26. Homo- and Co-Polymerization of Ethylene with the Highly Active TiCl4/THF/MgCl2 Catalyst

XAS and XES Techniques Shed Light on the Dark Side of Ziegler–Natta Catalysts: Active‐Site Generation

“…This observation provides evidencet hat AlR 3 causes ad rastic structural change in the precatalyst. This finding is valid not only for the MgCl 2 phase, as previouslyd emonstrated by X-ray powderd iffraction (XRPD), [14,25] but also for all the Ti species, whose structure can not be elucidated by XRPD because of the absence of al ong-range order.T he effect is the same irrespectiveo ft he activator used. Moreover,t he spectra of the catalysts are very similar to that of violet TiCl 3 ,a lthough 1/3 less intense, suggesting that the local structure of the new Ti phase is analogous to that of TiCl 3 .Acloser inspection of the FT of the k 3 -weighted c(k) functions (Figure 1b,c) reveals that the first shell signal is split into two components,e videnced as ab eat at 1.6 i nt he Im(FT).…”

“…[14] For aM g/Tim olar ratio equal to 2, ac rystalline bimetallic adduct wasf ormed:[ TiCl 4 (thf) 2 ]r emoves aC l À ion from [MgCl 2 (thf) 1.5 ]t of orm the [TiCl 5 (thf)] À anion;t he counter cation is ad imeric Mg-chloride tetrahydrofuranate. [14,25,26] The local structure around the Ti atoms was elucidated by the extended X-Ray absorption fine structure (EXAFS) data ( Figure 1). The Fourier transform (FT) of the k 3 -weighted c(k) function (Figure 1a,b)i sd ominated by af irst shell peak centered around1 .8 ( not phase corrected), whichi st he result of the sum of the six single-scattering contributionsd eriving from the first shell ligands (i.e.…”

Spectroscopic Investigation of Heterogeneous Ziegler–Natta Catalysts: Ti and Mg Chloride Tetrahydrofuranates, Their Interaction Compound, and the Role of the Activator

“…These two bands shift at lower wavenumber and are split in several components when thf form a complex with a metal center. The splitting into multiple bands is a consequence of the change in the symmetry of thf molecule; whereas the red-shift of the C-O-C stretching bands with respect to the free thf molecule can be explained by considering that an electron transfer occurs from the oxygen of thf to the metal, as already suggested by Kim et al [32] As a matter of fact, in both IR (Figure 3 a) and Raman (Figure 3 b) spectra, the shift of both n asym A C H T U N G T R E N N U N G (C-O-C) and n sym A C H T U N G T R E N N U N G (C-O-C) absorption bands with respect to those of pure thf increases in the order Mg < Ti III < Ti IV complexes, in agreement with the increasing Lewis acidity of the metal site. Coming in more details, the maxima of the IR absorption band due to [ 34] It has been demonstrated that Raman spectroscopy permits the unambiguous identification of cis (having C 2v symmetry) and trans (having D 2h symmetry) isomers: the spectrum of the cis conformer shows two absorption bands at 384 and 306 cm…”

“…shows new diffraction peaks (the most intense ones at 11.5 and 18.18) indicative of formation of a new phase, which likely corresponds to the [21,32] although the presence of a residual small amount of the [MgCl 2 A C H T U N G T R E N N U N G (thf) 1.5 ] precursor cannot be excluded. It has been already demonstrated that reaction between Ti and Mg chlorides in thf yields several different yellow bimetallic complexes having a defined structure, depending on the Mg/Ti molar ratio and on the crystallization conditions.…”

“…It has been already demonstrated that reaction between Ti and Mg chlorides in thf yields several different yellow bimetallic complexes having a defined structure, depending on the Mg/Ti molar ratio and on the crystallization conditions. [32] In all the cases, […”