Catalyst composition and polymerization process using mixture of silane electron donors

1. A catalyst composition for the polymerization of one or more olefins comprising the combination of one or more Ziegler-Natta procatalysts comprising one or more transition metal compounds; one or more aluminum containing cocatalysts; and a mixture comprising two or more selectivity control agents (SCA's), each SCA corresponding to the formula: (CH 3 O) n Si(OR) 4-n ,

wherein R, independently each occurrence, is C 2-12 alkyl, C 3-12 cycloalkyl, C 6-18 aryl or poly(alkyl)-substituted aryl, or C 7-18 poly(aryl)-substituted alkyl, and

n is an integer from 1 to 4.

2. A catalyst composition according to claim 1 wherein n is an integer from 1 to 3.

3. A catalyst composition according to claim 1 wherein each SCA corresponds to the formula: (CH 3 O) n Si(OR) 4-n ,

wherein R is C 2-6 alkyl or C 5-6 cycloalkyl, and

n is an integer from 1 to 4.

4. A catalyst composition according to claim 1 wherein the mixture of SCA's comprises a mixture of homologs resulting from exchange of tetramethylorthosilicate with one or more C 2-6 alkanols or C 3-6 cycloalkanols.

5. A catalyst composition according to claim 3 wherein the mixture of SCA's comprises at least three tetraalkoxysilane homologs and no single homolog is present in the SCA mixture in an amount greater than 75 weight percent.

6. A catalyst composition according to claim 5 wherein no single homolog is present in the SCA mixture in an amount greater than 70 weight percent.

7. A catalyst composition according to claim 6 wherein no single homolog is present in the SCA mixture in an amount greater than 65 weight percent.

8. A catalyst composition according to claim 7 wherein the SCA mixture comprises:

a) tetramethoxysilane in an amount from 0-15 percent;

b) trimethoxy(C 2-6 alkoxy)silane in an amount from 10-40 percent;

c) dimethoxydi(C 2-6 alkoxy)silane in an amount from 15-60 percent;

d) methoxytri(C 2-6 alkoxy)silane in an amount from 10-40 percent; and

e) tetra(C 2-6 alkoxy)silane in an amount from 0-15 percent, the total of components a), b), c), d) and e) equaling 100 weight percent.

9. A catalyst composition according to claim 8 wherein the SCA mixture comprises:

a) tetramethoxysilane in an amount from 0-12 percent;

b) trimethoxy(C 2-6 alkoxy)silane in an amount from 15-35 percent;

c) dimethoxydi(C 2-6 alkoxy)silane in an amount from 20-50 percent;

d) methoxytri(C 2-6 alkoxy)silane in an amount from 15-35 percent; and

e) tetra(C 2-6 alkoxy)silane in an amount from 0-12 percent, the total of components a), b), c), d) and e) equaling 100 weight percent.

10. A catalyst composition according to claim 9 wherein the SCA mixture comprises:

a) tetramethoxysilane in an amount from 0-10 percent;

b) trimethoxy(C 2-6 alkoxy)silane in an amount from 15-30 percent;

c) dimethoxydi(C 2-6 alkoxy)silane in an amount from 25-45 percent;

d) methoxytri(C 2-6 alkoxy)silane in an amount from 15-30 percent; and

e) tetra(C 2-6 alkoxy)silane in an amount from 0-10 percent, the total of components a), b), c), d) and e) equaling 100 weight percent.

11. A polymerization process for the polymerization of one or more olefin monomers comprising contacting said monomer or monomers under polymerization conditions with a polymerization catalyst composition according to any one of claims 1 and 2 - 10 .

12. A process according to claim 11 which is a gas phase polymerization process.

13. A process according to claim 11 which is conducted in more than one reactor operating in series.

14. A process of claim 11 wherein propylene is polymerized to form polypropylene.