The MgCl 2 ‐supported titanium catalyst MTK ST was characterized by X‐ray diffraction measurement and electron spin resonance (ESR) spectroscopy. To obtain a better understanding of the nature of active sites the process of catalyst formation was interrupted at several stages to obtain samples for ESR measurements. Likewise some systematic modifications of the standard system were performed and the results were studied by ESR technique. The structural data were correlated with the activity pattern in the polymerization of ethylene. It is suggested that the precursor of an active site is a clustered titanium species containing titanium ions of mixed valence state which is complexed and alkylated by AIR 3 giving a polymerization active species of high efficiency. A comparison with the polymerization data suggests that the active species or its immediate precursor is characterized by an ESR signal with rhombic anisotropy with g 1 = 1,94, g 2 = 1,89, g 3 = 1,76.
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Fuhrmann et al. (1994) studied this question.
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