Poly(methyl methacrylate) (PMMA) containing 50 wt% of anthracene (anth) labeled reactive PMMA chains [thus end-capped by an anhydride (anh) or an isocyanate (NCO)] has been melt blended with polystyrene containing 33 wt% of chains end-capped by an aliphatic amine (PS-NH2) at 180degreesC. Conversion of anth-PMMA-anh and anth-PMMA-NCO into PMMA-b-PS copolymers has been determined by SEC with a UV detector. The interfacial reaction mainly occurs in the initial mixing stage (less than or equal to1.0 min), although at a rate which depends on the reactive groups attached to PMMA. Moreover, this interfacial reaction rate controls how the phase morphology is developed.