Kinetics of ethylene polymerization initiated by free radicals

Abstract

AbstractEthylene polymerization can be carried out under milder conditions than heretofore because of the development of peroxydicarbonate catalysts. This simplifies study of polymerization kinetics. Diethyl peroxydicarbonate undergoes first‐order decomposition in tert‐butyl alcohol with an energy of activation of 33 kcal./mole, and a half‐life of 1.5 hr. at 60°C. The rate of decomposition, 1.3 × 10−4 sec.−1, does not change in the presence of polymerizing ethylene. The rate of polymerization can be followed by pressure change. But, because the concentration of ethylene in tert‐butyl alcohol as a function of pressure is not known accurately, it is better to measure polymerization rate by isolating the polymer. A mathematical relationship was developed between initiator decomposition and polymer formation. It is based on a conventional free‐radical polymerization mechanism and is supported by the experimental data. Polymer molecular weight is independent of initiator concentration and increases only slightly with duration of polymerization. The effects of reaction variables of molecular weight are explained by normal free‐radical transfer reactions. The polymer contains ethyl carbonate groups from homolytic scission of diethyl peroxydicarbonate, and it exhibits the same absorption bands in the infrared as does ethyl n‐octyl carbonate. These bands disappear on saponification and are not present in polyethylene made with other peroxide catalysts.