Ziegler-natta catalyst systems containing organometallic compounds

Ziegler-Natta-type catalyst systems containing, as active constituents, a) a titanium-containing solid component which also contains magnesium, halogen and a carboxylic acid ester as electron donor, b) an aluminum component, c) a further electron-donor component, and, as an additional component, d) an organometallic compound of the general formula A or B M1R1(A) M2R1R2(B) where M1 is an element f rom main group I, and M2 is an element from main group II of the Periodic Table, and R1 and R2 are identical or different C1-C10-alkyl, C6-C12-aryl, C6-C12-arylalkyl, or 5- to 7-membered cycloalkyl, which may itself carry C1-C8-alkyl groups, can be used to prepare polymers which have, in particular, a low proportion of very fine grains.