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 MgCl 2 -supported Ziegler–Natta catalysts for regulating stereoselectivity. In this work, propylene polymerization was conducted using four TiCl 4 /Di/MgCl 2 -type Z–N catalysts containing different Di ions and a TiCl 4 /MgCl 2 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-MgCl 2 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.