AbstractA study has been made of the influence of several variables on the performance of a Ziegler catalyst, prepared from triisobutylduminum and TiCl, in the polymerization of ethylene. Polymerizations were run at atmospheric pressure, with catalyst formedin situ, and with reaction rate unrestricted by gas‐liquid mass transfer rate. The influence of impurities on the catalyst performance has been minimized by a concerted effort to eliminate moisture, oxygen, etc., from the system as a whole; by the use of triisobutylaluminum scrubbers to purify the ethylene; and through the purification of the solvent, isooctane, by distillation in presence of triisobutylaluminum. Conditions leading to reproducible results and high catalyst efficiency have been established. A catalyst prepared in the absence of ethylene is relatively inefficient depending on Al/Ti ratio. Under the conditions studied, the most efficient catalysts were obtained by the addition of triisobutylaluminum to the TiCl4. The reverse mode of addition gave poor catalysts, particularly if the triisobutylaluminum was exposed to ethylene for a short time a t 50°C. However, under special conditions insuring complete mixing before significant reaction or aging of the catalyst components, the order of addition had no effect. A study of the performance of catalyst prepared by the rapid addition of triisobutylaluminum to TiC14, as a function of its components concentrations, at 50°C., revealed regions of maximum efficiencies. Maximum efficiency of 600 to 850 g. polyethylene/g. triisobutylaluminum was reached with Al/Ti ratios between 1.4 and 2.8. The efficiency with respect to TiCl4reached a plateau of 1800 g. polyethylene/g. TiCl4above an Al/Ti ratio of 2.8. The ratio of polymer yield to maximum polymerization rate was constant above an Al/Ti ratio of 1.4. The highest efficiency, 1500 g. polyethylene/g. triisobutylaluminum and 2100 g. polyethylene/g. TiCl, was obtained by incremental addition of triisobutylaluminum. These results have been interpreted consistently with a previously proposed mechanism of heterogeneous catalysis of polymerization. It appears that under conditions of rapid triisobutylaluminum addition, all intrinsic properties of the catalyst become constant above an Al/Ti ratio of 2.8. That is, the surface of the solid phase (presumably TiCL) is apparently a constant molar fraction of the total titanium content and is completely covered by adsorbed triisobutylaluminum. The same effect can be obtained a t a lower AI/Ti ratio of about 1.4 when triisobutylaluminum is added incrementally. Otherwise, a t AI/Ti ratios below 2.8, the adsorbed co‐catalyst is partly alkylaluminum chlorides, resulting in decreased activity per polymerization center. At a ratio of 0.7 the surface is less than completely covered with adsorbed aluminum compounds. It is indicated that the polyethylene obtained at high Al/Ti ratios is partially crosslinked. The physical nature of the polymer varies with operating conditions. A new, simple method for determining the quantity of catalyst poisons in the solvent is desc