Process for producing a prepolymerized solid Ziegler-Natta catalyst

A process for producing a propylene polymer, the process comprising the steps of:

• I) producing in batch mode a prepolymerized Ziegler-Natta catalyst by
• a) providing a solid Ziegler-Natta catalyst component being free of phthalic compounds, wherein the solid Ziegler-Natta catalyst component comprises:
• (a1) a compound of a transition metal (TM), which transition metal is selected from one of the groups 4 to 6 of the periodic table (IUPAC),
• (a2) a compound of a metal (M) which metal is selected from one of the groups 1 to 3 of the periodic table (IUPAC), and
• (a3) an internal donor (ID) being a non-phthalic compound, wherein the internal donor (ID) is selected from optionally substituted malonates, maleates, succinates, glutarates, citraconates and derivatives and/or mixtures thereof;
• and
• b) subjecting the solid Ziegler-Natta catalyst component of step a) into a catalyst prepolymerization step, wherein the solid Ziegler-Natta catalyst component is prepolymerized in the presence of linear C4 olefin monomers to a polymerization degree from 0.2 to 50 gPO/g cat to obtain the prepolymerized solid Ziegler-Natta catalyst, and
• II) producing a propylene polymer by
• c) subjecting the prepolymerized Ziegler-Natta catalyst of step b) into a polymerization reactor, wherein propylene and optionally at least one other α-olefin selected from C2 and/or C4 to C8-α-olefin is/are polymerized in the presence of the prepolymerized solid Ziegler-Natta catalyst, wherein step c) comprises a process prepolymerization step and is carried out in a prepolymerization reactor and one or more polymerization reactor(s), and
• d) obtaining the propylene polymer from step c) being free of phthalic compounds originating from the catalyst, wherein the propylene polymer is a propylene homopolymer or a propylene random copolymer having an amount of xylene cold soluble fraction, determined according to ISO 16152 (25° C.), of below 5.5 wt. %.