The state of Ti3+ ions formed at various steps of preparation and activation of highly active titanium-magnesium catalysts of various composition (dry milling of TiC13 with MgCl,; reduction of T i c 4 by nonsolvated magnesium dialkyl; interaction of the TiC14/MgCl s stem with an organoaluminium compound) was studied by the ESR method. It was found that In all systems studied the same three types of Ti3+ ions in the chlorine environment are formed, differing in coordination and localization sites in MgCI,. The content of these ions depends on the composition and preparation conditions of the catalysts, and does not exceed 20% of the total content of titanium. Most of the Ti3+ ions formed in all systems enter the composition of particles or surface aggregates of TiC13 and appear in the ESR spectra only after dissolution of the sample in pyridine. The active centers of titanium-magnesium catalysts are supposed to be constituents of these particles or of surface aggregates. Y

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