Multicenter nature of titanium‐based Ziegler–Natta catalysts: Comparison of ethylene and propylene polymerization reactions

Abstract

This article discusses the similarities and differences between active centers in propylene and ethylene polymerization reactions over the same Ti-based catalysts. These correlations were examined by comparing the polymerization kinetics of both monomers over two different Ti-based catalyst systems, ␦-TiCl 3 -AlEt 3 and TiCl 4 / DBP/MgCl 2 -AlEt 3 /PhSi(OEt) 3 , by comparing the molecular weight distributions of respective polymers, in consecutive ethylene/propylene and propylene/ethylene homopolymerization reactions, and by examining the IR spectra of "impact-resistant" polypropylene (a mixture of isotactic polypropylene and an ethylene/propylene copolymer). The results of these experiments indicated that Ti-based catalysts contain two families of active centers. The centers of the first family, which are relatively unstable kinetically, are capable of polymerizing and copolymerizing all olefins. This family includes from four to six populations of centers that differ in their stereospecificity, average molecular weights of polymer molecules they produce, and in the values of reactivity ratios in olefin copolymerization reactions. The centers of the second family (two populations of centers) efficiently polymerize only ethylene. They do not homopolymerize ␣-olefins and, if used in ethylene/␣-olefin copolymerization reactions, incorporate ␣-olefin molecules very poorly.

Application of Monte Carlo simulation method to polymerization kinetics over Ziegler–Natta catalysts

“…However, because of deactivation reactions, it becomes deactivated and its contribution to the final polymer decreases as the reaction proceeds. Figure 2B, as well as previous findings, confirms the fact that this center makes no significant contribution to the production of the final polymer [4,6]. Later, center II is the most substantial center-producing polymer; however, the contribution of center II to the polymerization shortly disappears and center III substitutes it after a short time.…”

“…3B. It can be seen that the polydispersity index is about 2 for each active center except for the beginning of the reaction, as reported in the literature [1][2][3]6]. In fact, each center follows a Schultz-Flory distribution as already was shown for Ziegler-Natta heterogeneous catalysts.…”

“…4 after the polymerization proceeds for 1 h. It can be inferred that centers I and II produce the majority of low molecular weight chains, whereas centers IV and V synthesize most of high molecular weight chains as the surface area under the molecular weight distribution curves of these centers shows. In general, centers I and II play an insignificant role in synthesizing polymer, which is also reported elsewhere [4][5][6]. Table II shows polymer number average molecular weight of centers I, II, III, IV, and V and total polymer number average molecular weight.…”

“…in the reaction medium. Finally, they can influence catalyst system, polymer molecular weight, and polymer molecular weight distribution, as well as playing the role of transfer agents [1][2]6,13]. Table VII shows the contribution of each active center to the polymer synthesized.…”

Propylene Polymerization with Titanium‐Based Ziegler‐Natta Catalysts: Effects of Temperature and Modifiers on Molecular Weight, Molecular Weight Distribution and Stereospecificity

“…In a situation when different components of crystalline fractions have different molecular weights (Table 2) and different isotacticities ( Figure 4B and 4C, Table 3), it is unwarranted to assume that different types of steric mistakes observed by NMR (although their contents are always low) belong to the same polymer chains. [22] Positions of 13 C NMR signals in various steric sequences in polypropylene are well known from the literature, starting with the pioneering works of A. Zambelli and his coworkers [15] and continuing up to the present time. [12c,13] When 13 C NMR spectra are recorded at 125.6 MHz, the measurement of the contents of steric pentads is straightforward.…”