This paper discusses a new class of maleic anhydride modified PP polymers (PP-MA), having a relatively well-defined molecular structure. The chemistry involves the borane-terminated PP intermediate that is prepared by hydroboration of the chain-end unsaturated PP. The borane group at the chain end was selectively oxidized by oxygen to form a "stable" polymeric radical which then in situ reacts with maleic anhydride to produce maleic anhydride terminated PP (PP-t-MA) with a single MA unit. In the presence of styrene, the polymeric radical initiated an alternative copolymerization of styrene and maleic anhydride to produce PP-b-SMA diblock copolymers, and the incorporated MA units are proportional to the concentration of styrene and the reaction time. No detectable side reaction was observed in the PP polymer chain. The resulting relatively well-defined PP-MA copolymers, with controllable PP molecular weight and MA concentration, were evaluated systematically in the reactive PP/PP-MA/polyamide blends. The compatibility is dependent on not only the microstructure of the compatibilizer, formed in situ by reacting PP-MA with polyamide chains, but also the composition of the PP/polyamide blend. In polyamide-rich blends, all PP-MA copolymers show limited compatibility. Higher MA concentrations result in poor blend morphologies. Most of the compatibilizers formed fail to position themselves at the interfaces between the PP and polyamide domains. On the other hand, in PP-rich blends the in situ formed compatibilizers are generally very effective, especially those involving the PP-b-SMA with high PP molecular weight and multiple MA units.