Multi-stage process for producing a C2 to C8 olefin polymer composition

1. A continuous multi-stage process for producing a C 2 to C 8 olefin polymer composition in at least two reactors, the continuous multi-stage process comprising the steps of:

a) introducing a stream of a pre-polymerized solid Ziegler-Natta catalyst and a monomer of C 2 to C 8 olefin into a first polymerization reactor, thereby producing in a first polymerization a first C 2 to C 8 olefin polymer;

b) withdrawing a stream comprising the first C 2 to C 8 olefin polymer from the first polymerization reactor and passing it into a second polymerization reactor;

c) introducing a stream of a monomer of C 2 to C 8 olefin into the second polymerization reactor, thereby producing in a second polymerization a first polymer mixture comprising the first C 2 to C 8 olefin polymer and a second C 2 to C 8 olefin polymer produced in the second polymerization reactor; and

d) withdrawing a stream comprising the first polymer mixture from the second polymerization reactor, thereby obtaining the C 2 to C 8 olefin polymer composition;

wherein the pre-polymerized solid Ziegler-Natta catalyst is prepared by off-line pre-polymerization of a solid Ziegler-Natta catalyst component with a C 2 to C 4 olefin monomer before feeding to the continuous multi-stage process, and wherein the pre-polymerized solid Ziegler-Natta catalyst has a particle size distribution span ((d 90 -d 10 )/d 50 ) of below 1.5.

2. The continuous multi-stage process according to claim 1 , 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),

(a3) optionally an internal donor (ID) and

(a4) optionally a compound of a group 13 metal of the periodic table (IUPAC).

3. The continuous multi-stage process according to claim 1 , wherein the solid Ziegler-Natta catalyst component is in the form of particles having a mean particle size ranging from 3 to 200 μm.

4. The continuous multi-stage process according to claim 1 , wherein the off-line pre-polymerization is carried out in the presence of a co-catalyst (Co) and optionally in the presence of an external electron donor (ED); and/or

wherein the off-line pre-polymerization is carried out with a total residence time of 5 to 55 minutes.

5. The continuous multi-stage process according to claim 1 , wherein the off-line pre-polymerization degree is from 0.1 to 50 g polymer /g cat .

6. The continuous multi-stage process according to claim 1 , wherein the off-line pre-polymerization is carried out in a medium, wherein the medium is an oil or a hydrocarbon solvent.

7. The continuous multi-stage process according to claim 1 , wherein the off-line pre-polymerization is carried out at a temperature in a range from 0 to 50° C.

8. The continuous multi-stage process according to claim 1 , wherein pre-polymerized solid Ziegler-Natta catalyst has a particle size distribution span ((d 90 -d 10 )/d 50 ) of below 1.2.

9. The continuous multi-stage process according to claim 1 , wherein the first polymerization reactor in step a) is a loop reactor.

10. The continuous multi-stage process according to claim 1 , wherein the process comprises at least one gas phase reactor.

11. The continuous multi-stage process according to claim 1 , wherein the process comprises a further step e) of introducing the stream of step d) and a stream of a monomer of C 2 to C 8 olefin into a third polymerization reactor, thereby producing a second polymer mixture.

12. The continuous multi-stage process according to claim 11 , wherein the process further comprises a step f) of withdrawing a stream comprising the second polymer mixture from the third polymerization reactor.

13. The continuous multi-stage process according to claim 11 , wherein the process comprises two loop reactors and one gas phase reactor.

14. The continuous multi-stage process according to claim 11 , wherein the process comprises one loop reactor and two gas phase reactors.

15. The continuous multi-stage process according to claim 12 , wherein the process further comprises a step g) of introducing the second polymer mixture into a fourth polymerization reactor together with additional monomers selected from C 2 to C 8 olefins, thereby producing a third polymer mixture.