AbstractIn the reaction of methyl methacrylate (2) with methyl 2‐sodioisobutrate (1) (a model of the active center in the anionic polymerization of2) in a mole ratio of 2:1, the concentrations of all known initial compounds and reaction products were estimated. This reaction was studied in a flow reactor with reaction times between 0,02 and 1,4 s using GLC, LC and GPC as analytical methods. At the same time, a rise in temperature of the reaction mixture by 6°C was observed (for [2] = 0,2 mol/1); at a higher concentration of2(0,6 mol/1), this rise was 27°C and hence not negligible in the interpretation of the results. The concentration changes of the individual products were in accordance with the concept of a stepwise addition of2, to the metalloesters. At the end of the reaction the mean molecular weight of the addition products corresponded roughly to the ratio2/effective1. The cyclization of Na‐trimer4ato a cyclic trimer9was shown to be an important side reaction. The Na‐tetramer5aand the higher Na‐oligomers6aprobably undergo a similar cyclization.2added1very quickly; the following approximate order of reaction rates (Scheme 1) is proposed:R1>R2,R3