Chain “stationary” insertion mechanism and production of isotactic polypropylene with C 1 symmetric catalyst systems

Razavi, Abbas; Bellia, Vincenzo; Baekelmans, Didier; Slawinsky, M.; Sirol, Sabine; Peters, Liliane; Thewalt, Ulf

doi:10.1134/s0023158406020169

Cited by 15 publications

(14 citation statements)

References 61 publications

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“…The s PP samples obtained with 1a and 2a featured 63.5–78.7% rrrr contents, assigned to enantiomorphic site control mechanism with chain migratory insertion; this is consistent with the data previously reported in the literature . [11c] As expected, the experimental stereosequences distribution for the syndioselective systems fits well with the two‐parameter statistic model implying two concomitant mechanisms of stereoerrors formation (entries 1–4): enantiofacial misinsertion (probability σ > 0) and site epimerization (probability k > 0).…”

Section: Resultssupporting

confidence: 77%

iPP–sPP Stereoblocks or Blends? Studies on the Synthesis of Isotactic–Syndiotactic Polypropylene Using Single C₁‐Symmetric {Ph₂C‐(Flu)(3‐Me₃Si‐Cp)}ZrR₂ Metallocene Precatalysts

Theurkauff

Roisnel

Waassenaar

et al. 2014

Macro Chemistry & Physics

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Single isoselective C 1 -symmetric {Ph 2 C-(Flu)(3-Me 3 Si-Cp)}ZrR 2 (R = Cl ( 1b ), Me ( 2b )) metallocene precatalysts, upon activation with MAO or [PhNMe 2 H] + [B(C 6 F 5 ) 4 ] − / i Bu 3 Al, produce isotactic polypropylene (PP) (up to 80% mmmm ), which is more enriched with syndiotactic pentads (0.9-2.2% rrrr ) than the statistical model assumes. Similar or even more syndiotactically enriched PPs (3.0-25.4% rrrr ) are obtained using combinations of 1b or 2b with C s -symmetric syndioselective {Ph 2 C-(Flu)(Cp)}ZrR 2 (R = Cl ( 1a ), Me ( 2a )) metallocenes. While NMR spectroscopy, differential scanning calorimetry (DSC), and gel permeation chromatography (GPC) analyses do not provide explicit evidence on the nature of these polymers, temperature rising elution fractionation (TREF) unambiguously identifi es them as blends of i PP and s PP polymers and not discrete i PP− b − s PP stereoblock copolymers. The formation of s PP fractions from the a priori isoselective 1b and 2b systems is attributed to a desilylation side reaction, which takes place in the precatalyst synthesis step and possibly also during the polymerization, resulting in the generation of the Me 3 Si-free syndioselective catalytically active species. Activation of 2a with B(C 6 F 5 ) 3 and [PhNMe 2 H] + [B(C 6 F 5 ) 4 ] − selectively leads to the corresponding cationic complexes 3a and 4a , as confi rmed by NMR spectroscopic methods and the solid-state structure of {Ph 2 C-(Flu)(Cp)} ZrMe(μ-Me)B(C 6 F 5 ) 3 ( 3a ). On the other hand, the activation of 2b with the same molecular activators is less selective, and the formation of the corresponding cationic products 3b and 4b along with unidentifi ed impurities is monitored by NMR spectroscopy. A possible chain-shuttling process between isoselective and syndioselective active species, mediated by Me 3 Al or i Bu 3 Al, appears to be inoperative under given conditions.

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Section: Resultssupporting

confidence: 77%

iPP–sPP Stereoblocks or Blends? Studies on the Synthesis of Isotactic–Syndiotactic Polypropylene Using Single C₁‐Symmetric {Ph₂C‐(Flu)(3‐Me₃Si‐Cp)}ZrR₂ Metallocene Precatalysts

Theurkauff

Roisnel

Waassenaar

et al. 2014

Macro Chemistry & Physics

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“…On the other hand, the molecular weight of the polypropylene obtained from 3a is significantly lower, about 20% of those obtained from III-Me 2 and 3b (entry 2). This trend is not unexpected and can be accounted for by the absence of the 5-methyl substituent on the Cp ring of 3a , which is known to be responsible for destabilizing a transition state leading to chain transfer to monomer or β-H elimination. ,, …”

