A process for preparing propylene-ethylene random copolymers for pipe applications

A process for the preparation of a monophasic propylene-ethylene random copolymer composition (R-PP) having a melt flow rate (MFR2), determined according to ISO 1133 at 230° C. at a load of 2.16 kg, in the range from 0.01 to 1.00 g/10 min, comprising:

• a) polymerizing propylene and ethylene comonomer units in a first polymerization reactor in the presence of a single-site catalyst to produce a first polymerization mixture comprising a first propylene-ethylene random copolymer fraction (R-PP1) and the single-site catalyst;
• b) withdrawing the first polymerization mixture from the first polymerization reactor;
• c) transferring the first polymerization mixture into a second polymerization reactor;
• d) polymerizing propylene and ethylene comonomer units in the second polymerization reactor in the presence of the single-site catalyst to produce a second polymerization mixture comprising the first propylene-ethylene random copolymer fraction (R-PP1), a second propylene-ethylene random copolymer fraction (R-PP2) and the single-site catalyst;
• e) withdrawing the second polymerization mixture from the second polymerization reactor;
• and
• f) optionally compounding the second polymerization mixture, optionally with the addition of additives (A),
• wherein the single site catalyst comprises:
• (i) a metallocene complex of the general formula (I)
• wherein each X independently is a sigma-donor ligand,
• L is a divalent bridge selected from —R′2C—, —R′2C—CR′2—, —R′2Si—, —R′2Si—SiR′2—, —R′2Ge—, wherein each R′ is independently a hydrogen atom or a C1-C20-hydrocarbyl group optionally containing one or more heteroatoms from groups 14-16 of the periodic table or fluorine atoms, or optionally two R′ groups taken together can form a ring,
• each R1 are independently the same or can be different and are hydrogen, a linear or branched C1-C6-alkyl group, a C7-20-arylalkyl, C7-20-alkylaryl group or C6-20-aryl group or an OY group, wherein Y is a C1-10-hydrocarbyl group, and optionally two adjacent R1 groups can be part of a ring including the phenyl carbons to which they are bonded,
• each R2 independently are the same or can be different and are a CH2—R8 group, with R8 being H or linear or branched C1-6-alkyl group, C3-8-cycloalkyl group, C6-10-aryl group,
• R3 is a linear or branched C1-C6-alkyl group, C7-20-arylalkyl, C7-20-alkylaryl group or C6-C20-aryl group,
• R4 is a C(R9)3 group, with R9 being a linear or branched C1-C6-alkyl group,
• R5 is hydrogen or an aliphatic C1-C20-hydrocarbyl group optionally containing one or more heteroatoms from groups 14-16 of the periodic table;
• R6 is hydrogen or an aliphatic C1-C20-hydrocarbyl group optionally containing one or more heteroatoms from groups 14-16 of the periodic table;
• or
• R5 and R6 can be taken together to form a 5 membered saturated carbon ring which is optionally substituted by n groups R10, n being from 0 to 4;
• each R10 is same or different and may be a C1-C20-hydrocarbyl group, or a C1-C20-hydrocarbyl group optionally containing one or more heteroatoms belonging to groups 14-16 of the periodic table;
• R7 is H or a linear or branched C1-C6-alkyl group or an aryl or heteroaryl group having 6 to 20 carbon atoms optionally substituted by one to three groups R11,
• each R11 are independently the same or can be different and are hydrogen, a linear or branched C1-C6-alkyl group, a C7-20-arylalkyl, C7-20-alkylaryl group or C6-20-aryl group or an OY group, wherein Y is a C1-10-hydrocarbyl group,
• (ii) a co-catalyst system comprising a boron containing co-catalyst and/or an aluminoxane co-catalyst, and
• (iii) a silica support.