omega -Isocyanate PMMA, alpha -anhydride PMMA, and PS-co-PSNH2 have been prepared by atom transfer radical polymerization (ATRP) with controlled molecular weights (10(4) and 3.5 x 10(4)) and low polydispersity (1.2). They have been used as precursors of PS-g-PMMA copolymers and let to react in the melt (170 degreesC, for 10 min) under moderate shear rate. The well-controlled molecular characteristics of these precursors are a substantial advantage to study the effect of the kinetics of the interfacial reaction on the phase morphology. When the grafting reaction is fast (NH2/anhydride pair) and low molecular weight chains are used, the interfacial reaction is quasi-complete and a nanophase morphology is observed, whereas limited reaction and formation of microphases are observed in all the other cases. A high reaction yield requires not only that the functional groups are highly mutually reactive but also that the interface is anytime made available to the functional polymers for the reaction to progress. Then, a nanophase morphology may be observed, which is that of the copolymer formed by the interfacial reaction. A low reaction yield is dictated by either a slow interfacial reaction or a slow diffusion of the copolymer away from the interface. In the latter case, the phases formed by the unreacted precursors are stabilized by the copolymer which resides at the interface.