Section: Resultsmentioning

confidence: 91%

“…For instance, in the series of single-carbon-bridged systems, C s -symmetric precatalysts (Scheme ; I ), highly syndiotactic polypropylene is commonly produced ([ r ] 4 75−91%; liquid propylene, MAO, T polym = 40−80 °C) . Simple modification of the ligand skeleton in the precatalyst, namely, installation of a bulky substituent ( t Bu group) at the 3-Cp position that imposes an overall C 1 symmetry of the molecule (Scheme ; II and III ), results in highly isospecific systems for polymerization of propylene ([ m ] 4 79−98%; liquid propylene, MAO, T polym = 40−80 °C). ,, All these stereoselective polymerization catalyst systems operate via an enantiomorphic-site control mechanism.…”

Section: Introductionmentioning

confidence: 99%

New C₁-Symmetric Ph₂C-Bridged Multisubstituted ansa-Zirconocenes for Highly Isospecific Propylene Polymerization: Synthetic Approach via Activated Fulvenes

et al. 2010

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The synthesis of multisubstituted diphenylmethylene-bridged fluorenyl-cyclopentadienyl proligands Ph 2 C(3,6-tBu 2 FluH)(3-R 1 -5-R 2 C 5 H 3 ) (Flu = fluorenyl; R 1 = tert-butyl, R 2 = H (2a); R 1 = tert-butyl, R 2 = Me (2b); R 1 = cumyl, R 2 = Me (2c)) was developed using a nucleophilic addition protocol based on regular and activated fulvenes. Two highly congested proligands (2b,c) were prepared in a two-step procedure, starting first by addition of [3,6-tBu 2 Flu] þ Lionto 6,6 0 -bis(pchlorophenyl)fulvenes, followed by a Pd-catalyzed reductive dechlorination. The X-ray crystal structures of 2a and 2b 3 CH 2 Cl 2 were determined. These revealed a preorganized sandwichlike positioning of the fluorenyl and cyclopentadienyl plane fragments, similar to that observed in the metallocenes. The corresponding dichlorozirconium complexes {Ph)) were prepared by salt metathesis and characterized by elemental analysis, NMR spectroscopy, and X-ray crystallography (for 3a,c). These C 1 -symmetric zirconocenes exist as racemic mixtures of two enantiomers, arising from planar chirality at the Cp ring. When activated with MAO, 3a,b showed high activity in the polymerization of propylene (5020 and 3580 kg of iPP mol -1 h -1 , respectively; toluene solution, 60 °C), affording highly isotactic ([m] 4 94.0%) polymers with molecular weights in the range M w = 30 000-175 000, which are similar to those obtained with the corresponding CMe 2 -bridged catalysts. No activity was observed with the 3c/MAO system, which was proposed to be due to a deactivation involving a cumyl phenyl ring. This was supported by DFT computations on the cationic species [{Ph 2 C(3,6-tBu 2 Flu)(3-cumyl-5-Me-C 5 H 2 )}ZrMe] þ , which revealed facile coordination of the phenyl ring to the metal center, followed by C-H(phenyl) activation at the ortho position with concomitant elimination of methane; propylene insertion into the resulting ortho-metalated species was found to be thermodynamically unfavorable.

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“…Thus, even though recent DFT computational studies conclude that it is difficult to assess whether a site epimerization-controlled mechanism takes place in these polymerizations [ 25 ], our proposed hypothesis regarding the polymerization mechanism with catalyst 3 is illustrated in Figure 7 : the enantiomorphic site control with chain migratory mechanism followed by back-skip of the polymer chain to its original position (site epimerization) after every insertion leads to isotactic polypropene. A similar explanation termed “chain stationary” was proposed by Razavi [ 26 , 27 ] to explain the isotacticity of PP with C 1 symmetric catalysts based on the assumption that the subsequent incoming monomers would “see“ the polymer chain always residing at the same enantio-topic site.…”

