The progress of the interfacial reaction of polystyrene chains end-capped by a primary amine (PS-NH2) and PMMA chains end-capped by an anhydride (PMMA-anh) has been monitored by SEC-UV, by using anthracene-labeled polystyrene chains (anth-PS-NH2) as a probe. Assemblies of an anth-PS-NH2 layer and a PMMA-anh layer were annealed at 200 degreesC for various periods of time. The interfacial reaction rate depends on the molecular weight (MW) of the reactive precursors in relation to the gammaN value of the chains. For chains of low gammaN (chiN = 6), the reaction is faster because the interface becomes more diffuse with time, as observed by TEM and AFM, consistent with compatibilization of the weakly immiscible polymers by the copolymer formed in-situ. For chains of higher molecular weight and chiN (10, instead of 6), the interface is much sharper and the residence time at the interface of the symmetric diblock copolymer of higher molecular weight is also increased, which dramatically restricts the progress of the interfacial reaction under the annealing conditions used in this work.