The existence of multiple stationary values of the fully, filled length in reactive extrusion is studied on the basis of a mathematical model describing dynamic extruder behavior The extruder is modelled as a plug flow type of reactor Through the introduction of travel time for a fluid element in the extruder, time dependent viscosity models following from rheokinetics can be used. Special attention is paid to ionic and radical polymerization reactions. Hysteresis behavior for stationary values of the fully filled length exists when die resistance and rotational speed are varied. The effects of reaction rate, viscosity, levels and degree-of-fill on stable operation at high conversions are studied. The mathematical dynamic model presented here can account for some of the experimentally, observed dynamic phenomena in the reactive extrusion of radical polymers.