About Dendrimers: Structure, Physical Properties, and Applications

One of the most exciting developments in the areas of catalysis, organometallic chemistry, and polymer science in recent years has been the intense exploration and commercialization of new polymerization technologies based on single-site and metallocene coordination olefin polymerization catalysts.1 The vast number of specifically designed/synthesized transition metal complexes (catalyst precursors) and main-group organometallic compounds (cocatalysts) allows unprecedented control over polymer microstructure, the generation of new polymer architectures, and the development of new polymerization reactions. Commercialization of new generations of single-site and metallocene catalyst-based technologies has provided the multibillion pound per year polyolefins industry with the ability to deliver a wide range of new and innovative olefin-based polymers having improved properties.2-4 The intense industrial activity in the field and the challenges to our basic understanding that have come to light have in turn 1391 Chem. Rev. 2000, 100, 1391−1434

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Cocatalysts for Metal-Catalyzed Olefin Polymerization: Activators, Activation Processes, and Structure−Activity Relationships

: The unpolarized absorption and circular dichroism spectra of the fundamental vibrational transitions of the chiral molecule, 4-methyl-2-oxetanone, are calculated ab initio. Harmonic force fields are obtained using Density Functional Theory (DFT), MP2, and SCF methodologies and a 5S4P2D/3S2P (TZ2P) basis set. DFT calculations use the Local Spin Density Approximation (LSDA), BLYP, and Becke3LYP (B3LYP) density functionals. Mid-IR spectra predicted using LSDA, BLYP, and B3LYP force fields are of significantly different quality, the B3LYP force field yielding spectra in clearly superior, and overall excellent, agreement with experiment. The MP2 force field yields spectra in slightly worse agreement with experiment than the B3LYP force field. The SCF force field yields spectra in poor agreement with experiment.The basis set dependence of B3LYP force fields is also explored: the 6-31G* and TZ2P basis sets give very similar results while the 3-21G basis set yields spectra in substantially worse agreements with experiment. jg

Ab initio calculation of vibrational absorption and circular dichroism spectra using density functional force fields

Sterically encumbered Lewis acid and Lewis base combinations do not undergo the ubiquitous neutralization reaction to form "classical" Lewis acid/Lewis base adducts. Rather, both the unquenched Lewis acidity and basicity of such sterically "frustrated Lewis pairs (FLPs)" is available to carry out unusual reactions. Typical examples of frustrated Lewis pairs are inter- or intramolecular combinations of bulky phosphines or amines with strongly electrophilic RB(C(6)F(5))(2) components. Many examples of such frustrated Lewis pairs are able to cleave dihydrogen heterolytically. The resulting H(+)/H(-) pairs (stabilized for example, in the form of the respective phosphonium cation/hydridoborate anion salts) serve as active metal-free catalysts for the hydrogenation of, for example, bulky imines, enamines, or enol ethers. Frustrated Lewis pairs also react with alkenes, aldehydes, and a variety of other small molecules, including carbon dioxide, in cooperative three-component reactions, offering new strategies for synthetic chemistry.

Frustrated Lewis pairs: metal-free hydrogen activation and more.

Although reversible covalent activation of molecular hydrogen (H2) is a common reaction at transition metal centers, it has proven elusive in compounds of the lighter elements. We report that the compound (C6H2Me3)2PH(C6F4)BH(C6F5)2 (Me, methyl), which we derived through an unusual reaction involving dimesitylphosphine substitution at a para carbon of tris(pentafluorophenyl) borane, cleanly loses H2 at temperatures above 100°C. Preliminary kinetic studies reveal this process to be first order. Remarkably, the dehydrogenated product (C6H2Me3)2P(C6F4)B(C6F5)2 is stable and reacts with 1 atmosphere of H2 at 25°C to reform the starting complex. Deuteration studies were also carried out to probe the mechanism.

Reversible, Metal-Free Hydrogen Activation

Selectivity in propene polymerization with metallocene catalysts.

