a process for preparing polymers and copolymers based on vinyl chloride by mass polymerization of a monomer composition based on vinyl chloride , comprising carrying out at least part of the polymerization reaction in a polymerization zone defined by an essentially spherical wall by effecting in the said polymerization zone a controlled and regulated circulation of the polymerization reaction mixture by means of a rotated blade - type turbine assembly , where the said blades pass by in the vicinity of the said wall over 10 % to 60 % of its surface , the reaction mixture being caused by the centrifugal force of said rotated blade - type turbine assembly to perform an at least partially ascending movement over at least part of the spherical wall and falling back , essentially under the action of gravity in the central part of the spherical polymerization zone , thus ensuring intensive mixing without dead space .