  Preserving the superior mechanical properties of monolayer two-dimensional
(2D) materials when transitioning to bilayer and layer-stacked structures poses
a great challenge, primarily arising from the weak van der Waals (vdW) forces
that facilitate interlayer sliding and decoupling. Here, we discover that
mechanically interlocked 2D polymers (2DPs) offer a means for structural
reinforcement from monolayer to bilayer. Incorporating macrocyclic molecules
with one and two cavities into 2DPs backbones enables the precision synthesis
of mechanically interlocked monolayer (MI-M2DP) and bilayer (MI-B2DP).
Intriguingly, we have observed an exceptionally high effective Young's modulus
of 222.4 GPa for MI-B2DP, surpassing those of MI-M2DP (130.1 GPa), vdW-stacked
MI-M2DPs (2 MI-M2DP, 8.1 GPa) and other reported multilayer 2DPs. Modeling
studies demonstrate the extraordinary effectiveness of mechanically interlocked
structures in minimizing interlayer sliding (~0.1 {\AA}) and energy penalty
(320 kcal/mol) in MI-B2DP compared to 2 MI-M2DP (~1.2 {\AA}, 550 kcal/mol),
thereby suppressing mechanical relaxation and resulting in prominent structural
reinforcement.
