Abstract This paper explains the reasons behind the very low polydispersity index (PDI) obtained in living anionic polymerizations in microstructured reactors. From the results, it can be explained that a narrow molecular weight distribution can be obtained due to the presence of a highly segregated flow behavior, even in microflow conditions, provided that the mean residence time is high enough. This paper investigates the feasibility of a living anionic polymerization reaction under micro-fluidic conditions. This is accomplished using a multiphysics model that accounts for the changes in viscosity and diffusivity. These properties descend with the increase in weight of the polymer, and could not be un-coupled from hydrodynamics and mass transfer. The results of the model are used to understand the reasons behind the very low PDI that can be experimentally obtained in microflow conditions. This leads to the conclusion that the increased viscosity almost "suppresses" the diffusion of the monomer, even at the very short characteristic lengths of a micro-device. These conditions generate a fully segregated flow that yields an almost monodisperse polymer regardless of the effective residence time distribution encountered in the reactor.