The influence of molecular weight and chemical structure of several polycarbonate diols on the kinetics of condensation reaction with p-tolyl isocyanate has been studied. Size exclusion chromatography has been used to monitor the reaction. The condensation reaction kinetics is adequately described by an autocatalysed third order rate equation. The values obtained for rate constants, using a Runge–Kutta mathematical model, suggest an association phenomena by hydrogen bonding implying hydroxyl groups but also urethane groups. In bulk and in stoichiometric conditions, the association phenomena observed increases proportionally, on one hand to the decrease of molecular weight of macrodiol, and on the other hand to the tendency to form intramolecular hydrogen bonds.