Autoclave

An autoclave for preparation in bulk of polymers and copolymers based on vinyl chloride comprising a vessel, at least one agitator mounted for rotational movement within said vessel consisting essentially of a ribbon wound in helicoidal turns on a substantially vertical rotatable shaft, the upper surface and the lower surface of said ribbon being respectively surfaces S.sub.1 and S.sub.2 each generated by displacement of a straight segment of a line of substantially constant length, when one of the ends describes a helix on a cylinder which forms a surface of the rotatable shaft, said segments being situated during their displacement in planes tangential to said cylinder, upper surface S.sub.1 extending from the shaft at a negative angle .phi..sub.1 with the plane perpendicular to the axis of the rotatable shaft and the lower surface S.sub.2 extending at a positive angle .phi..sub.2 with the plane perpendicular to the axis of the rotatable shaft.