Ziegler-Natta-katalysaattori

A catalyst of the Ziegler-Natta type is prepared using a preactivated support containing (A) from 80 to 95 mol % of magnesium chloride and (B) 5 to 20 mol % of at least one electron donor compound which contains no labile hydrogen and no ester function; the preactivated support consisting of spherical particles having a mass average diameter, Dm of 10 to 100 microns and a particle size distribution Dm/Dn of less than 2. The catalyst is prepared by (i) treating the preactivated support with at least one electron donor compound containing labile hydrogen, (ii) optionally treating the resulting activated support with at least one ester of an aromatic acid, (iii) impregnating the treated support with titanium tetrachloride and (iv) after washing the impregnated catalyst activating it with titanium tetrachloride. The catalyst has a high titanium content and a high activity when used to polymerise olefins.