Isocyanate-free polyurethane resins biosourced to a very high percentage level were prepared by the reaction of aminated mimosa tannin extract with commercial mimosa tannin extract prereacted with dimethyl carbonate. The reaction took place with ease at ambient temperature. Indications were that the polyurethanes obtained formed a hard film when cured at a temperature higher than 100 °C. Furthermore, the carbohydrate fraction of the tannin extract also appeared to be carbonated and reacted to generate isocyanate-free polyurethane linkages with the aminated tannins. This indicated that not only the polyphenolic fraction of the tannin extract, but also its other major component, can be used to prepare polyurethane resins. Flavonoid monomers and oligomers carbonated at different levels, both unreacted and urethane-linked to aminated flavonoid monomers and oligomers, were identified by FTIR and MALDI-TOF spectrometry.