Reactive compatibilization of immiscible polymer blends is typically accomplished by grafting reactions between functional groups distributed randomly on one polymer and end-functional groups on the other polymer. A number of model studies have focused on end coupling in polymer melts. In this work we compare directly reaction rate constants for an end-functional chain reacting with an end-functional chain, k E, vs reacting with a mid-functional chain, k M, using competitive reaction of phthalic anhydride end- and mid-functional poly(methyl methacrylate) (PMMA-eAn and PMMA-mAn) with amine terminal PMMA and polystyrene (PMMA-NH 2 and PS-NH 2). PMMA-eAn was labeled with 7-nitrobenz-2-oxa-1,3-diazole (NBD) while PMMA-mAn was labeled with anthracene. We measured the extent of coupling to block and graft copolymers selectively at the characteristic excitation and emission wavelengths of NBD and anthracene using a fluorescence detector coupled with GPC. We found that coupling with the mid-functional PMMA was slower under all reaction conditions investigated and had the increasing order of k E/k M: homogeneous melt (1.7), solution (2.8), heterogeneous blend prepared in the mixer (2.6-3.2), and static flat interface (>10). The kinetic excluded-volume effect and steric hindrance due to the polymer chain are considered to be the reasons for k E/k M > 1 in the homogeneous case. k E/k M in solution was in agreement with the value (2.1) predicted by the kinetic excluded-volume theory. The large value of k E/k M in the static flat interface was attributed to end-group segregation at the interface. Interestingly, we found that flow affected the interfacial reaction tremendously, resulting in over 1000 times higher rate constant in heterogeneous melt blending than that in the static bilayer film.