On the Mechanism of Stereospecific Polymerization—Development of a Universal Model to Demonstrate the Relationship Between Metallocene Structure and Polymer Microstructure

Abstract

Abstract:With the discovery of stereorigid bridged metallocencs, soluble catalysts became available for the stereospecific polymerization of cc-olefins. A relatively simple mechanism was used to explain the stereospecificity, primarily in terms of the catalyst symmetry. In this paper we demonstrate that the simple rule of thumb that C,-symmetric catalysts produce isotactic and C',-symmetric catalysts syndiotactic polypropylene is too narrow. The introduction of one methyl group at the Cp ring in the [jiPr(CpFlu))ZrCI,]/ M A 0 system (Flu = fluorenyl, MA0 = methylalumoxan) rcduces the C, syninietry to C , , and the resulting catalyst products hemiisotactic polypropylene. The analogous catalyst with a bulkier tcrtbutyl group at the Cp ring gives isotactic polypropylene. When the C, symmetry of [{Me,Si(lnd),)ZrCl,] (Ind = indenyl) is reduced to C , , a metallocene can be obtained that produces atactic polypropylene. We have broken away from the symmetry-based model and developed a universal model, which accurately describes

Mechanism‐Based Design of a ROMP Catalyst for Sequence‐Selective Copolymerization

“…Both the viscosity and refractive index traces show a single, smooth, monomodal peak at the same retention time, which is significant because norbornene homopolymer would not appear in the latter trace, its refractive index being coincidentally the same as that of the 1,3,5-trichlorobenzene solvent. Moreover, the 13 C NMR spectra (Figure 1) of the homopolymers of norbornene and cyclooctene, as well as the copolymers with various ratios of the two monomers, demonstrate that alternate incorporation of cyclooctene and norbornene is preferred for complex 3 but not for ROMP catalyst 2.…”

“…In copolymerizations with 3, the weight of the dry polymer corresponded to the weight of norbornene, plus up to one equivalent of cyclooctene. The polymer samples were characterized by 1 13 C resonances were assigned with the help of group equivalents, which gave good predictions for the homopolymers. The relaxation time was set to t 2 = 5 s so that reliable integrations could be taken from the 13 C NMR spectra.…”

The role of intermediate chain migration in propene polymerization using substituted {iPr(CpFlu)}ZrCl2/MAO catalysts

“…Fig. 1) is caused by insertion from one of the coordination sites only, whereas in a previous paper 7) we noted that the isospecificity of this catalyst can be explained without assuming such intermediate chain Comparison of pentad distributions obtained from NMR spectra and from a molecular mechanics-based modeling approach is performed for the catalysts { i Pr(3-XCpFlu)}ZrCl 2 (X = H, Me, Et, i Pr, t Bu) at a range of different temperatures. In order to model the temperature dependency of the pentad distributions the variation in steric influence along with the change of the rotational energy level for catalysts with substituents displaying relatively low barriers to rotation is treated approximately by calculating energy profiles of 360 8 rotation of the alkyl groups.…”

“…And indeed, energies of the different conformers of metalmonomer complexes correlate nicely with the microstructure of the polypropene, as demonstrated in forcefieldbased studies of metallocene-based catalysts, see e. g. ref. [3][4][5][6][7] One mechanistic subject still being debated is the question of to what extent intermediate chain migration (also termed "back-skip") is of importance for explaining stereoerrors in syndiotactic polymers 1,8,9) obtained with C s symmetric catalysts, or as a regular mechanism for polymerization with asymmetric catalysts for which insertion from one coordination site may be hindered by a large substituent 1,6,7,10,11) . It has been claimed 1,6) that the isotactic polymer produced by for example the catalyst 12) { i Pr(3-t BuCpFlu)}ZrCl 2 (5, cf.…”

Mechanismus‐basierte Entwicklung eines sequenzselektiven ROMP‐Katalysators für die Copolymerisation von Alkenen

“…Des Weiteren soll erwähnt werden, dass unser Konzept eines Katalysators mit alternierender Selektivität schon bei der Synthese von syndiotaktischem Polypropylen mit C 1 -oder C s -symmetrischen Metallocenkatalysatoren Anwendung fand. [13] In diesen Systemen polymerisiert der Katalysator die Monomere abwechselnd von der re-und von der si-Seite. Es wurde aber nie über chemoselektive Copolymerisation mit diesen Katalysatoren berichtet.…”