Process for the preparation of a propylene polymer

1. A process for the preparation of a propylene copolymer in a multistage polymerisation process in the presence of a single site catalyst comprising:

(I) prepolymerising a single site catalyst in the presence of propylene as sole monomer and in the presence of hydrogen or in the absence of hydrogen;

(II) in a slurry polymerisation step, polymerising propylene only with the prepolymerised catalyst of step (I) in the presence of hydrogen; and subsequently

(III) in a first gas polymerisation step polymerising propylene only in the presence of catalyst and polymer from step (II) and in the presence of hydrogen; and subsequently

(IV) in a second gas polymerisation step, polymerising propylene and at least one C2-10 alpha olefin comonomer in the presence of the catalyst and polymer from step (III) and optionally in the presence of hydrogen;

wherein the amount of hydrogen fed to the prepolymerisation step is at most 15% of the total amount of hydrogen fed to steps (I) and (II) of the polymerisation process.

2. A process as claimed in claim 1 wherein the amount of hydrogen fed to the prepolymerisation step is at most 10% of the total amount of hydrogen fed to steps (I) and (II) of the polymerisation process.

3. A process as claimed in claim 1 wherein no hydrogen is added in prepolymerisation step (I).

4. A process as claimed in claim 1 wherein the propylene polymer formed in the process is a heterophasic propylene copolymer.

5. A process as claimed in claim 1 wherein the weight split between slurry polymer and gas phase polymer is slurry/GP1(+GP2)) 25:75 to 60:40.

6. A process as claimed in claim 1 wherein said catalyst comprises a complex of formula (II)

wherein

M is zirconium or hafnium;

each X is a sigma 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, C 1 -C 20 -hydrocarbyl, tri(C 1 -C 20 -alkyl)silyl, C 6 -C 20 -aryl, C 7 -C 20 -arylalkyl or C 7 -C 20 -alkylaryl;

R 2 and R 2 ′ are each independently a C 1 -C 20 hydrocarbyl radical optionally containing one or more heteroatoms from groups 14-16;

R 5 and R 5 ′ are each independently hydrogen, C 1-20 hydrocarbyl group optionally containing one or more heteroatoms from groups 14-16 and optionally substituted by one or more halo atoms;

R 6 and R 6 ′ are each independently hydrogen or a C 1-20 hydrocarbyl group optionally containing one or more heteroatoms from groups 14-16; or

R 5 and R 6 taken together can form a 5 or 6 membered saturated or unsaturated carbon ring fused to the 6-membered ring of the indenyl group; or

R 5 ′ and R 6 ′ taken together can form a 5 or 6 membered saturated or unsaturated carbon ring fused to the 6-membered ring of the indenyl group;

R 7 and R 7 ′ are each independently hydrogen or C 1-20 hydrocarbyl group optionally containing one or more heteroatoms from groups 14-16;

Ar is an aryl or heteroaryl group having up to 20 carbon atoms optionally substituted by one or more groups R 1 ;

Ar′ is an aryl or heteroaryl group having up to 20 carbon atoms optionally substituted by one or more groups R 1 ;

each R 1 is a C 1-20 hydrocarbyl group or two R 1 groups on adjacent carbon atoms taken together can form a fused 5 or 6 membered non aromatic ring with the Ar group, said ring being itself optionally substituted with one or more groups R 4 ; and

each R 4 is a C 1-20 hydrocarbyl group.

7. A process for the preparation of a propylene copolymer in a multistage polymerisation process in the presence of a single site catalyst as claimed in claim 1 , said process comprising:

(I) prepolymerising a single site catalyst in the presence of propylene as sole monomer and in the presence of hydrogen or in the absence of hydrogen;

(II) in a slurry polymerisation stage, polymerising propylene only with the prepolymerised catalyst of step (I) in the presence of hydrogen; and subsequently

(III) in a first gas polymerisation stage polymerising propylene only in the presence of the catalyst and polymer from step (II) and in the presence of hydrogen; and subsequently

(IV) in a second gas polymerisation stage, polymerising propylene and at least one C2-10 alpha olefin comonomer in the presence of the catalyst and polymer from step (III);

wherein the amount of hydrogen fed to the prepolymerisation step is at most 15% of the total amount of hydrogen fed to the steps I+II.

8. A process as claimed in claim 5 wherein the weight split between slurry polymer and gas phase polymer is slurry/GP1(+GP2)) 50:50 to 35:65.

9. A process as claimed in claim 1 wherein the catalyst is in solid particulate form free from an external carrier.