AbstractMeasurements have been made on the kinetics of the polymerization of ethylene using Ziegler‐type catalysts made from zine di‐n‐butyl and titanium tetrchloride in hydrocarbon solvents. The rate of polymerization was proportional to the square of the ethylene pressure. At catalyst ratios (the molar ratio of zine butyl to titanium tetrachoride in the reaction mixture) less than 0.5, an initial, fast polymerzation was followed by a slower but more persistent polymerization. Both processes increased in velocity with increasing ratio up to 0.5; above this ratio the fast process was not observed while the rate of the slower one decreased with increasing ratio and was very small at unit ratio. At higher ratios the rate increased again as though to reach a limiting value when a very large excess of zine butyl was used. It has not been possible to devise more than speculative interpretations of these results. They suggest that at ratios greater than unity the reaction\documentclass{article}\pagestyle{empty}\begin{document}$ {\rm TiCl}_4 + {\rm ZnBu}_2 \to {\rm TiCl}_3 + {\rm ZnBuCl} + ({\rm Bu radicals}) \to {\rm other products} $\end{document}may be completed and that the chain initiation step takes place between zine butyl and ethylene, both adsorbed equilibrium‐wise onto the solid, reduced titanium choloride. It is not yet known whether propagation of polymerization occurs along the catalyst surface or at single sites on the surface. If the former is the case and the mobile propagation center is an ion‐pair, the hydrocarbon end being negative and the positive charge lying in the surface, the mobility of this latter charge may limit the rate of propagation. It is possible to explain some of the results obtained most easily from th