AbstractApplication of thein situinitiator formation technique to the vinyl chloride–propylene system has been examined for peroxydicarbonates, acyl peroxycarbonates, diacyl peroxides, and a peroxy ester. Of these the peroxydicarbonates were found to be the most effective. Of the peroxydicarbonates examined, diethyl, diisopropyl, and di‐sec‐butyl were the most efficient, all being more efficient than lauroyl peroxide. A comparison between thein situmode and the use of preformed initiator showed the latter to be the more efficient of the two techniques when diethyl peroxydicarbonate was used for the homopolymerization of vinyl chl