2. The process of claim 1 wherein the activity limiting agent is a carboxylic acid ester, a diether, a poly(alkene glycol), a diol ester, or a combination thereof.

3. The process of claim 1 wherein the activity limiting agent is selected from a benzoate, a C₄-C₃₀ aliphatic acid ester and combinations thereof.

4. The process of claim 1 wherein the activity limiting agent is selected from a laurate, a myristate, a palmitate, a stearate, an oleate or combinations thereof.

5. The process of claim 1 wherein the selectivity control agent is selected from the group consisting of an alkoxysilane, an amine, an ether, a carboxylate, a ketone, an amide, a carbamate, a phosphine, a phosphate, a phosphite, a sulfonate, a sulfone, a sulfoxide, and combinations thereof.

6. The process of claim 1 wherein the selectivity control agent corresponds to the formula SiR_m(OR′)_4-m, where R is C_3-12 cycloalkyl, C_3-12 branched alkyl, or C_3-12 cyclic or acyclic amino group, R′ is C_1-4 alkyl, and m is 0, 1, or 2.

7. The process of claim 6 wherein the selectivity control agent is selected from dicyclopentyldimethoxysilane, di-tert-butyldimethoxysilane, methylcyclohexyldimethoxysilane, methylcyclohexyldiethoxysilane, ethylcyclohexyldimethoxysilane, diphenyldimethoxysilane, diisopropyldimethoxysilane, di-n-propyldimethoxysilane, diisobutyldimethoxysilane, diisobutyldiethoxysilane, isobutylisopropyldimethoxysilane, di-n-butyldimethoxysilane, cyclopentyltrimethoxysilane, isopropyltrimethoxysilane, n-propyltrimethoxysilane, n-propyltriethoxysilane, ethyltriethoxysilane, tetramethoxysilane, tetraethoxysilane, diethylaminotriethoxysilane, cyclopentylpyrrolidinodimethoxysilane, bis(pyrrolidino)dimethoxysilane, bis(perhydroisoquinolino)dimethoxysilane, and dimethyldimethoxysilane.

8. The process of claim 1 wherein the mixed external electron donor system is selected from the group consisting of: dicyclopentyldimethoxysilane and isopropyl myristate; diisopropyldimethoxysilane and isopropyl myristate; dicyclopentyldimethoxysilane and poly(ethylene glycol) laurate; dicyclopentyldimethoxysilane, isopropyl myristate and poly(ethylene glycol) dioleate; methylcyclohexyldimethoxysilane and isopropyl myristate; n-propyltrimethoxysilane and isopropyl myristate; dimethyldimethoxysilane, methylcyclohexyldimethoxysilane and isopropyl myristate; dicyclopentyldimethoxysilane and n-propyltriethoxysilane and isopropyl myristate; diisopropyldimethoxysilane, n-propyltriethoxysilane and isopropyl myristate; dicyclopentyldimethoxysilane, tetraethoxysilane and isopropyl myristate; dicyclopentyldimethoxysilane, diisopropyldimethoxysilane, n-propyltriethoxysilane and isopropyl myristate; and combinations thereof.

9. The process of claim 1 wherein the mixed external electron donor comprises three or more different electron donors.

10. The process of claim 1 wherein the reactor system has two or more flow regimes selected from the group consisting of homogeneous fluidization, bubbling fluidization, turbulent fluidization, fast fluidization, spouted bed, spout-fluid bed, pneumatic convey (dense-phase and dilute phase), packed bed (fixed bed), packed (or fixed) moving bed and systems with multiple flow patterns within a single polymerization zone.

11. The process of claim 10 where the reactor system operates under two flow regimes; one is fast fluidization, and the other is packed moving bed.

12. The process of claim 1 wherein at least one of the flow regimes has the superficial gas velocity less than 23 times of the minimum fluidization velocity.

13. The process of claim 1, wherein the reactor system is making polypropylene homopolymer, ethylene-propylene copolymer, butene-propylene copolymer or an ethylene-butene-propylene terpolymer.

14. The process of claim 1, wherein at least a part of the reactor system is running under the condensing mode.

15. The process of claim 1 wherein the selectivity control agent and the activity limiting agent are added to the reactor separately.

16. The process of claim 1 wherein all external electron donors in the mixed external electron donor system are mixed together prior to adding to the reactor.

17. The process of claim 10 wherein at least one of the flow regimes has a fluidized bulk density greater than 8 lb/ft³.