The bulk polymerization of hydroxyl-terminated polybutadiene with tolylene diisocyanate has been studied in uncatalyzed isothermal reactions. The conversion in urethane was monitored by C-13-NMR spectroscopy of the C=O function at level up to 70%. Effects of reagents ratio and temperature were investigated. The experimental data were modeled by a set of two competitive second-order reactions to account for the difference in reactivity of o-NCO and p-NCO groups of tolylene 2,4 diisocyanate. The apparent kinetic constants calculated appeared to be statistically higher than previous results found in literature from work in aromatic solvents. Deviations from second-order law were observed during the last stage of polymerization. Activation energies were estimated at 12.3 kJ . mol(-1) and 42.8 kJ . mol(-1) for the p-NCO and o-NCO groups, respectively. The difference in their reactivity tended to decrease as temperature of reaction was increased.