The bulk polymerization of monomers which produce polymer insoluble in the monomers, in which the polymerization is carried out in an autoclave mounted on a horizontal axis and provided with stirring means formed of at least two half-frames arranged symmetrically of the axis of the autoclave and fixed at each end to a rotatable stub shaft axially mounted in the autoclave, each half-frame having a peripheral member serving as a blade which, on rotation of the half-frames, passes in close proximity to the interior surface of the autoclave, and bracing means interconnecting the peripheral members of the half-frames.