Coupling reactions between functional group grafted macromolecules have been proven to be a powerful way to facilitate compatibilization and adhesion. Yet their reaction kinetics requires more fundamental understanding. The common wisdom for reactions between small molecules is that external mass transfer increases the reaction rate by increasing concentration of reactive species without affecting reaction mechanism and transition states at a certain temperature. However, for reactions between macromolecules, the kinetics is more complicated because of their relatively slow diffusion and complex configurations. It is generally difficult to study the effects of flow on macromolecules reactions during mixing because interfacial area generation is always convoluted and hard to isolate. In this paper, a reactive bilayer system was created to explore this issue. In an effort to study the reaction between polyethylene (PE) and thermoplastic polyurethane (TPU), maleic anhydride (MA), hydroxyl (OH) and secondary amine (NHR) functionalized PEs were synthesized via reactive extrusion and blended into non-modified PE. We bonded these PEs to TPU via lamination and coextrusion. In order to compare reaction rates of the two processes we determined the interfacial copolymer density Ó considering both advection and interfacial area generation. We found that the reaction rate in coextrusion was much faster in comparison with lamination at the same temperature. This difference was attributed to the extensional and compressive flow in coextrusion overcoming the diffusion barrier at the interface and forcing reactive species to penetrate into interface. The effects of functional group reactivity and processing variables on adhesion were correlated with interfacial copolymer coverage. Amine functionalized PE showed dramatic adhesion improvement even at 1 wt%. We also determined that flow accelerates the reaction between polyamide 6 and maleic anhydride high density polyethylene. New die design is investigated to isolate the effect of extensional and compressive flow.