Process for preparing propylene polymers

1. A process for polymerizing propylene monomers together with ethylene comonomers, the process comprising:

polymerizing propylene monomers together with ethylene comonomers in the presence of a single-site catalyst comprising:

(i) a complex of formula (I):

wherein,

M is zirconium or hafnium;

each X independently is a sigma-donor ligand,

L is a divalent bridge selected from —R′ 2 C—, —R′ 2 C—CR′ 2 —, —R′ 2 Si—, —R′ 2 Si—SiR′ 2 —, —R′ 2 Ge—, wherein each R′ is independently a hydrogen atom or a C 1 -C 20 -hydrocarbyl group optionally containing fluorine atoms, or optionally two R′ groups taken together can form a ring;

R 1 and R 1′ are each independently hydrogen, C 5 -C 10 -aryl or a group —CH(R x ) 2 wherein each R x is independently H or a C 1 -C 10 hydrocarbyl group, and optionally the two R x taken together can form a ring,

R 2 and R 2′ are each independently hydrogen, C 5 -C 10 -aryl or a group —C(R y ) 3 wherein each R y is independently H or a C 1-10 hydrocarbyl group, or optionally two or three R y groups taken together can form a ring,

whereby at least one of R 1 or R 2 and one of R 1′ or R 2′ is different from hydrogen, and

whereby R 2 together with one of R 1 , as well as R 2′ together with one of R 1′ can be part of a further mono- or multicyclic ring condensed to the phenyl ring,

R 3 and R 3′ are each independently a linear C 1 to C 6 hydrocarbyl group or a branched or cyclic C 4 to C 10 hydrocarbyl group, whereby the groups are not branched in α-position,

R 4 and R 4′ are each independently a tertiary C 4 to C 10 hydrocarbyl group,

R 5 and R 5′ are each independently a linear or branched C 1 to C 10 alkyl group or an C 5 -C 10 -aryl group, and

(ii) a cocatalyst system comprising a boron containing cocatalyst and an aluminoxane cocatalyst,

wherein the process is a multistage process comprising at least two reactors connected in series including at least one gas phase polymerization step.

2. The process according to claim 1 , wherein the aluminoxane cocatalyst is one of formula (X):

wherein n is from 6 to 20 and R is selected from a C 1 -C 10 alkyl, C 3 -C 10 cycloalkyl, C 7 -C 12 arylalkyl or alkylaryl and/or phenyl or naphthyl.

3. The process according to claim 1 , wherein the boron based cocatalyst is one of formula (Z):

BY 3   (Z)

wherein Y independently is the same or can be different and is a hydrogen atom, an alkyl group of from 1 to about 20 carbon atoms, an aryl group of from 6 to about 15 carbon atoms, alkylaryl, arylalkyl, haloalkyl or haloaryl each having from 1 to 10 carbon atoms in the alkyl radical and from 6-20 carbon atoms in the aryl radical or fluorine, chlorine, bromine or iodine.

4. The process according to claim 1 , wherein the boron based cocatalyst is one of compounds containing a borate anion.

5. The process according to claim 1 , wherein the molar ratio of boron in the boron containing cocatalyst to the metal ion M in the complex of formula (I) is in the range of 0.5:1 to 10:1 mol/mol.

6. The process according to claim 1 , wherein the molar ratio of aluminium in the aluminoxane cocatalyst to the metal ion M in the complex of formula (I) is in the range of 1:1 to 2000:1 mol/mol.

7. The process according to claim 1 , comprising the steps of:

a) introducing propylene monomer units, ethylene comonomer units and optionally hydrogen into a polymerization reactor;

b) polymerizing the propylene monomer units and ethylene units to form a polymer of propylene which is a random copolymer of propylene and ethylene.

8. The process according to claim 1 , wherein the single-site catalyst has an metallocene (MC) activity of at least 1000 kg/(g·h).

