Chromium polymer chains

In the propagation centers of chromium oxide catalysts as well as in other catalysts of olefin polymerization the growth of a polymer chain proceeds as olefin insertion into the transition metal-carbon tr-bond. Krauss (70) stated that he succeeded in isolating, in methanol solution from the... 

It should be mentioned that donor substitution of the phenylene backbone of the salphen ligand was shown to have a decreasing effect on activity [103], which explains the overall lower productivity compared with halogen-substituted chromium salphens. However, experiments clearly proved an increased activity upon dimerization. Whereas the monomeric complex m = 4) converts about 30% of p-BL in 24 h, producing a molecular weight of 25,000 g/mol, the corresponding dimer yields up to 99% conversion with > 100,000 g/mol. Moreover, the smaller polydispersity (PD < 2) shows the better polymerization control, which is attributed to the decreased rate of polymer chain termination. This behavior is caused by the stabilization of the coordinated chain end by the neighboring metal center, as recently reported for dual-site copolymerizations of CO2 with epoxides [104-106]. The polymeric products feature an atactic microstructure since the... 

Arasabenzene, with chromium, 5, 339 Arcyriacyanin A, via Heck couplings, 11, 320 Arduengo-type carbenes with titanium(IV), 4, 366 with vanadium, 5, 10 (Arene(chromium carbonyls analytical applications, 5, 261 benzyl cation stabilization, 5, 245 biomedical applications, 5, 260 chiral, as asymmetric catalysis ligands, 5, 241 chromatographic separation, 5, 239 cine and tele nucleophilic substitutions, 5, 236 kinetic and mechanistic studies, 5, 257 liquid crystalline behaviour, 5, 262 lithiations and electrophile reactions, 5, 236 as main polymer chain unit, 5, 251 mass spectroscopic studies, 5, 256 miscellaneous compounds, 5, 258 NMR studies, 5, 255 palladium coupling, 5, 239 polymer-bound complexes, 5, 250 spectroscopic studies, 5, 256 X-ray data analysis, 5, 257... 

The difference in polymerization mechanism between one-component metal oxide catalysts and traditional Ziegler-Natta two-component catalysts seems to exist only in the initiation stage, while the mechanism of continued propagation of polymer chain has many common features for all the catalyst systems based on transition metal compounds. Thus most studies of the chromium oxide catalyst system, for example, deal either with the nature... 

The typical dimensions involved in polymerization catalysis are sometimes difficult to grasp. A catalyst particle that is 100 pm in diameter contains pores that are typically about 100-200 A in diameter. These pores expose internal surfaces containing active chromium centers from which 100,000-A polymer chains can grow. Thus, a typical polymer chain formed on Cr/silica is often hundreds or thousands of times longer than the average pore diameter of the catalyst in which it is formed. Most (typically, 99%) of the surface area of the catalyst particle is internal. From this internal surface, each catalyst particle produces thousands of times its own mass (or volume) in polymer. Where does this polymer go These issues can be understood only when it is realized that polymerization catalysts must fracture and disintegrate in order to be active [52,445-472]. 

The chromium(III) complex [Cr(bipy)(ox)2] can act as a metaUoligand to form a coordination polymer chain with Mn(II) ions, in which each manganese ion is 8-coordinate... 

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