The relationship between the fracture toughness (G(c)) and the areal density of diblock copolymer at the interface (Sigma) was investigated for joints between polypropylene (PP) and polyamide-6 (PA6), two incompatible, semicrystalline polymers. Diblock copolymers were formed in situ by reaction between succinic acid groups terminally grafted onto 5% of the PP chains and the NH2 ends of the PA6 chains. Fracture toughnesses were measured using an asymmetric double cantilever beam test (ADCB). After the bulk PA6 had been completely rinsed from an adhered sample, X-ray photoelectron spectroscopy (XPS) was used to measure the areal density of copolymer at the interface. Above the melt temperature of PP, but below that of PA6, reaction at the interface was limited by diffusion of the reactive PP chains (D = 1.58 x 10(-11) cm(2) s(-1) at 213 degrees C). By controlling the temperature and the time of formation, G(c) could be varied in the range from 5 to 100 J/m(2). Dissipation during fracture was observed to occur by plastic deformation in the PP, and failure of the joint was determined to occur by chain scission of the PP part of the copolymer. The fracture toughness was found to vary as the square of the areal density of copolymer at the interface, a relationship similar to that observed and predicted for glassy polymers. This scaling behavior suggests that the stresses in the fracture are concentrated over a distance on the order of 10 nm at the head of the crack.