The effect of chain architecture of in situ formed copolymers on the interfacial morphology of reactive polymer blends was' investigated. We found that the chain architectures of copolymers at the interface significantly affected the reaction and interface roughness. Although the amount of in situ formed Y-shaped graft copolymers was smaller than that for diblock copolymers, the interface area generated by the former was larger than that generated by the latter.