The Photochemical Polymerization of Butadiene

The mercury photo-sensitized polymerization of butadiene and butadiene-acetone mixtures in the gas phase, and the photochemical reactions in solutions of butadiene and acetone in hexane, were studied. Metal mirror experiments demonstrated the absence of hydrogen atoms and the presence of alkyl radicals at low pressures. The mechanism proposed for butadiene vapor postulates an activated butadiene molecule produced by collision with activated mercury and competition between deactivation by collision, predissociation into free radicals, and a collisional process to give hydrogen gas. The polymerization chain proceeds by a mechanism involving either free radicals or hydrogen atoms. In the presence of acetone vapor, the rate is increased and the increase is directly proportional to the number of free radicals coming from acetone, indicating that polymerization chains are not broken by bimolecular gas phase reaction between free radicals. The quantum yield for butadiene alone at 53-mm pressure and 43°C was 0.24. The average number of molecules reacting per free radical formed was calculated to be between 3 and 8. The liquid solution results are explained on the basis of a modified Frank-Rabinowitch hypothesis for the primary photochemical process. Activated acetone molecules are deactivated by collision with hexane, and react with acetone to yield unsaturated compounds and with butadiene to initiate polymerization. The following kinetic equation is developed for the rate of polymerization, dP/dt=(kIabs/I0)(n[C4H6]–[(CH3)2CO]), where n is the average number of molecules reacting per free radical. n is taken equal to 4.