A better understanding of the reactive interphase is extremely important towards modifying interfaces for multilayer coextrusion. We therefore studied in situ interfacial reaction with copolymer development in multilayered systems from two to thousands of layers alternating two model polymers. Firstly, influence of interfacial reaction and formed interphase on the resulting macroscopic rheology, morphology and microscopic dynamics were investigated in terms of copolymer architecture and reaction extent/time, etc (Lu B., et al. Polym. Test., 2017, 61, 289-299). Interfacial reaction kinetics was investigated, as well as the length scale of the reactive interphase. Influence of the reactive interphase on melt rheology was then comparatively studied by melt shear and extensional rheology. Particularly by extensional rheology, we provided a more quantitative view of the contribution of reactive interphase. Effect of reaction extent was also examined. In line with macroscopic rheology, the retarded microscopic dynamics in reactively healed bilayers was revealed from a dielectric viewpoint. The reactive interphase also drastically altered the dielectric responses of the bilayer upon reaction (Lu B., et al. Soft Matter, 2017, 13, 2523-2535). Secondly, we further probed the role of interfacial reaction and intephase formation in multilayers with the layer thickness tuned from micro- to nanoscale by forced-assembly multilayer coextrusion. Layer continuity/integrity, interfacial stability and morphology/microstructure were examined. Interfacial distortions (i.e. layer breakup and droplet formation) observed in nanolayered systems was explained by the rheological mismatch of components and the presence of the graft copolymers (interfacial tension reduction). Besides, charge transport dynamics displayed different scenarios depending on the number of layers. Influence of reactive interphase on melt extensional rheology was further evaluated from micro- to nanolayers.