AbstractThe reaction between catalyst components has been studied and polymerization experiments carried out on ethylene using the systems pentyl‐sodium – TiCl4(I) and diethylaluminium bromide – TiCl4(II). Results of reduction of TiCl4can be interpreted on the basis of the following equations: AmNa + TiCl4→ TiCl3+ NaCl + Am•, in the case of I, Et2AlBr + TiCl4→ TiX3+ EtAlX3+ Et•and EtAlX2+ TiCl4→ TiX3+ + AlX3+ Et•(X = Cl, Br), in the case of II. The occurrence of the second reaction in II was confirmed by analyses of the precipitates formed in the reaction and by reacting TiCl4with EtAlBr2. The reducing power of the organometallic compounds examined decreases in the range: AmNa>Et2AlBr>EtAlBr2.Polymerization results with system I showed that active catalysts are obtained only with AmNa/TiCl4mole ratios>unity. The polymer yield appeared to increase with increasing catalyst ratio. This may reasonably be explained by postulating the existence of an AmNa‐TiCl3complex as the actual catalytic species. In this range of catalyst ratios, the intrinsic viscosities of the polyethylenes remain approximately constant.In the case of system II, polymerization results could also be interpreted by assuming the existence of one complex or a mixture of a number of complexes as the catalyst. With Et2AlBr/TiCl4mole ratios>∼ 2, a complex between Et2AlBr and TiCl3(Et2AlX‐TiX3) is postulated. With catalyst ratios in the vicinity of unity and lower, ambiguous results were obtained due to the occurrence of reactions between the catalyst component