Effects of Internal Electron Donors on Propylene–Ethylene Copolymerization with MgCl ₂ -Supported Ziegler–Natta Catalysts

In this work, copolymerization of propylene with a small amount of ethylene was conducted using four TiCl4/Di/MgCl2 type Z–N catalysts containing different Di (internal electron donors) and a TiCl4/MgCl2 blank catalyst (Cat-B) under the same conditions. For each catalyst, the distribution of three groups of active centers, producing copolymer chains with high, medium, and low ethylene content and their intrinsic reactivities in propylene and ethylene insertions were determined based on quench-labeling the propagation chains and extraction fractionation of the copolymer. Using bidentate internal donors (phthalate, 1,3-diether, and diol ester) as Di led to a sharp increase in the total number of active centers ([C*]/[Ti]) and enhancement in stereoselectivity of the three groups of active centers, while adding ethyl acetate caused minor effects on [C*]/[Ti] and stereoselectivity as compared with Cat-B. In contrast to Cat-B, the catalysts containing phthalate (Cat-P), diether (Cat-E), and diol ester (Cat-O) had similar active center distribution characterized by a higher fraction of the active centers with medium stereoselectivity and medium ethylene incorporation rate and an evidently lower fraction of the active centers with high stereoselectivity and low ethylene incorporation rate. As a result of stereoselectivity enhancement by the bidentate Di, intrinsic reactivity of the propylene insertions was remarkably increased, meanwhile that of the ethylene insertions was reduced. As compared with Cat-B, Cat-P, Cat-E, and Cat-O showed a narrower distribution of the kpE/kpP ratio (kpE and kpP are rate constants of ethylene and propylene insertions, respectively) among the three copolymer fractions. Among Cat-P, Cat-E, and Cat-O, Cat-O with diol ester-type Di showed the narrowest distribution of the kpE/kpP ratio, meaning that it can produce propylene–ethylene copolymers with the narrowest composition distribution.