Isoconversional analysis was applied to non-isothermal bulk cure kinetics of uncatalyzed polyurethane (PU) resins based on telechelic hydoxyl natural rubber (HTNR) and poly[(phenyl isocyanate)-co-formaldehyde] (P-MDI). The dependency of apparent activation energy on conversion led to the identification of two different kinetic models, each involving two activation energies: a phenomeno-logical model based on the Kamal-Sourour equation with non integer reaction exponents; and a new semi-mechanistic model assuming a main second order reaction between alcohols and isocyanates forming urethane linkages and a secondary reversible reaction forming allophanates that is known to become significant above 120 degrees C. Such a model based on mechanistic considerations allowed us to explain the variations of reactivity of the HTNR polyols with its average molecular weight. (C) 2015 Elsevier B.V. All rights reserved.