Typical preparation of mixed matrix membranes (MMM), where the pre-synthesized polymer and filler are simply mixed together in a suitable solvent before casting, often suffer from poor polymer-filler adhesion. This study introduces a novel generation of MMM where the fillers are covalently linked with the polymer matrix, hence generating defect-free membranes. To do so, a metal-organic framework (MOF) bearing polymerizable groups (UiO-66-NH2) was in-situ linked with a polymer of intrinsic microporosity (PIM-1) during the polymer synthesis. The PIM-MOF chemical cross interface linking was confirmed by combined analytical technics including 1H-NMR, 13C-MAS NMR, FTIR, SEM, XRD and contact angle. Compared with traditional non-crosslinked MMMs, the in-situ derived MMMs showed a significantly higher permeability and ideal selectivity for all the gases tested (CO2, CH4, N2 and O2). These improvements are highly related to the degree of cross interface linking between PIM-1 and UiO-66-NH2. For the same filler loading of 20 wt.%, the CO2/CH4 selectivity increases from 19.1 to 31.9 as the in-situ polymerization time is extended from 12 to 72 h. The physical aging was also effectively controlled for the in-situ crosslinked MMMs where gas permeability remained up to 80% of its initial value after 400 days of storage at ambient conditions.