These differences in the concentration dependence may be explained by differences in the catalytic mechanism between DBTDL and the remaining non-organotin catalysts (see Figure ). DBTDL catalyst acts as a Lewis acid that can interact with N=C=O groups-in this case coming from IPDI-and increase their electrophilicity (Figure ). Further, DBTDL is a Lewis acid with pKa 11 and therefore this catalyst can be expected to be more active at higher concentrations, resulting in reduced reaction times. In contrast, according to literature, the catalytic activity of both of the bismuth catalysts and the zinc one is based on an insertion mechanism (Figure ). In this case, the catalysts exchange their ligands with the alcohol-in this case the polyester polyol-or interact with hydrogen from the -OH in alcohol (here the polyester polyol). Therefore, steric effects may have a major effect on the reaction time. This can then account for the differences between BiHex and BiNDE; while both catalysts have 3 ligands, BiHex is less bulky (8 vs 10 carbons), explaining the faster reaction rate. The increase in total reaction time (from 7 to 26 hours) observed after increasing the concentration of BiHex catalyst to 4 mol%. Longer reaction time in II step for PrBiHex at 4 mol% can result from side reaction manifested by formation of allophanate crosslinks. The effect of formed allophanate crosslinks during the second step effected an increased reaction time. BiHex is used an allophanatization catalyst for polyurethane formation within the referred patent . Additionally, allophanatization reactions are reversible and dissociation of the obtained allophanates occurs quickly , therefore, we did not observe allophanates in the final structure (confirmed by 1 Hand C-NMR spectroscopy, see in Figure ).