Process for preparing propylene/1-butene copolymers

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

wherein the polypropylene is a terpolymer and comprises 1-butene-derived comonomer units in an amount of from 5 to 20 wt % and ethylene-derived comonomer units in an amount of up to 3 wt %, and

wherein 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 or 2; y is 2or 3; and z is 0 or 1; under the provision that x+y+z=4;

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

wherein

a carbon atom bonded to the Si atom is a tertiary carbon atom and each of the residues R 4 and R 5 bonded to the tertiary carbon atom is, independently, a C 1-2 alkyl;

each of the residues R 6 and R 7 is, independently, a C 1-2 alkyl;

R 8 is hydrogen or a C 1-4 alkyl;

R 2 is C 1-2 alkyl; and

R 3 is C 1-4 alkyl.

2. The process according to claim 1 , wherein R 3 is methyl or ethyl.

3. The process according to claim 1 , wherein x is 1, R 4 , R 5 , R 6 and R 7 are methyl, and R 8 is hydrogen or C 1-2 alkyl.

4. The process according to claim 1 , wherein the polypropylene has an amount of xylene solubles XS of 10.0 wt % or less and/or a melt flow rate MFR 2 of from 1.0 to 100 g/10 min.

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

a Ziegler-Natta procatalyst which comprises a titanium compound, and

an organometallic cocatalyst which comprises an aluminium compound.

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

7. The process according to claim 5 , 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 and

b) reacting the product of a) with a dialkylphthalate of formula:

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 takes place;

a) optionally washing the product of b) and/or

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

8. A process for preparing a film comprising preparing a polypropylene by the olefin polymerization process according to claim 1 and processing the polypropylene to a film.

9. The process according to claim 3 , wherein the polypropylene has an amount of xylene solubles XS of 10.0 wt % or less and/or a melt flow rate MFR 2 of from 1.0 to 100 g/10 min.

10. The process according to claim 3 , wherein the Ziegler-Natta catalyst system comprises:

a Ziegler-Natta procatalyst which comprises a titanium compound, and

an organometallic cocatalyst which comprises an aluminium compound.

11. The process according to claim 3 , wherein a molar ratio of aluminium to Ti in the Ziegler-Natta catalyst system is from 10/1 to 1000/1 and/or a molar ratio of the external donor to Ti in the Ziegler-Natta catalyst system is from 1/1 to 100/1.

12. The process according to claim 3 , 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 and

b) reacting the product of a) with a dialkylphthalate of formula:

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 takes place;

a) optionally washing the product of b) and/or

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