Process for preparing propylene polymer compositions

1. An olefin polymerization process, wherein propylene and C4 to C10 α-olefin and optionally ethylene are reacted in the presence of a Ziegler-Natta catalyst so as to obtain a propylene polymer composition,

wherein the polypropylene comprises C4 to C10 α-olefin-derived comonomer units in an amount of from 0.5 wt % to 15 wt % and ethylene-derived comonomer units in an amount of 0 wt % to 3 wt %, and

the Ziegler-Natta catalyst comprises:

i) 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 2 or 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,

the carbon atom bonded to the Si atom is a secondary carbon atom and the residues R 4 and R 5 bonded to the secondary carbon atom are, independently from each other, a C 1-2 alkyl group,

R 2 is a linear C 1-4 alkyl and

R 3 is a linear C 1-4 alkyl, and

ii) a solid Ziegler-Natta catalyst component, which is free of any external carrier material; wherein the solid Ziegler-Natta catalyst component ii) is prepared according to the procedure comprising:

a) providing a solution of

a 1 ) at least a Group 2 metal alkoxy compound (Ax) being the reaction product of a Group 2 metal compound and an alcohol (A) comprising in addition to the hydroxyl moiety at least one ether moiety optionally in an organic liquid reaction medium; or

a 2 ) at least a Group 2 metal alkoxy compound (Ax′) being the reaction product of a Group 2 metal compound and an alcohol mixture of the alcohol (A) and a monohydric alcohol (B) of formula ROH, with R being a straight-chain or branched C 2 -C 16 alkyl residue, optionally in an organic liquid reaction medium; or

a 3 ) a mixture of the Group 2 metal alkoxy compound (Ax) and a Group 2 metal alkoxy compound (Bx) being the reaction product of a Group 2 metal compound and the monohydric alcohol (B), optionally in an organic liquid reaction medium; or

a 4 ) Group 2 metal alkoxy compound of formula M(OR 1 ) n (OR 2 ) m X 2-n-m or mixture of Group 2 alkoxides M(OR 1 ) n′ X 2-n′ and M(OR 2 ) m′ X 2-m′ , where M is Group 2 metal, X is halogen, R 1 and R 2 are different alkyl groups of C 2 to C 16 carbon atoms, and 0≤n<2; 0≤m<2 and n+m≤2, provided that both n and m≠0, 0<n′≤2 and 0<m′≤2; and

b) adding said solution from step a) to at least one compound of a transition metal of Group 4 to 6 and

c) obtaining the solid catalyst component particles, and

adding a non-phthalic internal electron donor at any step prior to step c).

2. The process according to claim 1 , wherein y is 2 or 3, z is 0, R 2 is a linear C 1-4 alkyl.

3. The process according to claim 1 , wherein the amount of C4 to C10 α-olefin-derived comonomer units in the polypropylene composition is from 2 wt % to 12 wt % and the amount of the ethylene-derived comonomer units is from 0 wt % to 2.5 wt %.

4. The process according to claim 1 , wherein the propylene polymer composition is a propylene-1-butene copolymer or propylene-1-butene-ethylene terpolymer.

5. The process according to claim 1 , wherein the propylene polymer composition is prepared in a process comprising a liquid phase polymerization.

6. The process according to claim 1 , wherein the particles of the solid Ziegler-Natta catalyst component ii) have a surface area less than 20 m 2 /g.

7. The process according to claim 1 , wherein the solid Ziegler-Natta catalyst component ii) is obtainable or obtained by a precipitation or emulsion-solidification method.

8. The process according to claim 1 , wherein the process of preparing the solid Ziegler-Natta catalyst component ii) where no external carrier material is used and further comprises the steps:

preparing a solution of Group 2 metal complex by reacting the solution from step a) and the non-phthalic internal electron donor or a precursor thereof in a reaction medium comprising C 6 -C 10 aromatic liquid;

reacting said Group 2 metal complex with at least one compound of a transition metal of Group 4 to 6 and

obtaining the solid catalyst component particles.