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-tetramethy lpiperidine-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 phthala te) is studied by ESR with the purpose to determine the concentration of titanium-containing sites in these catalysts exhibiting the propert ies of a Lewis acid. Only a part of surface titanium species is found to interact with TEMPO and TEMPON. The fraction of titanium species th at show the acidic properties and interact with nitroxyl radicals rang es from 20 to 70 mel. % of the overall titanium content and is governe d by the catalyst composition (the titanium content and the presence o f a stereoregulating additive, DBP). The interaction of the catalyst w ith a cocatalyst [AlEt3 or Al(iso-Bu)(3)] hardly changes the fraction of the acid sites. Plausible structures of surface titanium species ar e discussed from the viewpoint of the difference in their acidic prope rties.