| Abstract: | Chain transfer constants in the homogeneous free-radical polymerization of ethylene at 1360 atm. and 130°C. have been determined for over 50 compounds, including nearly 300 hydrocarbons. The effects of substitution, unsaturation, and ring strain in the transfer agent molecule on the reactivity of its C? H bonds in chain-transfer reactions with a polyethylene growing chain are analyzed. Qualitatively, these trends parallel those found for simple radicals attacking simple molecules. However, the principle that the reactivity of a compound is the sum of the reactivities of all reactive bonds, which is well established for simple radicals and transfer agents, is found not to be true in ethylene polymerization. It is postulated that the deviations from this principle are due to steric factors which become very important when the free radical is bulky. The transfer constants measured in ethylene polymerization are also compared with transfer constants in other systems. A strong correlation is found between the transfer constants in ethylene and published data on rates of abstraction of hydrogen atoms by methyl radicals. |
