Method for making propylene impact copolymers with balanced impact strength and stiffness

A method of producing a propylene impact copolymer comprising an unbalanced polypropylene matrix, comprising:

• (a) contacting a catalyst composition with propylene, a first amount of a chain terminating agent and optionally ethylene or C4 to C12 α-olefins in a first reactor to form a first polypropylene having a melt flow rate, MFR1, comprising no more than 5 wt %, by weight of the first polypropylene, of ethylene or α-olefin derived units;
• (b) contacting a catalyst composition and first polypropylene with propylene, a second amount of a chain terminating agent, and optionally ethylene or C4 to C12 α-olefins in a second reactor to form a second polypropylene having a melt flow rate, MFR2, comprising no more than 5 wt %, by weight of the second polypropylene, of ethylene or α-olefin derived units, wherein the second amount of chain terminating agent is greater than the first amount of chain terminating agent and is sufficient to control the ratio MFR2/MFR1 to be greater than 1.5;
• and
• (c) contacting a catalyst composition, first polypropylene and second polypropylene in a third reactor with propylene and ethylene to form ethylene-propylene copolymer elastomer, comprising sufficient ethylene-derived units such that the overall ethylene content of the impact copolymer is within the range from 35 to 70 wt, and the MFR of the impact copolymer, MFRICP, is from 6 to 18 dg/min.