482 research outputs found
End-capped Oligomers of Ethylene, Olefins and Dienes, by means of Coordinative Chain Transfer Polymerization using Rare Earth Catalysts
In situ generated half-lanthanidocene based catalysts for the controlled oligomerisation of styrene: Selectivity, block copolymerisation and chain transfer
International audienceThe Cp*Nd(BH4)2(THF)2/n-butylethylmagnesium combination affords a controlled and syndioselective oligomerisation of styrene. Living oligostyrenes can be used as macromonomers for block copolymerisation, leading to the unprecedented synthesis of a (polystyrene)-block- (1,4-trans polyisoprene) copolymer. Reversible transmetallation between the neodymium and the magnesium atom is further established with a transfer efficiency close to 100%
One-pot synthesis of an ionic half-sandwich complex of Neodymium. Application to stereospecific isoprene polymerization catalysis.
International audienceThe reaction of one equivalent of Nd(BH4)3(THF)3 with an half equivalent of dialkylmagnesium in the presence of a stoechiometric amount of pentamethylcyclopentadiene cleanly affords a new kind of half-sandwich of neodymium that is stable toward comproportionation. This strategy can be advantageously applied to generate in situ catalysts allowing the controlled polymerisation of isoprene
Controlled Trans-stereospecific Polymerisation of Isoprene with Neodymium (III) Borohydride-dialkylmagnesium system
International audienceNeodymium trisborohydride, associated with MgR2 alkylating agents, affords an efficient initiator for the stereospecific polymerization of isoprene. This system presents good activity (150 kg of polyisoprene/mol of Nd/h; 95% yield in 2 h) and is trans-stereospecific (up to 97.7%), and its polymerization shows a quasi-living character. The molecular weights are controlled by the [Isoprene]/[Nd] ratio, with a polydispersity index lower than 1.6. A full study is presented here, including the influence of the nature of the alkylating reagent (MgR2), the solvent, the reaction temperature, the [MgR2]/[Nd] ratio, and the replacement of neodymium by other lanthanides. This work also includes a kinetic study of the system and a detailed NMR study of the microstructure of the trans-polyisoprene obtained, providing mechanistic insights about the polymerization reaction
Borohydrido rare earth complexes as precatalysts for the polymerisation of styrene
Borohydrido based lanthanide complexes associated with n-butylethylmagnesium are efficient catalytic systems for the polymerisation of styrene. The reaction is quantitative at 50°C after several hours. Two growing chains per magnesium atom can be evidenced, and chain transfer between the lanthanide and the magnesium atoms may occur. The activity of such catalytic systems is found to be related to the nature of the rare earth
A New Family of Styrene/Diene Rubbers
International audienceThe insertion of single styrene units into polyisoprene is demonstrated using borohydrido rare earth/dialkylmagnesium systems. This yields a new family of styrene/diene copolymers (SBR rubbers). The resulting poly[(1,4-trans-isoprene)-co-styrene] exhibits quite narrow molecular weight distributions, up to 30% inserted styrene, and a 96â98% 1,4-trans-microstructure. The presence of a bulky and electron-rich ligand in the coordination sphere of the metal leads to an increase of the amount of styrene inserted and narrower chemical composition and molecular weight distributions. The presence of significant quantities of styrene in the medium does not alter the selectivity of the reaction, in contrast with cis-specific polymerizations
Highly trans-stereospecific Isoprene Polymerization by Neodymium Borohydrido Catalysts
Highly stereospecific polymerization of isoprene was achieved using borohydrido neodymium complexes. In combination with stoichiometric amounts of dialkylmagnesium, Nd(BH4)3(THF)3 (1) and Cp*'Nd(BH4)2(THF)2 (2) (Cp*' = C5Me4nPr) afford very efficient catalysts. Activity reaches 37300 g polyisoprene/mol Nd/h. Half-lanthanidocene 2 gives rise to polyisoprene 98.5 % trans-regular, the highest content yet described for a homogeneous organometallic catalyst. NMR experiments argue for the formation of bimetallic Nd(m-BH4)Mg active species
âHalf-lanthanidocenes catalysts via the "borohydride/alkyl" route: A simple approach of ligand screening for the controlled polymerization of styrene.â
International audienceThe ââborohydride/alkyl'' (B/A) route initially reported for isoprene has been applied successfully to the polymerization of styrene. This method provides via an in situ approach an interesting tool for the assessment of the influence of a ligand on the performance of half-lanthanidocene catalysts. All systems lead to well controlled oligomerization/polymerization processes. This method is thus a convenient tool for the controlled polymerization of styrene starting from a common trisborohydride precursor and commercial ligands. The influence of the nature of several ligands on the activity could be established, with trends corresponding to those obtained starting from the isolated precursors: HCp = HCpPh3 > HCp* (Cp = C5H5; CpPh3 = 1; 2; 4-Ph3C5H2; Cp* = C5Me5). These results suggest an influence of the electron donating ability of the ligand rather than steric requirements
New ionic half-metallocenes of early lanthanides
International audienceWe present in this study a new and one-step method allowing the preparation of an unprecedented family of stable half-lanthanidocenes. X-ray analysis shows that the isolated compounds all display the same ionic LnâMg bimetallic structure consisting of two anionic (CpR)Ln(BH4)3 species and one cationic Mg(THF)6 (CpR =C5Me5, Ln = Nd, 1a, Ln = La, 1b; CpR =C5H5, Ln = Nd, 2a; CpR =C5H2Ph3, Ln = Nd, 3a). Such complexes display high stability with respect to disproportionation in solution. Combined with dialkylmagnesium, neodymium complexes provide powerful catalysts for stereospecific isoprene polymerisation
Tout sur lâĂcole, Alain Bentolila
Au moment oĂč le rapport ThĂ©lot, conclusif du grand dĂ©bat sur lâĂ©cole en France Ă©tait rendu public, paraissait le livre dâAlain Bentolila au titre Ă la fois provocant et totalisant Tout sur lâĂcole. La compĂ©tence scientifique attestĂ©e et la connaissance approfondie du systĂšme Ă©ducatif français de ce brillant universitaire, professeur de linguistique Ă PARIS V, ancien instituteur, auteur de nombreux ouvrages, ne permettent pas de mettre en doute le bien fondĂ© dâun Ă©tat des lieux sans concession..
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