Gas phase olefin polymerizations using dual donor catalyst systems

1. A gas-phase process for polymerization of olefins in a plug flow reactor, in which at least one olefin monomer is contacted with a catalyst system comprising a magnesium halide supported titanium-containing component, an organoaluminum component, and at least one external electron donor component, wherein catalyst system components are added through injection points positioned axially along the reactor, comprising:

adding a first external donor component to the reactor at an injection point axially near an injection point for the supported transition metal containing component, and

adding at least a second external donor component to the reactor axially downstream from the injection point for the first external donor component.

2. The process of claim 1 wherein the second external electron donor is more stereoregulating than the first electron donor.

3. The process of claim 1 wherein the first and second external electron donor compounds are organic silicon compounds having a formula Si(OR) n R′ 4−n , where R and R′ are selected independently from C 1 -C 10 alkyl and cycloalkyl groups and n=1-4.

4. The process of claim 3 wherein the first and second external electron donor compounds are selected from the group consisting of tetraethoxysilane, dicyclopentyldimethoxysilane diisopropyldimethoxysilane diisobutyldimethoxysilane, isobutylisopropyldimethoxysilane di-tert-butyldimethoxysilane, n-propyltriethoxysilane, isopropyltriethoxysilane, and octyltriethoxysilane.

5. The process of claim 1 wherein the first external electron donor is tetraethoxysilane.

6. The process of claim 1 wherein the second external electron donor is dicyclopentyldimethoxysilane, diisopropyldimethoxysilane, diisobutyldimethoxysilane, or isobutylisopropyldimethoxysilane.

7. The process of claim 1 wherein the olefin monomer is propylene.

8. The process of claim 7 wherein propylene is polymerized with a minor amount of ethylene, C 4+ alpha olefins, or mixtures thereof.

9. The process of claim 1 wherein the plug flow reactor is a cylindrical horizontal, subfluidized stirred-bed reactor in which the ratio of length to diameter is greater than 2.

10. The process of claim 1 wherein the injection point of the second external donor component is at least 25% of the length of the reactor downstream from the injection point of the first external donor component.

11. A gas-phase process for polymerization of olefins in a cylindrical, horizontal, subfluidized stirred-bed plug flow reactor, in which at least one olefin monomer is contacted with a catalyst system comprising a magnesium halide supported titanium-containing component, an organoaluminum component, and at least one external silicon-containing electron donor component,wherein catalyst system components are added through injection points positioned axially along the reactor, comprising:

adding a first external donor component to the reactor at an injection point axially near an injection point for the supported transition metal containing component, and

adding at least a second external donor component, which is more stereoregulating than the first electron donor, to the reactor at least 25% of the length of the reactor axially downstream from the injection point for the first external donor component.

12. The process of claim 11 in which at least 95 mole % of the olefin monomer is propylene.

13. The process of claim 12 in which the first electron donor is tetraethoxysilane.

14. The process of claim 13 in which the second electron donor is dicyclopentyldimethoxysilane, diisopropyldimethoxysilane, diisobutyldimethoxysilane, or isobutylisopropyldimethoxysilane.

15. The process of claim 14 wherein the injection point of the second external donor component is between 25% to 75% of the length of the reactor downstream from the injection point of the first external donor component.

16. The process of claim 15 wherein the injection point of the second external donor component is between 40% to 60% of the length of the reactor downstream from the injection point of the first external donor component.

17. The process of claim 11 wherein a second cylindrical horizontal, subfluidized stirred bed plug flow reactor is operated in series with the first reactor.

18. The process of claim 11 wherein the organoaluminum component is a trialkylaluminum.

19. The process of claim 18 wherein the organoaluminum component is a triethylaluminum.

20. The process of claim 17 in which a mixture of propylene and ethylene is polymerized in the second reactor.