Process for preparing propylene copolymers

1. An olefin polymerization process comprising: reacting propylene and 1-butene and ethylene in the presence of a Ziegler-Natta catalyst system so as to obtain a polypropylene, which is a 1-propylene-1-butene-ethylene terpolymer,

wherein the polypropylene comprises 1-butene-derived comonomer units in an amount of from 0.5 to 15 wt % and ethylene-derived comonomer units in an amount of up to 3 wt %, and

the Ziegler-Natta catalyst system comprises an external donor of the following formula (I):

(R 3 ) z (R 2 O) y Si(R 1 ) x   (I)

wherein

x is 1; y is 2; and z is 1; under the provision that x+y+z=4;

R 1 is an organic residue of the following formula (II):

wherein

the carbon atom C bonded to the Si atom is a tertiary carbon atom and each of the residues R 4 , R 5 and R 6 bonded to the tertiary carbon atom is, independently from each other, a linear C 1-4 alkyl, or two of R 4 , R 5 and R 6 , together with the tertiary carbon atom C they are attached to can be part of a carbocycle of 4-10 carbon atoms;

R 2 is a linear C 1-4 alkyl;

R 3 is linear C 1-4 alkyl; and

wherein the process is operated in liquid phase or by mixed liquid-gas techniques.

2. The process according to claim 1 , wherein R 4 , R 5 , R 6 are methyl, R 2 is methyl or ethyl and R 3 is methyl or ethyl.

3. The process according to claim 1 , wherein, R 4 , R 5 and R 6 are methyl and R 2 is methyl.

4. The process according to claim 1 , wherein the polypropylene does not comprise any monomer units derived from a C 5-10 alpha-olefin.

5. The process according to claim 1 , wherein the amount of 1-butene-derived comonomer units in the polypropylene is 1 to 12 wt %.

6. The process according to claim 1 , wherein the 1-butene content is 1 to 12 wt %, and ethylene content is 0.5 to 2.5 wt %.

7. The process according to claim 1 , wherein the MFR 2 of the propylene polymer is in the range of 0.5 to 100 g/10 min.

8. The process according to claim 1 , wherein the Ziegler-Natta catalyst system comprises:

a Ziegler-Natta procatalyst comprising a titanium and a magnesium compound, and

an organometallic cocatalyst comprising an aluminum compound.

9. The process according to claim 1 , wherein the molar ratio of aluminum to Ti in the Ziegler-Natta catalyst system is from 10 to 1000; and/or the molar ratio of the external donor to Ti in the Ziegler-Natta catalyst system is from 1 to 100.

10. The process according to claim 1 , wherein the Ziegler-Natta procatalyst is obtained by:

a) reacting a spray crystallized or emulsion solidified adduct of MgCl 2 and a C 1 -C 2 alcohol with TiCl 4

b) reacting the product of stage a) with a dialkylphthalate of formula (I):

wherein R 1′ and R 2′ are independently an alkyl group having at least 5 carbon atoms,

under conditions where a transesterification between said C 1 to C 2 alcohol and said dialkylphthalate of formula (I) takes place;

c) optionally washing the product of stage b) and/or

d) optionally reacting the product of step b) or step c) with additional TiCl 4 .

11. The process of claim 1 , further comprising:

processing the polypropylene to a film.