The monomeric A2 plus monomeric B3 approach was employed to prepare hyperbranched polyurethanes (HBPUs) based on 4,4-methylenediphenyl diisocyanate and trimethylolpropane with the molecular weights ranging from 20,000 to 59,800 g mol1. Through controlling the concentration and stoichiometric ratio of monomers, gel was successfully prevented in the monomeric A2 plus monomeric B3 system. The structures of HBPUs were characterized by 13C NMR. Molecular weights of HBPUs were characterized by gel permeation chromatography. A gradual increase in HBPU molecular weight as well as degree of branching was observed when the stoichiometric ratio of A2 to B3 increased. It was exhibited that the solution concentration of the reaction had great effect on the gel point and cyclization of the monomeric A2 plus monomeric B3 system. This novel isocyanate terminated HBPUs are expected to be capable of modifying the curing performance of benzoxazine resin. (c) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013