The synthesis of a novel water-soluble hyperbranched polyurethane is described. The synthetic strategy involves a new approach to hyperbranched polymerization through the selective activation of the A functionality of the unprotected of AB(2) monomer. 1,1'-Carbonyl diimidazole is added at room temperature to an amino-diol monomer with the corresponding selective formation of an active carboxamide and no detectable reaction at the hydroxyl B functional groups. The active monomer self-condenses to form hyperbranched polyurethanes with measured degrees of branching equaling 0.6 and M-n up to 11 000 Da. A series of model reactions has been conducted to confirm the polymerization mechanism, and ideally branched model molecules have been used to assign the N-15 NMR spectra of the hyperbranched polymers. Reaction conditions have been varied, and the factors affecting the polymerization are discussed.