In this investigation, a new method for step-growth polymerization reactions of 4-phenylurazole (PHU) and 4-(4-methoxyphenyl)urazole (MPU) with various diisocyanates were developed under solvent-free conditions. The reaction of these monomers with hexamethylene diisocyanate (HMDI), isophorone diisocyanate (IPDI), and tolulyene-2,4-diisocyanate (TDI) were performed in the presence or absence of dibutyltin dilaurate (DBTDL) as a catalyst. The results of polymerization revealed that DBTDL did not have considerable effect on the polymerization process. The resulting polyureas showed good yields and moderate inherent viscosities ranging of 0.17-0.30 dL/g in NN-dimethylformamide (DMF) at 25 degrees C. They are soluble in most polar organic solvents. All of the above polymers were characterized by H-1-NMR, FTIR spectroscopy, and thermogravimetric analysis (TGA). This method was compared with the polymerization reaction in N,N-dimethylacetamide as a solvent via solution polymerization. Under solvent-free conditions, higher yields and inherent viscosities were obtained. In addition, in this method we do not need to use any solvents and the polymerization reaction can be classified as a green and environmentally friendly method. (c) 2008 Wiley Periodicals, Inc.