Stereospecific Nature of Active Sites on TiCl₄/MgCl₂ Ziegler–Natta Catalyst in the Presence of an Internal Electron Donor

Abstract

Based on our previous study on the donor‐free TiCl4/MgCl2 catalyst, the stereospecific nature of active sites on this catalyst after the addition of an internal donor, ethylbenzoate (EB), was investigated by stopped‐flow propylene polymerization combined with temperature rising elution fractionation (TREF) and gel permeation chromatography (GPC) methods. The addition of donor EB was observed to significantly restrain the formation of aspecific sites (AS sites), to drastically promote formation of sites with the second highest isospecificity (IS2 sites) and to construct some new active sites with the highest isospecificity (IS3 sites). It was first observed that the formation of IS3 sites still strongly depends on the interaction between the catalyst and cocatalyst (up to 10 s of pretreatment) even in the presence of internal donor. Based on Busico and co‐workers' three sites model and our previous report, a plausible mechanism for the transformation of stereospecific active sites through some secondary bimetallic complexation reactions was proposed. The extraction of EB from the catalyst by the cocatalyst was found to initiate after 10 s of pretreatment resulting in partial transformation of IS3 sites into AS sites and IS1 sites.

Effect of diether as internal donor on MgCl2‐supported Ziegler–Natta catalyst for propylene polymerization

“…Liu et al 15 proposed four types of active sites based on TREF analysis of produce. Kissin et al 16 considered that polymers produced with any heterogeneous Tibased catalyst consist of at least four or five Folry components.…”

Reductive Formation of Isospecific Ti Dinuclear Species on a MgCl2 (110) Surface in Heterogeneous Ziegler‐Natta Catalysts

“…[1] In the three-sites and its modified models, [2,3] a series of active Ti species whose bulkiness is increased by Cl, donors, or alkylaluminum compounds coordinating at one or two neighboring metal ions to a Ti 3þ site were proposed as isospecific centers.…”

Investigation of Alkoxysilanes in the Presence of Hydrogen with Ziegler-Natta Catalysts in Ethylene Polymerization

“…Internal donor is added during the catalyst synthesis, while external donor is added during polymerization reaction. The main functions of external donor are to replace the lost internal donor, which is removed from the alkylation and/or complexation reaction with cocatalyst and protect the stereoregularity of active sites in propylene polymerization [11,12]. In part of ethylene polymerization, Zhang et al observed that electron donor has a large effect not only on the isospecificity, but also on the catalyst activity, molecular weight and molecular weight distribution in ethylene homopolymerization [13].…”

“…The results showed that the addition of internal donor can improve the activity. As described previously, the various kinds of electron donors were used in olefin polymerization such as monoester, diester, alkoxysilane, diether and etc [11,14]. Recently, phthalate based type or diester and alkoxysilane are typically used as internal and external donor, respectively [15].…”