The invention relates to a modified Ziegler catalyst for preparing a
poly-1-olefin in suspension, in solution or in the gas phase, which
catalyst comprises the reaction product of a magnesium alkoxide
(component a) with a transition metal compound (component b) and an
organometallic compound (component c) together with an additional
component (d) comprising a compound of the chemical formula M-R.sub.x
where M is an element of main group IV of the Periodic Table, R is
halogen or an organic radical such as alkyl having from 1 to 10 carbon
atoms, oxyalkyl having from 1 to 10 carbon atoms, cycloalkyl having from
4 to 8 carbon atoms in the ring and, if desired, from 1 to 6 substituents
R' on the ring, aryl having from 6 to 10 carbon atoms in the aromatic
and, if desired, from 1 to 6 substituents R' on the aromatic, where R' is
a halogen or an alkyl radical having from 1 to 4 carbon atoms or an OH
group or an NO.sub.2 group or an oxyalkyl radical having from 1 to 4
carbon atoms, and x is an integer from 1 to 4. The invention also relates
to a process for preparing the Ziegler catalyst and to the
homopolymerization or copolymerization of 1-olefins in its presence.