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.

Automated summary

This study investigated the distribution of active centers in ethylene-propylene copolymerization and identified the formation mechanism of long-chain olefin segments in the polymer.