to traditional metallic and ceramic materials. [1] Therefore, it has been desirable to create high-performance polymeric materials that are mechanically robust, thermostable, and even healable for meeting practical applications in industry. The creation of cross-linked polymers has been regarded as one promising approach for creating strong and thermally stable polymers. [3][4][5] Cross-linked polymers normally exhibit superior mechanical and thermostable performances to their uncrosslinked counterparts. [6][7][8][9][10] Traditionally, a chemically covalent approach has been the primary means for realizing the cross-linking of polymeric materials. As compared with the parent polymer, cross-linked polymers normally gain enhanced mechanical strength and thermal stability. [6][7][8][9][10][11] This strategy, however, gives rise to significantly reduced extensibility because of the exclusive molecular mechanism between strength, extensibility, and toughness. [12][13][14][15] By contrast, noncovalent cross-linking can lead to increased mechanical strength without sacrificing the extensibility and toughness of polymeric materials. Such cross-linking can be realized by adopting strong noncovalent interactions including Coulombic interactions, [11] dipoledipole interactions, [12] metal coordination, [13] and hydrogenbonding (H-bonding) interactions. [5,16] To date, H-bonding is among the most extensively employed noncovalent interactions for the creation of cross-linked polymeric materials with tunable microstructure and tailor-made performances because of its directionality, versatility, and reversibility. In 1997, Meijer and co-workers [17] pioneered the synthesis of the first reversible cross-linked polymer by reacting a trifunctional copolymer of hydroxyl-terminated poly(propylene oxide) with diisocyanate and then 2-ureido-4-pyrimidone (UPy) units capable of forming quadruple H-bonds. The final poly mer shows a relatively high plateau modulus because such H-bond cross-links can serve as entanglements in a linear polymer. Following the synthesis of some uncross-linked polymeric materials containing UPy motifs, [17][18][19] Meijer's group has further synthesized many cross-linked polymeric materials based on UPy motifs. [20][21][22] On the other hand, in many natural biological materials, such as silk [23][24][25][26] and muscle, [27] multiple H-bonding interactions have recently been revealed to play a critical part in realizing a unique combination of high strength, great It has always been critical to develop high-performance polymeric materials with exceptional mechanical strength and toughness, thermal stability, and even healable properties for meeting performance requirements in industry.
Conventional chemical cross-linking leads to enhanced mechanical strength and thermostability at the expense of extensibility due to mutually exclusive mechanisms. Such major challenges have recently been addressed by using noncovalent cross-linking of reversible multiple hydrogen-bonds (H-bonds) that widely exist in biological...
