Reactive blending of immiscible polymers is an important process for synthesizing polymer blends with superior properties. We use a phase-field model to understand reaction dynamics and morphology evolution by diffusive transport in layered films of incompatible, end-reactive polymers. We thoroughly investigate this phenomenon over a large parameter space of interface shapes, layer thicknesses, reaction rates specified by a Damkohler number (Da_f), and Flory–Huggins interaction parameter (χ), under static conditions with no external fields. For films of the same thickness, the dynamics of the system is not significantly influenced by the length of the film or the initial shape of the interface. The interface between the polymers is observed to roughen, leading to the formation of a spontaneous emulsion. The reaction progresses slower and the interface roughens later for thicker films, and systems with higher χ. Increasing Da_f increases the reaction rate and hastens the onset of roughening. The quasi-static interfacial tension decreases with the extent of reaction, but does not become vanishingly small or negative at the onset of roughening. Simulations with reversible reactions and systems where only a fraction of the homopolymers have reactive end groups show that a critical diblock (reaction product) concentration exists, below which interfacial roughening and spontaneous emulsification is not observed. We also demonstrate that thermal fluctuations accelerate the onset of interfacial roughening, and help sustain the system in an emulsified state.