ESR study of surface acid sites of titanium-magnesium catalysts for olefin polymerization: Adsorption of nitroxyl radicals

The interaction of stable nitroxyl radicals (TEMPO, 2,2,6,6-tetramethylpiperidine-1-oxyl, and TEMPON, 2,2,6,6-tetramethyl-4-oxopiperidine-1-oxyl) with titanium-magnesium catalysts of the composition TiCl4/MgCl2 (0.4-2.1 wt % Ti) and TiCl4+DBP/MgCl2 (DBP stands for dibutyl phthalate) is studied by ESR with the purpose to determine the concentration of titanium-containing sites in these catalysts exhibiting the properties of a Lewis acid. Only a part of surface titanium species is found to interact with TEMPO and TEMPON. The fraction of titanium species that show the acidic properties and interact with nitroxyl radicals ranges from 20 to 70 mel. % of the overall titanium content and is governed by the catalyst composition (the titanium content and the presence of a stereoregulating additive, DBP). The interaction of the catalyst with a cocatalyst [AlEt3 or Al(iso-Bu)(3)] hardly changes the fraction of the acid sites. Plausible structures of surface titanium species are discussed from the viewpoint of the difference in their acidic properties.