Progress of the reactions was monitored using FTIR, as described in our previous study . Briefly, the first step of the reactionError! Reference source not found. was considered to be completed when the NCO/NH ratio reached approx. 5, which corresponds to ~50% of NCO groups being consumed and ensuring the complete conversion of -OH groups. In comparison to our previous work, where the range was approx. 3-6, here the range of values for the (A2262)/(A1526) ratio was similar with exception of 0.1 mol% catalyst concentration. The heterometallic catalyst that used in this study can at the same time act as a Lewis acid catalyst and also according to insertion mechanism. Therefore, the observed differences in kinetics relative to our previous work may be explained by differences in selectivity and activity of a catalyst. Further, the higher ratio (~11) NCO/NH observed for the case of synthesis using 0.1 mol% of catalyst may be also related to the pot life of Mg-Ti butoxide catalyst: at this low concentration the reaction proceeds slowly and the catalytic activity of the catalyst may begin to decrease before the lower ratio can be reached. All of the FT-IR spectra for all reactions are available in the SI, Figures .