The reaction efficiency of single unit monomer insertion (SUMI) reactions via the reversible addition fragmentation chain transfer (RAFT) method is investigated in detail by the determination of obtained product yields of optimized batch and microflow synthesis procedures in combination with kinetic simulations of the radical insertion process. A method is developed to obtain exact concentration information on different SUMI products from calibration of the corresponding electrospray ionization mass spectra that are recorded on-line during synthesis. Experimental data show that isolated yields decrease for each subsequent SUMI reaction. This effect is investigated via kinetic modelling to understand which parameters have a beneficial or negative influence on the reaction outcome. Although most reaction conditions (such as monomer concentration or radical flux) do not play a considerable role in the obtainable yield of the insertion reaction, the model clearly shows that the propagation rate coefficient must display a strong chain-length dependency in order to explain the experimental observations. When taken into account, the simulations very well fit the experimental data obtained from optimized microreactor flow synthesis and recommendations for SUMI reactions are formulated. Finally, the optimized SUMI conditions obtained from microreactor experiments and kinetic modelling insights have been applied to upscale the SUMI synthesis reactions in a mesoflow reactor. This demonstrates the simple upscalability of continuous flow reactions and opens the pathway towards future synthesis of longer sequence controlled oligomers.