The present invention relates to a new polymerization process which is suitable for polymerizing high molecular weight products of high viscosity such as polycarbonates and polystyrenes in reasonable time. The present invention achieves the above-identified object by means of a polymerization process, wherein at least in the final stages of the polymerization reaction, two different types of mechanical mixing means are employed as alternatives, designated first and second mixing means. The first mixing means do generate new surfaces in the material to be treated by means of mechanical mixing and by means of gravity driven mixing, while the second mixing means do generate new surfaces in the material to be treated not by means of mechanical mixing and only by means of gravity driven mixing.