A new generation of MgCl2-supported catalysts for the polymerization of propene without any external donors was prepared. Two diethers, 9,9-bis-(methoxymethyl)fluorene (for Cat-A) and 2,2-dipropyl-1,3-dimethoxypropane (for Cat-B) differing in the bulkiness of alkyl substituents in position 2, have been used as internal donors in MgCl2/TiCl4/diether-AlR3 catalysts. The weight-average molecular weights produced with both catalysts were over 3.5 x 10(5) at low temperature in slurry polymerization (<40 C). Cat-A showed higher activity and produced higher isotactic polypropene than Cat-B. The activity of both catalysts proved to be dependent on the temperature.