Section: Resultsmentioning

confidence: 77%

Propene Polymerization with C1-Symmetric Fluorenyl-Metallocene Catalysts

et al. 2017

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Propene homopolymers have been produced by employing three C1-symmetric metallocene molecules (1, 2 and 3), each having t-butyl substituent(s) on the Cp, on the fluorenyl or on both aromatic moieties activated with methylaluminoxane at different polymerization temperatures and monomer concentrations. Polymers’ microstructures determined by 13C NMR spectroscopy suggest that the otherwise dominant alternating mechanism governed by the chain migratory insertion is largely replaced by the competing site epimerization mechanism, as a direct result of the imposing steric bulk of the t-butyl substituent on one of the distal positions of the Cp moiety. This phenomenon is more pronounced with 3 when a second t-butyl is present in the same half-space of the molecule making the site epimerization mandatory. The lower activity of catalyst 3 with respect to catalyst 2 is also in line with the necessity for the polymer chain to back-skip (or the site to epimerize) to its original position before the subsequent monomer insertion. Chain end group analyses by 1H NMR spectroscopy have revealed that the formation of vinylidene end groups, either via β-H elimination or as a result of direct chain transfer to the monomer after an ordinary 1,2-insertion is the prevailing chain termination route. A correlation between the relative concentrations of vinylidene end groups of polypropene (PP) polymers produced with each catalyst and the corresponding polypropenes’ molar masses was found, indicating the lower the relative concentrations of vinylidene end groups, the higher the molar masses.

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Chain “stationary” insertion mechanism and production of isotactic polypropylene with C 1 symmetric catalyst systems

Cited by 15 publications

References 61 publications

iPP–sPP Stereoblocks or Blends? Studies on the Synthesis of Isotactic–Syndiotactic Polypropylene Using Single C₁‐Symmetric {Ph₂C‐(Flu)(3‐Me₃Si‐Cp)}ZrR₂ Metallocene Precatalysts

iPP–sPP Stereoblocks or Blends? Studies on the Synthesis of Isotactic–Syndiotactic Polypropylene Using Single C₁‐Symmetric {Ph₂C‐(Flu)(3‐Me₃Si‐Cp)}ZrR₂ Metallocene Precatalysts

New C₁-Symmetric Ph₂C-Bridged Multisubstituted ansa-Zirconocenes for Highly Isospecific Propylene Polymerization: Synthetic Approach via Activated Fulvenes

Propene Polymerization with C1-Symmetric Fluorenyl-Metallocene Catalysts

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Chain “stationary” insertion mechanism and production of isotactic polypropylene with C 1 symmetric catalyst systems

Cited by 15 publications

References 61 publications

iPP–sPP Stereoblocks or Blends? Studies on the Synthesis of Isotactic–Syndiotactic Polypropylene Using Single C1‐Symmetric {Ph2C‐(Flu)(3‐Me3Si‐Cp)}ZrR2 Metallocene Precatalysts

iPP–sPP Stereoblocks or Blends? Studies on the Synthesis of Isotactic–Syndiotactic Polypropylene Using Single C1‐Symmetric {Ph2C‐(Flu)(3‐Me3Si‐Cp)}ZrR2 Metallocene Precatalysts

New C1-Symmetric Ph2C-Bridged Multisubstituted ansa-Zirconocenes for Highly Isospecific Propylene Polymerization: Synthetic Approach via Activated Fulvenes

Propene Polymerization with C1-Symmetric Fluorenyl-Metallocene Catalysts

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iPP–sPP Stereoblocks or Blends? Studies on the Synthesis of Isotactic–Syndiotactic Polypropylene Using Single C₁‐Symmetric {Ph₂C‐(Flu)(3‐Me₃Si‐Cp)}ZrR₂ Metallocene Precatalysts

iPP–sPP Stereoblocks or Blends? Studies on the Synthesis of Isotactic–Syndiotactic Polypropylene Using Single C₁‐Symmetric {Ph₂C‐(Flu)(3‐Me₃Si‐Cp)}ZrR₂ Metallocene Precatalysts

New C₁-Symmetric Ph₂C-Bridged Multisubstituted ansa-Zirconocenes for Highly Isospecific Propylene Polymerization: Synthetic Approach via Activated Fulvenes