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

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.