Four catalysts, viz. 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD), potassium bis(trimethylsilyl)amide (KHMDS), potassium methoxide (KOMe), and potassium tert-butoxide (KO-t-Bu), are screened for a low-toxicity route toward (poly)urethane/urea preparation from carbonate/carbamate esters. On the basis of similar kinetics, the catalytically active species for KHMDS, KO-t-Bu, and KOMe is inferred to be the alkoxide anion RO–, depending on the R group in the carbonate/carbamate. Its activity is much higher than that of TBD. Computational simulation for MeO–-catalyzed urethane/urea formation has been carried out using density functional theory. The activation energies calculated are in close match with the experimental results for the three MeO–-based catalysts. TBD and KOMe are applied in the isocyanate-free polyurea preparation by polycondensation. Again, KOMe shows a higher efficiency than TBD.