A process for mass polymerization in an autoclave having rotatable ribbon agitators (see Group II) is described with reference to an example in which 210kg. vinyl chloride were introduced into an autoclave of 500 litres capacity and 0,06 kg. or 0,03% of azodiisobutyronitrile added as catalyst. 10 kg. of monomer were removed in degassing to expel air and the agitator rotates at 70 r.p.m. The pressure mounted to and was maintained at 9,05 kg./cm2, and the temperature was maintained at 54,5 DEG C., by cooling and heating as required, until after 16 hours the pressure fell to 8,5 kg./cm2. The pressure was then released, the apparatus swept by nitrogen and the product discharged, being a finely pulverized polymer of vinyl chloride representing a yield of 73,21% of the monomer, having an apparent density of 0,480 and more than 88% passing through a 250 micron mesh sieve. The resin had a polymerization coefficient K of 63,1, a low polymer rate of 4,8% by weight and a stability to heat (200 DEG C.) of 19 minutes. The physical properties of the product may be controlled by selecting the rate of rotation of the agitators. The apparatus may also be used for polymerization in suspension or emulsion or applied to other monomers, e.g. vinyl fluoride, vinylidene chloride, acrylic nitriles, ethylene and propylene.