Coordination compounds polymerization

Chromium(III) Chemistry. The most characteristic reactions of Cr(III) in aqueous solution at >4 pH, eg, in the intestine and blood, and hydrolysis and olation (147). As a consequence, inorganic polymeric molecules form that probably are not able to diffuse through membranes. This may be prevented by ligands capable of competing for coordination sites on Cr(III) (see Coordination compounds) (147). Thus any large fraction of ingested Cr(III) should be absorbed. Chromium (ITT) in the form of GTF may be more efficiendy absorbed. 

The stereospecific polymerization of alkenes is catalyzed by coordination compounds such as Ziegler-Natta catalysts, which are heterogeneous TiCl —AI alkyl complexes. Cobalt carbonyl is a catalyst for the polymerization of monoepoxides several rhodium and iridium coordination compounds... 

Polymers with much higher average molecular weights, from 90,000 to 4 x 10 , are formed by a process of coordinate anionic polymerization (43—45). The patent Hterature describes numerous organometaUic compounds, aLkaline-earth compounds, and mixtures as polymerization catalysts. Iron oxides that accumulate in ethylene oxide storage vessels also catalyze polymerization. This leads to the formation of nonvolatile residue (NVR) no inhibitor has been found (46). 

Polymerization of olefins such as styrene is promoted by acid or base or sodium catalysts, and polyethylene is made with homogeneous peroxides. Condensation polymerization is catalyzed by acid-type catalysts such as metal oxides and sulfonic acids. Addition polymerization is used mainly for olefins, diolefins, and some carbonyl compounds. For these processes, initiators are coordination compounds such as Ziegler-type catalysts, of which halides of transition metals Ti, V, Mo, and W are important examples. 

Addition polymerization is employed primarily with substituted or unsuhstituted olefins and conjugated diolefins. Addition polymerization initiators are free radicals, anions, cations, and coordination compounds. In addition polymerization, a chain grows simply hy adding monomer molecules to a propagating chain. The first step is to add a free radical, a cationic or an anionic initiator (I ) to the monomer. For example, in ethylene polymerization (with a special catalyst), the chain grows hy attaching the ethylene units one after another until the polymer terminates. This type of addition produces a linear polymer ... 

Polymerizations catalyzed with coordination compounds are becoming more important for obtaining polymers with special properties (linear and stereospecific). The first linear polyethylene polymer was prepared from a mixture of triethylaluminum and titanium tetrachloride (Ziegler catalyst) in the early 1950s. Later, Natta synthesized a stereoregular polypropylene with a Ziegler-type catalyst. These catalyst combinations are now called Zieglar-Natta catalysts. 

Investigations of silicon-metal systems are of fundamental interest, since stable coordination compounds with low valent silicon are still rare [64], and furthermore, silicon transition-metal complexes have a high potential for technical applications. For instance, coordination compounds of Ti, Zr, and Hf are effective catalysts for the polymerization of silanes to oligomeric chain-silanes. The mechanism of this polymerization reaction has not yet been fully elucidated, but silylene complexes as intermediates have been the subject of discussion. Polysilanes find wide use in important applications, e.g., as preceramics [65-67] or as photoresists [68-83],... 

Polymeric coordination compounds. 1. Haiduc, Russ. Chem. Rev. (Engl. Transl.), 1961, 30,498-526 (483). 

The ability of complexes to catalyze several important types of reactions is of great importance, both economically and intellectually. For example, isomerization, hydrogenation, polymerization, and oxidation of olefins all can be carried out using coordination compounds as catalysts. Moreover, some of the reactions can be carried out at ambient temperature in aqueous solutions, as opposed to more severe conditions when the reactions are carried out in the gas phase. In many cases, the transient complex species during a catalytic process cannot be isolated and studied separately from the system in which they participate. Because of this, some of the details of the processes may not be known with certainty. 

Chain-reaction mechanisms differ according to the nature of the reactive intermediate in the propagation steps, such as free radicals, ions, or coordination compounds. These give rise to radical-addition polymerization, ionic-addition (cationic or anionic) polymerization, etc. In Example 7-4 below, we use a simple model for radical-addition polymerization. 

Coordination compound. In polymerizations, a metal complex compound whose central atom provides a base for the monomer and the growing polymer to attach to as ligands. 

Ligand. As it is used in polymerization, a ligand is a molecule or a group that attaches itself to a central atom of a catalyst in a complex and arcane way called a coordination compound, Ziegler catalyst as an example. 

Metallocene, An organo-metallic coordination compound, or more specifically a cyclopentadienyl derivative of a transition metal or metal halide. Metallocenes are best known as catalysts for polymerizing ethylene and propylene. 

Equation (3)]. Polymerization of PO also proceeds rapidly to give the low molecular weight PPO [J/n = 4050 g mol— 1.2]. Since the molecular weight of polymer indicates that less than 10% of aluminum sites are active for producing the polymer, an oligomeric aluminum complex is supposed to be an active species although the characterization is not fully achieved. In this system, exclusion of a coordinated compound such as THE is essential for the high catalytic activities. Thus, aluminum complex prepared by the reaction of APBus and methylphosphonic acid in the presence of THE exhibits much less catalytic activity. 

CT excitation is more common in the photochemistry of coordination compounds. Besides the d — d transition (ligand field band), the CT band is often observed and the redox reaction between central metal ion and ligand is frequently induced by photoirradiation. This process has been used to initiate vinyl polymerization. 

Klaus H. Theopold was born in Berlin, studied at the Universitat Hamburg for his Vordiplom in 1977, and at UC Berkeley, where he obtained his PhD in 1982 under the direction of Professor R. G. Bergman. After spending a year as postdoctoral fellow in the laboratory of Professor R. R. Schrock at MIT, he began his independent career in 1983 as an Assistant Professor at Cornell University. In 1990 he moved to the University of Delaware, where he is currently Professor of Chemistry. His scientific interests encompass synthetic and mechanistic studies of transition metal compounds, in particular paramagnetic organometal-lics, polymerization catalysis, and coordination compounds relevant to the activation of O2. 

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