Catalysts useful in the polymerization of olefins, more particularly of ethylene and mixtures thereof with higher alpha-olefins and/or diolefins, are disclosed. The new catalysts are obtained by mixing a hydride or organometallic compound of a metal belonging to Groups I to III of the Mendelyeev Periodic System with the product obtained by contacting a titanium tetrahalide with a support comprising an anhydrous magnesium or zinc halide in an active form, under particular conditions.

Catalysts for the polymerization of olefins

Chain transfer via {beta}-CH{sub 3} elimination by a homogeneous bimetallic Ziegler-Natta propylene polymerization catalyst is reported. Propylene is converted by Cp{sup {double_dagger}}{sub 2}MCl{sub 2}/MAO catalysts (Cp{sup {double_dagger}} = pentamethylcyclopentadienyl; M=Zr, Hf; MAO = methylalumoxane) to atactic propylene oligomers and low polymers. GC-MS and {sup 1}H and {sup 13}C NMR analyses of the oligomers obtained at {degrees}C (P{sub n} {approx} 4.5 for Zr, 3.4 for Hf) show these products to be mainly allyl- and isobutyl-terminated (1/1 ratio). The allyl/vinylidene ratio is 92/8 for Zr and 98/2 for Hf. No other unsaturated end groups could be detected. This end group structure is produced by first monomer insertion into the M-CH{sub 3} bond and then chain transfer by {beta}-CH{sub 3} elimination. On the contrary, Cp{sup {double_dagger}}{sub 2}MCl{sub 2}/MAO promotes 1-butene polymerization with the chain transfer being exclusively {beta}-H elimination and transfer to Al: no {beta}-ethyl elimination could be detected. The behavior of these catalysts toward propylene and 1-butene is compared with known Cp{sub 2}MCl{sub 2}/MAO catalysts. 37 refs., 11 figs., 5 tabs.

Olefin polymerization at bis(pentamethylcyclopentadienyl)zirconium and -hafnium centers: chain-transfer mechanisms

A series of MAO-activated C(1)-symmetric indenyl-ansa-dithienocyclopentadienyl-based zirconocenes have been used to produce isotactic polypropylenes of medium to high molecular weights, of different degrees of stereoregularity, and free from regioerrors. The effect of the presence of rr defects on the polymorphic behavior and mechanical properties of polypropylene is analyzed. The presence of rr defects induces crystallization of gamma form and of disordered modifications intermediate between alpha and gamma forms. A linear relationship between the amount of gamma form and the average length of isotactic sequences has been found. Samples with low concentration of rr defects, up to 3-4%, present high melting temperatures, in the range 160-130 degrees C, and behave as stiff-plastic materials; sample with higher rr content, in the range 4-6% and melting temperatures around 115-120 degrees C are highly flexible thermoplastic materials, and, finally, samples with concentration of rr defects in the range 7-11% and melting temperatures in the range 80-110 degrees C are thermoplastic elastomers with high strength. The fine-tuning of the chain microstructure, achieved by a tailored design of new metallocene catalysts, has allowed production of new polypropylenes having desired properties, intermediate between those of stiff plastic and elastomeric materials.

Structure-property correlations in polypropylene from metallocene catalysts: stereodefective, regioregular isotactic polypropylene.

Etude comparative par 13 C RMN de la structure stereochimique d'echantillons prepares dans differentes conditions de reaction

Luigi Resconi

Papers

Selectivity in propene polymerization with metallocene catalysts.

Catalysts for the polymerization of olefins

Olefin polymerization at bis(pentamethylcyclopentadienyl)zirconium and -hafnium centers: chain-transfer mechanisms

Structure-property correlations in polypropylene from metallocene catalysts: stereodefective, regioregular isotactic polypropylene.

Microstructure of isotactic polypropylene prepared with homogeneous catalysis: stereoregularity, regioregularity, and 1,3 insertion