Abstract: An apparatus for carrying out polymerizations from the gas phase comprising a sealable reactor with a spiral stirrer which is rotatable in the reactor about a vertical axis to generate circulatory streams of the materials employed, in which the stirrer rotating within the reactor is surmounted by a stirrer rod, which extends upward beyond the uppermost flight circle of the stirrer and is attached to the stirrer at a certain distance from the axis of the stirrer, the distance of the side face of the stirrer rod from the reactor wall is at least 10 mm, and the surface of revolution of the moving rod matches the contour of the reactor in this zone.