Process for preparing propylene/1-butene copolymers

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)
• (R3)z(R2O)ySi(R1)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;
• R1 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 R4 and R5 bonded to the tertiary carbon atom is, independently, a C1-2 alkyl;
• each of the residues R6 and R7 is, independently, a C1-2 alkyl;
• R8 is hydrogen or a C1-4 alkyl;
• R2 is C1-2 alkyl;
• and
• R3 is C1-4 alkyl.