9. The process according to claim 1 , comprising the steps of:

(A′) polymerizing in a first reactor propylene monomer units and optionally ethylene comonomer units in the presence of hydrogen for obtaining a first propylene homopolymer fraction (hPP1) or a first propylene-ethylene random copolymer fraction (cPP1),

(B′) transferring said first propylene homopolymer fraction (hPP1) or first propylene-ethylene random copolymer fraction (cPP1) in a second reactor,

(C′) polymerizing in said second reactor in the presence of the first propylene homopolymer fraction (hPP1) or first propylene-ethylene random copolymer fraction (cPP1) propylene monomer units and optionally ethylene comonomer units in the presence of hydrogen for obtaining a second propylene homopolymer fraction (hPP2) or a second propylene-ethylene random copolymer fraction (cPP2), said first propylene homopolymer fraction (hPP1) or first propylene-ethylene random copolymer fraction (cPP1) and said second propylene homopolymer fraction (hPP2) a second propylene-ethylene random copolymer fraction (cPP2) forming the matrix phase (M),

(D′) transferring said matrix phase (M) into a third reactor, said third reactor being a gas phase reactor,

(E′) polymerizing in said third reactor in the presence of the matrix (M) propylene monomer units and ethylene comonomer units for obtaining an elastomeric propylene copolymer (E), wherein said matrix (M) and said elastomeric propylene copolymer (E) form the heterophasic propylene copolymer.

10. A process for the production of a polymer of propylene, which is a random copolymer of propylene and ethylene or a heterophasic propylene copolymer and comprises a homopolymer of propylene or a random copolymer of propylene and ethylene, the process comprising:

polymerizing propylene monomers together with ethylene comonomers in the presence of a single-site catalyst comprising:

(i) a complex of formula (I):

wherein

M is zirconium or hafnium;

each X independently is a sigma-donor ligand

L is a divalent bridge selected from —R′ 2 C—, —R′ 2 C—CR′ 2 —, —R′ 2 Si—, —R′ 2 Si—SiR′ 2 —, —R′ 2 Ge—, wherein each R′ is independently a hydrogen atom or a C 1 -C 20 -hydrocarbyl group optionally containing fluorine atoms, or optionally two R′ groups taken together can form a ring;

R 1 and R 1′ are each independently hydrogen, C 5 -C 10 -aryl or a group —CH(R x ) 2 wherein each R x is independently H or a C 1-10 hydrocarbyl group, and optionally the two R x taken together can form a ring,

R 2 and R 2′ are each independently hydrogen, C 5 -C 10 -aryl or a group —C(R y ) 3 wherein each R y is independently H or a C 1-10 hydrocarbyl group, or optionally two or three R y groups taken together can form a ring

whereby at least one of R 1 or R 2 and one of R 1′ or R 2′ is different from hydrogen and

whereby R 2 together with one of R 1 , as well as R 2′ together with one of R 1′ can be part of a further mono- or multicyclic ring condensed to the phenyl ring

R 3 and R 3′ are each independently a linear C 1 to C 6 hydrocarbyl group or a branched or cyclic C 4 to C 10 hydrocarbyl group, whereby the groups are not branched in α-position

R 4 and R 4′ are each independently a tertiary C 4 to C 10 hydrocarbyl group

R 5 and R 5′ are each independently a linear or branched C 1 to C 10 alkyl group or an C 5 -C 10 -aryl group and

(ii) a cocatalyst system comprising a boron containing cocatalyst and an aluminoxane cocatalyst,

wherein the process is a multistage process comprising at least two reactors connected in series including at least one gas phase polymerization step.

11. A polymer of propylene obtainable from the process according to claim 1 , which is a random copolymer of propylene and ethylene or a heterophasic propylene copolymer comprising a homopolymer of propylene or a random copolymer of propylene and ethylene.

12. The polymer of propylene according to claim 11 , wherein the homopolymer of propylene has a melting temperature Tm of at least 150.0° C.

13. The polymer of propylene according to claim 11 , wherein the polymer of propylene is a heterophasic propylene copolymer comprising a matrix (M) comprising the homopolymer of propylene and/or the random copolymer of propylene and ethylene, and an elastomeric copolymer (E) dispersed in the matrix (M).