Responses of a Supported Ziegler–Natta Catalyst to Comonomer Feed Ratios in Ethylene–Propylene Copolymerization: Differentiation of Active Centers with Different Catalytic Features

Ethylene–propylene (E/P) copolymerization with a TiCl4/Di/MgCl2–TEA/De (Di, internal donor; De, external donor; TEA, triethylaluminum)-type Ziegler–Natta catalyst was conducted at various comonomer feed ratios (E/P ratios). Distribution of active centers among three copolymer fractions (fractions soluble in room-temperature n-octane (C8-sol) and boiling n-heptane (C7-sol) and fraction insoluble in boiling n-heptane (C7-insol)) and their change with the E/P ratio were studied by quench-labeling the copolymerization, fractionating the copolymer, and measuring the labeled group in each fraction. By comparing with the active-center distribution of propylene homopolymerization, active centers in copolymerization were differentiated into three categories having low, medium, and high stereoselectivities. At E/P ≥ 40/60, the category of isospecific active centers produced the majority of the C8-sol fraction composed of the random copolymer (rEP). The majority of the active centers with medium isoselectivity also produced rEP, while a small fraction produced segmented copolymers (sEP) containing crystalline polyethylene (PE) segments. The active-center category of low stereoselectivity produced copolymers with very long PE segments in C7-insol. There is evidence that a small proportion of high isospecific centers produced sEP chains containing long crystallizable polypropylene (PP) segments, which can work as compatibilizers in high-impact polypropylene (PP/EP reactor alloy). Using a De type enabling production of PP with higher isotacticity in homopolymerization, more sEP chains having longer crystallizable PP segments were formed in copolymerization.