Mixed donor system for high melt flow and high activity

A catalyst system for polymerizing an olefin to form a polyolefin, comprising:

• a Ziegler-Natta catalyst, with the proviso that the Ziegler-Natta catalyst does not comprise an internal electron donor;
• and
• at least two organosilicon compounds, wherein one of the at least two organosilicon compounds has a structure according to Formula VI and another of the at least two organosilicon compounds has a structure according to Formula VII:
• where R13, R14, and R15 are independently one substituent selected from the group consisting of alkyl substituents having from about 1 to about 10 carbon atoms, alkoxy substituents having from about 1 to about 10 carbon atoms, and aryl substituents having from about 1 to about 10 carbon atoms, R16 and R17 are independently one substituent selected from the group consisting of alkyl substituents having from about 1 to about 10 carbon atoms, aryl substituents having from about 1 to about 10 carbon atoms, and hydrogen, and where R20, R21, R22, and R23 are independently one substituent selected from the group consisting of alkyl substituents having from about 1 to about 10 carbon atoms, and alkoxy substituents having from about 1 to about 10 carbon atoms, and
• wherein the catalyst system has a property that when the catalyst system is contacted with an olefin monomer, and at pressure of about 3.0 Mpa or less, the ratio of MFR for the polyolefin expressed in units of g (10 min)−1 to the mole percentage of hydrogen expressed in percent units is greater than about 14:1, and
• the mole ratio of the organosilicon compound of Formula VI to the organosilicon compound of Formula VII is from about 2.3:1 to about 19:1.

A method of making a polyolefin, comprising:

• contacting an olefin with a catalyst system comprising a solid titanium catalyst component, the solid titanium catalyst component consisting essentially of a titanium compound and a support;
• and at least two organosilicon compounds, wherein one of the at least two organosilicon compounds has a structure according to Formula VI:
• where R13, R14, and R15 are independently one substituent selected from the group consisting of alkyl substituents having from about 1 to about 10 carbon atoms, alkoxy substituents having from about 1 to about 10 carbon atoms, and aryl substituents having from about 1 to about 10 carbon atoms, R16 and R17 are independently one substituent selected from the group consisting of alkyl substituents having from about 1 to about 10 carbon atoms, aryl substituents having from about 1 to about 10 carbon atoms, and hydrogen, and
• wherein the ratio of MFR for the polyolefin expressed in units of g (10 min)−1 to the mole percentage of hydrogen in percent units is greater than about 14:1 when the mole percent of hydrogen is from about 0.2 to about 2%, the ratio of MFR for the polyolefin expressed in units of g (10 min)−1 to the mole percentage of hydrogen expressed in percent units is greater than about 25:1 when the mole percent of hydrogen is from about 2 to about 3%, and the ratio of MFR for the polyolefin expressed in units of g (10 min)−1 to the mole percentage of hydrogen expressed in percent units is greater than about 35:1 when the mole percent of hydrogen is from about 3 to about 6%,
• wherein the mole ratio of the organosilicon compound of Formula VI to the other of the at least two organosilicon compounds is from about 2.3:1 to about 19:1.

A multidonor catalyst system for polymerizing an olefin, comprising:

• a solid titanium catalyst component consisting essentially of a titanium compound and a support;
• an organoaluminum compound having at least one aluminum-carbon bond;
• a first external electron donor and a second external electron donor, wherein the first external electron donor combined with a reference system for polymerizing an olefin produces a first polyolefin having a melt flow rate of MFR(1), and the second electron donor combined with the reference system for polymerizing an olefin produces a second polyolefin having a melt flow rate of MFR(2), where the reference system comprises the solid titanium catalyst and the organoaluminum compound,
• wherein the molar amount of the first external electron donor present in the multidonor catalyst system is greater than the molar amount of the second external electron donor present in the multidonor catalyst system, and
• the value of log [MFR(1)/MFR(2)] is from about 0.5 to about 0.8, and
• the mole ratio of the first external electron donor to the second external electron donor is from about 2.3:1 to about 19:1.