A-Mass or bulk polymerisation.It is a, solvent free two stage process.Vinyl chloride and an initiator (persulfate, monomer soluble peroxide) roughly at 0.015-0.1% concentration, are charged and vigorously stirred (130 rpm) in a vertical, stainless-steel autoclave (prepolymerizer) of 8 - 25 m3 capacity, (see sketch). The pressure in the polymerisation vessel reaches 10 bars.Very obviously the initiation step consists of the fomation of radicals.CH2=CH-Cl + catalyst------->CH2*-CH*-Cl Rapid polymerization takes place at 62 - 75 °C to give 60µm aggregated spherical flocs composed of 0.1 µm primary particles, which form the basis for seeds in the final PVC grain. Conversion is taken to 7 - 12 % in about 30 min, by which time the initiator used is exhausted and the unreacted monomer is recycled.

The slurry from the prepolymeriser is discharged in a 12 - 50 m3 autoclave, with fresh initiator and more VCM. In more recent plant designs the autoclave is replaced by a vertical reactor equipped with stirrers. In the second stage the 0.1 µm primary particles making up each seed grow in size, fuse together to give the final grain of PVC of 130 - 180 µm diameter. The reaction is terminated at a predetermined stage, either by adding a chain terminator and/or venting off the unreacted monomer to the recovery plant. The second stage takes 3-9 hours, depending on the molecular weight targeted. About 10 % of world PVC production is provided by the mass process. Finally, it must be added that the PVC produced, cannot be processed unless it is compounded. This topic will be addressed in the future.