One of the key requirements of the supported catalysts used for the preparation of propylene polymers is the capability of giving high yields coupled with high stereospecificity. The use of electron donor compounds in the preparation of the Ziegler/Natta catalysts comprising a titanium compound on a magnesium-containing support is very well known. Such electron donor compounds are very often used in order to increase the stereospecificity of the catalyst in particular when the polymerization of prochiral olefins is carried out. Generally, in order to achieve the desired effect, the electron donor compound is used in such amounts that at the end of the preparation process an appreciable and effective amount of it remains on the catalyst component. In order to obtain the explained effect the electron donor compounds generally disclosed in the art are mono or polyfunctional ethers, esters and amines. Sometimes, as disclosed in WO99/57160, is used a combination of two or more electron donor compounds in order to obtain a catalyst endowed with specific properties. Also in this case however, the amounts of donor and the preparation conditions are selected in such a way of having, on the final catalyst, a substantive amount of both donors. We have now surprisingly found that certain supported catalyst components obtainable by a specific preparation procedure are capable to give increased yields over the catalysts of the prior art. The specific procedure comprises the use of such a low amount of certain electron donor compounds that at the end of the preparation process it may also be not present on the final catalyst component. The catalyst component of the invention, however, results to be greatly improved with respect to the catalysts that have not been contacted with such electron donor compound.
USPTO Backgrounds
USPTO Backgrounds