Effects of Internal Electron Donor on the Distribution of Active Centers and Their Intrinsic Reactivities in Propylene Polymerization with MgCl2-Supported Ziegler–Natta Catalysts

Abstract

Internal electron donors (Di) play crucial roles in MgCl₂-supported Ziegler–Natta catalysts for regulating stereoselectivity. In this work, propylene polymerization was conducted using four TiCl₄/Di/MgCl₂-type Z–N catalysts containing different Di ions and a TiCl₄/MgCl₂ blank catalyst (Cat-B) under the same conditions. For each catalyst, the distribution of three groups of active centers producing PP chains with low, medium, and high isotacticity and their k_p values were determined based on quench-labeling the propagation chains and extraction fractionation of the polymer product. 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]), while adding ethyl acetate (Cat-A) caused no change in [C*]/[Ti] as compared with Cat-B. The catalysts containing phthalate (Cat-P), 1,3-diether (Cat-E), and diol ester (Cat-O) had similar active center distribution (expressed as fractions of active centers forming atactic (C_a*), medium isotactic (C_m*), and isotactic (C_i*) polypropylene). In contrast, Cat-A and Cat-B had more C_a* and less C_m*. The influences of Di on intrinsic reactivities (k_p values without distortion by monomer diffusion limitation) were more evident than those on the active center distribution. Though Di exerted weak influences on the k_p of C_a*, it strongly influenced the k_p of C_m* and C_i*. k_p values of C_m* centers in the four catalysts containing Di (Cat-P, Cat-E, Cat-O, and Cat-A) were close to each other, but Cat-B had smaller k_p values of C_m* and large k_p values of C_i*. Judging by the k_p of C_i* and C_m* centers and the characteristics of Di-MgCl₂ interactions, the catalysts studied in this work can be divided into three categories: (1) Cat-P and Cat-A with medium k_p of C_i*; (2) Cat-E and Cat-O with large k_p of C_i*; and (3) Cat-B with large k_p of C_i* but very small k_p of C_m*. The comparative studies on different Di have provided enlightening information on the mechanism of internal donor effects in supported Z–N catalysts.