We report here a biomimetic design of a reversibly unfolding modular cross-linker to enhance mechanical properties of 3D network polymers. The inspiration comes from the modular biopolymers observed in nature. A cyclic modular cross-linker based on the quadruple hydrogen bonding 4-ureido-2-pyrimidone (UPy) motif was synthesized via multistep organic synthesis. The modular crosslinker was incorporated into poly(n -butyl acrylate) by free radical polymerization. Stress-strain measurements show that the samples containing our modular cross-linker exhibit significantly enhanced mechanical properties over the control samples. Most strikingly, with increasing cross-linker density, both modulus and tensile strength are significantly improved without sacrificing extensibility. The enhanced tensile properties are attributed to the increased energy dissipating ability of the reversibly unfolding cross-linker. This introduces a novel biomimetic concept to enhance network mechanical properties through design of molecularly engineered cross-linkers.