Process for the polymerization of olefins

1. A gas-phase process for the homopolymerization or copolymerization of olefins comprising the following steps:

1) contacting, in a liquid hydrocarbon and in the absence of olefins, (a) a solid catalyst component comprising Ti, Mg, and Cl, and optionally an internal electron donor compound (ID), (b) an aluminum alkyl compound and optionally (c) an external donor compound; and

2) feeding a product of the contacting step 1) to a gas-phase polymerization reactor under polymerization conditions, wherein the contacting step 1) is carried out in the presence of H 2 in an amount ranging from 0.1 to 8 g per kg of solid catalyst component (a).

2. The gas-phase process according to claim 1 , wherein the amount of H 2 ranges between 0.2 to 6 g per kg of solid catalyst component (a).

3. The gas-phase process according to claim 1 , wherein the contacting of the components (a)-(c) and hydrogen is carried out for a period of time ranging from 10 to 120 minutes.

4. The gas-phase process according to claim 1 , wherein a temperature of the contacting step 1) ranges from 20 to 65° C.

5. The gas-phase process according to claim 1 , wherein the liquid hydrocarbon is selected from aliphatic saturated hydrocarbons having from 3 to 10 carbon atoms.

6. The gas-phase process according to claim 1 wherein the liquid hydrocarbon medium contains a predetermined amount of hydrogen dissolved in the liquid hydrocarbon medium.

7. The gas-phase process according to claim 1 , wherein the solid catalyst component (a) further comprises an internal electron donor compound (ID) selected from the group consisting of ethers, esters, amines and ketones.

8. The gas-phase process according to claim 7 , wherein in which the ID is used in amount such as to yield an ID/Ti ratio from 0.1 to 8.

9. The gas-phase process according to claim 7 , wherein the ID is selected from esters of aliphatic carboxylic acids.

10. The gas-phase process according to claim 1 , wherein in step 1) the external donor compound is present as component (c) and is selected from the group consisting of esters of aliphatic or aromatic carboxylic acids, cyclic alkyl ethers and alkoxysilanes.

11. The gas-phase process according to claim 10 , wherein the external donor is tetrahydrofuran.

12. The gas-phase process according to claim 1 , wherein the aluminum alkyl compound (b) is selected from Al-triisobutyl/diethylaluminum chloride mixtures.

13. The gas-phase process according to claim 1 , wherein step 2) is carried out at a temperature ranging from 60 to 130° C. and at total pressure of the gas-phase from 10 to 40 bar.

14. The gas-phase process according to claim 1 , wherein step 2) comprises the following steps in any order:

i) polymerizing ethylene, optionally together with one or more comonomers, in a gas-phase reactor in the presence of hydrogen and a catalyst system coming from step 1);

ii) copolymerizing ethylene with one or more comonomers in another gas-phase reactor in the presence of an amount of hydrogen less than that of step i) and the catalyst system as defined in step i);

where, in at least one of the gas-phase reactors, growing polymer particles flow upward through a first polymerization zone (riser) under fast fluidization or transport conditions, leave the riser and enter a second polymerization zone (downcomer) through which the polymer particles flow downward under the action of gravity, leave the downcomer and are reintroduced into the riser, thereby establishing a circulation of polymer between the two polymerization zones.