Free‐radical copolymerizations of 1,2‐dichloroethylenes. Evidence for chain transfer by chlorine atom elimination

Trialkylboron–oxygen, an active, low‐temperature free‐radical initiator, has been employed to investigate the effects of very low temperatures on the copolymerizations of vinyl acetate with cis and trans‐1,2‐dichloroethylenes. The low temperatures favor the propagation rate relative to the transfer rate, such that high molecular weight copolymers containing substantial quantities of 1,2‐dichloroethylene can be prepared. The molecular weights of the copolymers depend only on the amounts of 1,2‐dichloroethylene in the copolymers, regardless of the isomer which takes part in the copolymerization. Since the double bond of the trans isomer is about six times as reactive as that of the cis isomer, this indicates that the dominating chain transfer reaction occurs by chlorine atom elimination subsequent to the addition of the dichloroethylene unit to the growing free radical chain. It is suggested that a similar chain‐transfer mechanism occurs in the polymerization of vinyl chloride, wherein an infrequent head‐to‐head placement of monomer unit is followed by ejection of a chlorine atom to form an olefinic bond and termination of that growing chain. The presence of the 1,2‐dichloroethylene unit in the copolymer increases the glass transition temperature approximately 1°C per weight per cent copolymerized with the vinyl acetate.