We present an investigation of the mechanisms of mechanical reinforcement at interfaces between polypropylene (PP) and polyamide-6 (PA6), associated with the incorporation of a small amount of maleic anhydride functionalized PP (PP-g-MA) which reacts with the NH2 groups of the PA6 to form a copolymer in situ. In a previous study we have demonstrated, for one molecular weight of PP-g-MA, that diblock copolymer molecules were indeed formed at the interface, with an areal density Sigma, controlled by the reaction temperature and the reaction time, and that the measured fracture toughness of the interface scaled as G(c) proportional to Sigma(2), regardless of the reaction temperature, but for similar sample cooling conditions. We report here the behavior of the same system for a higher molecular weight PP-g-MA : at a reaction temperature above 220 degrees C, very close to the melting point of the PA6, and above a given Sigma, the measured G(c) becomes 4 times higher than that for reaction temperatures below 220 degrees C, where the observed G, values are identical to what has been measured for the low molecular weight PP-g-MA. G(c) is therefore no longer uniquely dependent on C. Crystallographic analysis on the PP side of the intel face showed a correlation between the presence of the PP beta-phase in the 20 mu m near the interface and a high toughness; this crystalline phase was not present in the samples prepared at T less than or equal to 220 degrees C or with the low molecular weight PP-g-MA which always exhibited a low toughness even for samples prepared above 220 degrees C. It is argued that the presence of this beta phase of the PP is the main factor responsible for the very high fracture toughness, first evidence of the influence of the crystallinity of a semi crystalline polymer on its adhesive properties.