of dynamic iminoboronate by preparing small molecule models containing iminoboronate-based boroxine adduct and a macromolecular polymer matrix crosslinked by this linkage ( Figure 15B). The results of ESI-MS analysis illustrated that the equilibrium between boroxine and boronic acid also occurs in the iminoboronates with dative B-N bonds. At ambient humidity, the model macromolecular polymer based on PPG 2000 exhibited excellent mobility, and the crack was repaired autonomously after exposure to ambient conditions for 4 h without any other stimulus ( Figure 15C). After being dried at 70°C, the water was removed and boroxines reformed, causing the polymer to turn into stretchable rubber. Upon the reversible hydrolysis-dehydration process of iminoboronates, a dynamic crosslinking network was synthesized from PPG 400 and aryl boroxines. The increase in crosslinking density made the self-healing time of the polymer at ambient humidity become longer than that of the model material, while its mechanical property was stronger ( Figure 15D). Utilizing this excellent reversibility of iminoboronates with B-N coordination, Zloczower and Raquez et al. prepared a self-healing epoxy thermoset with changeable mechanical properties. 83 Under water stimulus, the stiff polymer transferred into soft rubber because of the dissociation of boroxines ( Figure 16A). Moreover, the incorporation of dynamic nitrogencoordinated boroxines endowed the epoxy thermoset with water-responsive self-healing, reshaped, and patterned abilities, which demonstrated potential applications in smart materials ( Figure 16B,C). They developed a wearable sensor by modifying the epoxy thermoset with branched carbon nanotubes (CNS). The composite showed tunable stiffness and self-healing capability which was used as conductive sensors ( Figure 16D). The group of Raquez also reported a dually responsible, stiff, and self-healable polyurethane based on iminoboronate and boroxines ( Figure 17A). 84 Owing to the dynamic hydrogen and imine-coordinated boroxine bonds in the polymeric network, the material emerged with reversible temperature-/humidity-response capabilities and reprocessability ( Figure 17B). Through controlling the humidity under ambient conditions, the mechanical property of the polymer was tunable, which provided an opportunity to develop a self-healable and versatile material for 3D printing applications ( Figure 17C). In 2020, Bai et al. reported a reprocessable vitrimer with high mechanical strength and shape memory ability based on imine-coordinated boroxines. 85 The vitrimer was obtained from the reaction between 2-aimnoethanethiol on polybutadiene and the formyl groups in phenylboronic acids ( Figure 18A). Through tuning the crosslinking density of polymer networks, the tensile strength of the vitrimer reached up to 12.35 MPa ( Figure 18B). The frequency sweep plots were measured at different temperatures to study the dynamic nature of vitrimers, and the E a was calculated according to the time-temperature superposition principle. The calculated E a became lower with the increase of crosslinking density, indicating that the iminecoordinated boroxines could improve the effective collision of dynamic exchanges in the polymeric networks ( Figure 18C). Due to the dual-dynamic structure (B-N coordination and boroxines) in the bulk networks, the exchange reactions were easier to happen under heating, and the materials exhibited excellent thermal reversibility. Therefore, the vitrimers could be reprocessed by hot pressing at 70°C under 4 MPa ( Figure 18D). Although the imine-coordinated boroxines were sensitive to humidity, the polybutadiene-based vitrimer demonstrated superior water resistance because of the hydrophobic polymer chains, which have expanded the application fields of boroxine-based materials. On account of the variable mechanical strength at different temperatures, the vitrimer also showed shape memory behavior and remodellable property ( Figure 18E). Therefore, iminoboronate chemistry containing dynamic boroxines and dative B-N bonds further provides a novel strategy for designing functional materials with self-healing and multi-responsive abilities.

| CONCLUSION AND OUTLOOK
With the growing demand for environmental protection, introducing dynamic chemistries to breathe life into traditional polymers has become a significant research topic. Herein, we focus on the development of dynamic polymeric materials based on reversible B-O bonds with dative B-N coordination interactions. Thanks to the comprehensive insights into the influence F I G U R E 16 (A) The structure change and mechanism of water-triggered self-healing property of the epoxy-boroxines polymeric network. (B) Self-healing, (C) reshape and patterned properties of the epoxy thermosets. (D) Composites containing boroxine-based epoxy and carbon nanotubes for wearable sensors. Reproduced with permission: Copyright 2019, American Chemical Society. 83 of dynamic B-N chemistry on B-O bonds in small molecular organoboron compounds, the combination of dynamic covalent boronic ester/boroxine crosslinks and dative B-N coordination bonds within the same polymer matrix has successfully led to essential improvement in hydrolytic stability and mechanical performance without compromising the dynamic exchange kinetics. As presented in this review, a large variety of DMs based on these interesting linkages have been reported in the past decade, showing great potential for real-world applications including underwater adhesives, smart coatings, wearable sensors, and artificial skins.
Despite considerable progress has been achieved in previous literature, a few critical issues, and challenges remain unexplored: (i) While the investigations on B-N coordination bonds in small molecular organoboron systems have made essential strides via multiple characterizations (e.g., DFT calculations, XRD analysis, variable temperature 1 H NMR, and 11 B NMR spectra), the specific contributions of B-N chemistry in dynamic polymeric networks (such as improving the hydrolysis stability and dynamic kinetics) has only been studied in limited works due to the short of effective measurement technologies. To better understand the structure-property relationships between molecular design and material behaviors, more attempts might be devoted to developing novel testing toolboxes including atomic force microscopy, 2D FTIR spectra, and computational simulations. (ii) Compared with the materials just containing conventional B-O bonds, the applications of aforementioned polymer systems based on the combination of dynamic B-O bonds and dative B-N interactions have not been expanded vigorously. New strategies to install nitrogen-coordinated boronic ester and boroxine moieties into the existing commercial polymers (e.g., epoxy resins, polyurethane films) might be the potential opportunities for further broadening the practical application scenarios. (iii) Majority of the research discussed above utilizes the well-designed organoboron linkages containing both dynamic B-O, and B-N bonds which are either expensive or prepared through complicated synthetic pathways, thus restricting the large-scale productions in industry. Further research towards developing low cost and easily processable organoboron cross-linkers will benefit the commercialization of B-N coordinated dynamic boronic esters/ boroxines materials as viable alternatives to contemporary commodity polymers.  85 Overall, opportunities and challenges coexist in the advancement of dynamic polymeric materials based on reversible B-O bonds with dative B-N interactions. Upon addressing the aforementioned challenges, it is for sure that the complementary contributions from dynamic B-O bonds and tailoring B-N coordination chemistry will help create novel environmentally friendly materials with fine control over thermal, mechanical, and dynamic characteristics, which can be seamlessly integrated into our daily life.