AbstractThe polymerization of 4 and 5 molar vinyl chloride in chlorobenzene solution has been studied using 0.005 to 0.1 molar14C labelled azoisobutyronitrile as initiator at temperatures of 30 and 40°C. The number of initiator fragments per polymer molecule increases with the rate of initiation and extrapolates to 1.15 at an infinite rate of initiation showing that 25% of mutual termination occurs by combination and 75% by disproportionation. Values found for the efficiency of initiation vary from about 70% at the lowest initiator concentration to 55% at the highest