1,721,021 research outputs found
Influence of chain microstructure on the crystallization kinetics of metallocene-made isotactic polypropylene.
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Complexes of tris(pentafluorophenyl)boron with nitrogen-containing compounds : Synthesis, reactivity and metallocene activation
No full textThe strong Lewis acid tris(pentafluorophenyl)boron, B(C6F5)(3), reacts with several nitrogen-containing Lewis bases (nitriles, amines, imines, pyridines, etc.) and also with non-basic substrates (such as pyrroles and indoles) producing in both cases the B-N coordination adduct. With particular substrates (some tertiary amines, the imine tBu(Me)C=NBn, N-methyl-pyrrole and -indole) the 1:1 borane/N-compound reaction produces zwitterions where a new B-C bond is generated. Some of the borane-N-compound adducts present Bronsted acidity and can be reacted with dimethyl group 4 complexes with generation of weakly associated ion pairs, which are active catalysts for the polymerization of olefins
Relationship between regiospecificity and type of stereospecificity in propene polymerization with zirconocene-based catalysts
No full textThe microstructures of polypropenes produced with several zirconocene-based catalyst systems are compared, to verify the possible correlation between the type of stereospecificity and the amount of regioirregularities. It is confirmed that, while syndiospecific and aspecific zirconocenes are highly regiospecific, isospecific systems produce substantial amounts of regioirregular monomeric units. The amount of these secondary units strongly depends on the nature of the pi-ligands and on the type of the bridge connecting them. Molecular mechanics calculations are reported, indicating that the intermediates which are energetically suitable for the secondary and primary insertions, for isospecific or syndiospecific complexes, coordinate monomer enantiofaces of the opposite or the same chirality, respectively. This difference accounts for the lower regiospecificity of the isospecific catalytic complexes, assuming that the energy barrier for the rotation of the coordinated monomer around the metal-olefin bond, between the orientations suitable for the primary and secondary insertions is lower than (or comparable to) the activation energy for secondary monomer insertion
NMR evidence for B(C6F5)(3) attack on the inward position of a highly hindered meso ansa-zirconocene
No full textThe reaction of the ansa-zirconocene meso-[C2H4(4,7-Me(2)Indenyl)(2)]ZrMe2 with B(C6F5)(3) afforded, besides the expected ion pair with outward MeB(C6F5)(3)(-) anion, also a minor isomer (molar fraction 0.043) with the bulky methylborate anion in the inward site. 2D H-1 EXSY experiments revealed exchange between the two isomers, at T > 320 K, through the migration of the MeB(C6F5)(3)(-) anion between outward and inward sites. The presence of free B(C6F5)(3) fastened the exchange, suggesting B(C6F5)(3) attack on the zirconium bound methyl ligand of the two ion pairs. (C) 2004 Elsevier B.V. All rights reserved
"Propylene copolymers containing styrene units." (Montell Technology Company Bv, Neth.).
No full textFrom stiff plastic to elastic polypropylene: Polymorphic transformations during plastic deformation of metallocene-made isotactic polypropylene
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