Process for producing ethylene copolymers under high pressure

A process for preparing copolymers of ethylene with C.sub.3 -C.sub.10 -alk-1-enes, wherein the polymerization is carried out at pressures in the range from 1000 to 3500 bar and at from 200 to 280.degree. C. in the presence of a catalyst system comprising as active constituents a donor-substituted monocyclopentadienylamido(transition metal) complex compound of the formula I:

• ##STR8## where the substituents have the following meanings:
• M is titanium, zirconium or hafnium,
• X is fluorine, chlorine, bromine, iodine, hydrogen, C.sub.1 -C.sub.10 -alkyl or C.sub.6 -C.sub.15 -aryl,
• R.sup.1 to R.sup.4 are hydrogen, C.sub.1 -C.sub.10 -alkyl, 5- to 7-membered cycloalkyl which may in turn bear a C.sub.1 -C.sub.10 -alkyl group as substituent, C.sub.6 -C.sub.15 -aryl or arylalkyl, where two adjacent radicals may also together form a cyclic group having from 4 to 15 carbon atoms, or Si(R.sup.6).sub.3 where
• R.sup.6 is C.sub.1 -C.sub.10 -alkyl, C.sub.3 -C.sub.10 -cycloalkyl or C.sub.6 -C.sub.15 -aryl,
• R.sup.5 is ##STR9## R.sup.7 to R.sup.10 are hydrogen, C.sub.1 -C.sub.10 -alkyl, C.sub.6 -C.sub.15 -aryl or R.sup.7 and R.sup.9 or R.sup.8 and R.sup.10, in each case together with the atoms connecting them, form a ring,
• Y is ##STR10## where R.sup.11 is hydrogen, C.sub.1 -C.sub.6 -alkyl or C.sub.6 -C.sub.15 -aryl which may in turn be substituted by C.sub.1 -C.sub.4 -alkyl groups
• and
• n is 1, 2 or 3,
• a compound capable of forming metallocenium ions and, optionally, a trialkylaluminum compound.