AbstractThe bulk polymerization ofn‐butylmethacrylate was investigated in a counter‐rotating twin screw extruder. It appeared that the gel effect, occurring with bulk polymerizations, affected the polymerization progress very strongly. Due to this effect the conversion of the reaction is independent of the rotation speed of the screws. The reactive extrusion diagram, presented in this paper, indicates how the different phenomena interact with each other and helps to understand the influences of extruder parameters and reaction parameters on the proc