AbstractA common technique for the manufacture of polymer alloys is melt blending of polymers with specific chemical interactions. For instance, an amido ester crosslink forms when oxazoline‐grafted PS is mixed with acid‐grafted PE. The crosslink builds melt viscosity and stabilizes product morphology. Reaction control is critical since overreacted alloys from gels and underreacted alloys have gross composition gradients and/or tend to phase coalesce so that product properties are variable. Polymer reactions in plasticating compounders are particularly difficult control problems since several unit operations involving plastication, heat exchange, mixing, and separating of by‐products must be sequenced properly to control a reaction. Reactive polystyrene (OPS) and reactive polyethylene (APE) with oxazoline and carboxylic acid functionality, respectively, were melt blended in a ZSK compounding extruder under a variety of conditions. The extent of crosslink was monitored by the strong shear viscosity increase of the reacted product, the infrared absorption by the crosslink, and reactant depletion in the melt. The phase morphology was characterized by optical phase contrast microscopy. A response surface of product shear viscosity vs. the primary operational parameters of extrusion rate and extruder RPM is interpreted in terms of characteristic reaction time and development of reactive interfaces during the mixing process. Response surfaces result from storing coordinates and responses while exercising a new feedback loop control s