AbstractVinyl chloride was polymerized at 59–92% of saturation pressure in a water‐suspended system at 45–65°C with an emulsion poly(vinyl chloride) (PVC) latex as a seed. A water‐soluble initiator was used in various concentrations. The monomer was continuously charged as vapor from a storage vessel kept at lower temperature. Characterization included determination of molecular‐weight distribution and degree of long‐chain branching by gel permeation chromatography (GPC) and viscometry, thermal dehydrochlorination, and microscopy. The polymerization rate decreases with decreasing pressure but is reasonable even at the lowest pressure. The molecular weight decreases with decreasing pressure and increasing initiator concentration and also with increasing polymerization temperature, if the initiator concentrations are chosen to give a constant initiator radical concentration. The degree of long‐chain branching increases with increasing initiator concentration and decreasing monomer pressure but is unaffected by the polymerization temperature, if the initiator radical concentration is kept constant. The thermal stability decreases with decreasingMn, while the degree of long‐chain branching has only a minor influence. The most important factor in the system influencing the molecular parameter is the monom