Hugh Brown has shown that interfacial entanglements govern adhesion between two polymers. We demonstrate this for three systems by adding interfacial chains via chemical coupling. The adhesion between polypropylene (PP)/amorphous polyamide (aPA) was reinforced by the coupling reaction of maleic anhydride grafted PP (PP-g-MA) and the primary amine groups on aPA; huge increases in adhesion were observed. A good correlation between critical fracture toughness, G(c), and PP-g-MA concentration squared follows Brown's crazing mechanism. For a polystyrene (PS)/aPA interface reinforced by the coupling reaction of poly (styrene-r-maleic anhydride) (PS-r-MA)/aPA only modest adhesion increases in G(c) were observed through the whole PS-r-MA concentration range. This different behavior of G(c) vs. functional polymer concentration is believed to be caused by segregation of the formed graft copolymers at the interface. The relationship between G(c) and the extent of coupling was studied quantitatively with a model PS/PMMA system. The interface was reinforced by the coupling reaction of 0 - 10% PS-NH2/PMMA- anh. G(c) was measured with the asymmetric dual cantilever beam test (ADCB) and the amount of copolymer formed at the interface was determined by a fluorescence labeling technique. G(c) is low and is linear in block copolymer interfacial coverage (Sigma), indicating a chain scission mechanism. Reasonable agreement was achieved between experiment and theoretical prediction based on the energy to break C - C bonds.