The emergence of polymers with intrinsic microporosity provides solutions for flexible gas separation membranes with both high gas permeability and selectivity. However, their applications are significantly hindered by the costly synthetic efforts, limited availability of chemical systems, and narrow window of microporosity sizes. Herein, flexible mixed matrix membranes with tunable intrinsic microporosity can be facilely fabricated from the coordination assembly of polymer brushes and coordination nanocages. Polymer brushes bearing isophthalic acid side groups can coordinate with Cu to assemble into polymer networks crosslinked by 2 nm nanocages. The semi-flexible feature of the polymer brush and the high crosslinking density of the network prevent the network from collapsing during solvent removal and the obtained aerogels demonstrate hierarchical structure with dual porosity from the crosslinked polymer network and coordination nanocage, respectively. The porosity can be facilely tuned via the amount of Cu by regulating the network crosslinking density and nanocage loadings, and finally, optimized gas separation that surpasses Robeson upper bound for H /CO can be achieved. The coordination-driven assembly protocol paves a new avenue for the cost-effective synthesis of polymers with intrinsic microporosity and the fabrication of flexible gas separation membranes.