Dynamic covalent urea bonds and their potential for development of self-healing polymer materials

Abstract

The highly resonance-stabilized urea covalent moiety can be made dynamic through mediation of metal salts, thus allowing the development of novel classes of self-healing polymer materials with excellent healing efficiency.

Dynamic multiphase semi-crystalline polymers based on thermally reversible pyrazole-urea bonds

“…Of particular note is that PPzU 7c with the highest cross-linking degree (average molecular weight between cross-links ( M c ) = 1.7 kg mol −1 , Supplementary Table 3) is still semi-crystalline although cross-linking process indeed interferes with the crystallization. This is markedly different from formerly reported ones which always exhibited amorphous behavior 8–33,40–43 . With the increase of cross-linking density (Supplementary Table 3), the decrease of the intensities of crystalline peaks (2 θ = 19° and 21°) in X-ray diffraction (XRD) patterns (Fig.…”

“…Harsh conditions are required to cleave urea bonds, such as acidic or basic solutions, high temperature, and catalysts 37–39 , which limits their applications in dynamic covalent materials. Metal catalyst such as zinc acetate shows effectiveness to increase the reversibility of normal urea bonds 40 . In addition, by introducing bulky substituents to a urea nitrogen atom, the hindered urea bonds are significantly destabilized owing to the steric inhibition of resonance via hindering the orbital co-planarity of the urea bond, thus resulting in reversibly dissociating to isocyanate and amine under ambient condition without a catalyst 41–43 .…”

Tough and Multi‐Recyclable Cross‐Linked Supramolecular Polyureas via Incorporating Noncovalent Bonds into Main‐Chains

“…[43] However, the synthesis of recyclable cross-linked polyureas while with negligible sacrificing of their high mechanical properties remains a big challenge. [44][45][46] In this work, quadruple hydrogen bonds with high binding strength between ureidopyrimidinone (UPy) units were chosen as exchangeable units to endow the polyurea networks with a balanced dynamic property and robustness. Then, we designed a set of cross-linked supramolecular polyureas CSPU-xUPy, where x referred to the molar fraction of the UPy units relative to total amine groups and equaled 0.050, 0.097, and 0.146, respectively.…”

The effect of polarity on the molecular exchange dynamics in imine-based covalent adaptable networks

“…[8][9][10] By stimulating the exchange reactions of the dynamic covalent bonds, for example by introduction of heat or light, a molecular flow is introduced in the material. Several types of bond exchange reactions have been studied over the last decade, such as transesterifications, 1,12 boronic ester exchange, [13][14][15][16] 1,2,3-triazolium exchange, 17,18 diketoenamine exchange, 19,20 (alkyl)urea exchange, [21][22][23] urethane exchange, [24][25][26][27][28] thiol-ene/yne exchange, 29,30 and imine exchange. [31][32][33][34] Individually, each type of bond exchange reaction possesses different dynamics according to the mechanism of the exchange reaction.…”