Information on the reaction kinetics in the melt is a valuable prerequisite for a successful application of self-healing ability to benzoxazine-based thermosetting resins. The goal of this research was to develop a kinetic model of the Diels–Alder (DA) reaction in a molten state between N-phenylmaleimide (PMI) and benzoxazine-containing furan group (G-f). Rheological data were found to be of significant importance in the kinetic model development and interpretation. The increase in viscosity, the occurrence of side reactions, and equilibrium between DA and r-DA reaction were identified as possible reasons for the deceleration of reaction rate and limiting conversion. After a thorough investigation of the studied system, accompanied by the knowledge from the literature, the potential reason for the incomplete conversion of reactants was reduced to the crossover between chemically and diffusion-controlled reaction due to the increasing viscosity of reaction system. Accordingly, the kinetic model based on the combination of Rabinowitch equation and Chern–Poehlein model was used to accurately describe the progress of the studied reaction in the molten state.