The future of thermosetting polymers relies both on the formulation of biobased systems and on the energetic optimization of the molding processes. This second task more than ever relies on accurate thermokinetic modelling of the cure reactions involved. Nevertheless, bridging that gap between mechanistic knowledge and the production of simple kinetic equations that can be used in process simulation softwares remains a difficult issue. In the present work, we revisit the modelling of polyurethane cure kinetics taking the case of a partially biobased system : 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 phenomenological model based on the Kamal-Sourour equation with non integer reaction exponents; and a new semi-mechanistic model. It assumes 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°C. Such a model based on mechanistic considerations gives more accurate simulation of experimental data, using less kinetic parameters and allows explaining the variations of reactivity of the HTNR polyols with its average molecular weight. Keywords: bio-based polyurethane, natural rubber, calorimetry, kinetics, modelling