Propene/Ethene-[1-¹³C] Copolymerization as a Tool for Investigating Catalyst Regioselectivity. 2. The MgCl₂/TiCl₄−AlR₃System

Abstract

The propene/ethene-[1-13 C] copolymerization method has been applied to measure precisely the regioselectivity in propene polymerization of the "simple" MgCl2/TiCl4 catalyst (activated with Al( i -Bu)3), which can be viewed as the ancestor of all present-day MgCl2-supported industrial catalysts containing Lewis bases as selective modifiers. It has been found that the average content of 2,1regiomistakes in the polypropylene produced is fairly high (ca. 0.7 mol %), althoughsnot unexpectedly, due to the multisite catalyst naturesthese are distributed rather nonuniformly throughout the polymer. The difficulty in detecting the 2,1-units, despite their relatively high concentration, in the 13 C NMR spectra of homopolymer samples at natural 13 C abundance has been traced to their occurrence in a variety of stereochemical environments, which leads to splitting and broadening of the corresponding resonances. The fraction of "dormant" polypropylene chains with a 2,1-last-inserted unit was estimated to be in the 20-50% range. QM calculations of regioselectivity on classical Corradini-type models of epitactic octahedral catalytic Ti species turned out to be in basic agreement with the experiment.

Isoselectivity Distribution of Isospecific Centers in Supported Titanium‐Based Ziegler‐Natta Catalysts

“…The possibilities vary from simple occasional mistakes in the stereo-linking of adjacent monomer units to fluxional stereo-control mechanisms pioneered by Busico [25][26][27][28] and Randall. [29] The latter mechanisms assume that several changes can occur in the stereo-control mechanism at some active centers during the lifetime of a single macromolecule resulting in the Figure 5.…”

Coadsorption model for first-principle description of roles of donors in heterogeneous Ziegler–Natta propylene polymerization

“…The proposal implicitly assumes that isospecific active sites are the same dinuclear species irrespective of donor structures and the degree of the stereospecific improvement depends on the selectivity of the donor adsorption on the (1 1 0) surface. Afterward, it became clear that polymer microstructures reflecting active site structures are sensitive to donor structures [40][41][42][43][44][45], suggesting that donor molecules are directly involved in the active sites. Since the coordination of donor molecules to TiCl 4 is spectroscopically excluded [46], researchers now agree that donor molecules adsorb on MgCl 2 surfaces and interact with nearby Ti species in a non-bonded manner to improve the stereospecificity of the Ti species.…”

External Donor Induced Direct Contact Effects on Mg(OC2H5)2‐Based Ziegler‐Natta Catalysts for Propylene Polymerization

“…[14][15][16] Even though many scientists explained isospecificity increase by the existence of donors according to these results, the effects of donors are considered to be more complicated. [17][18][19] For the further improvement of the Ziegler-Natta catalyst system, it is essential to understand how donors interact with other components of the catalyst. The internal donor coordinated to the MgCl 2 surface is desorbed from the solid catalyst by the reaction with alkylaluminum.…”