Structure development in highly branched segmented polyurethaneureas based on oligomeric A(2)+B-3 approach was investigated by experimental studies and kinetic Monte-Carlo simulations. In both simulations and experiments, hyperbranched polymers were produced by the slow addition of A(2) onto B-3. Experimental studies showed strong influence of solution concentration on the get point and the extent of cyclization in the polymers formed. In polymerizations conducted at a solution concentration of 25% by weight gelation took place at the stoichiometric ratio [A(2)]/[B-3] =0.886. This is somewhat higher than the theoretical ratio of 0.75. In very dilute solutions, such as 5% solids by weight, no gelation was observed although the stoichiometric amount of A(2) added well exceeded the theoretical amount for gelation. Both experimental studies by size exclusion chromatography (SEC) and kinetic Monte-Carlo simulations demonstrated a gradual increase in polymer molecular weights as more A(2) is added onto B-3. This was followed by a sharp increase in the polymer molecular weight as the gel point is approached. A very similar behavior was observed for the polydispersity values of the polymers formed. Kinetic Monte-Carlo simulations performed at different cyclization ratios showed very good agreement with experimental results. (c) 2005 Elsevier Ltd. All rights reserved.