Catalysis coordination

Coordination catalysts contain a metal atom that is activated by a variety of electron-wthdrawng species that are attached to it. Coordination catalysts for polyethylene fall into three main categories  

Within each category there are many variants. There are also hybrids, which combine chemical structures borrowed from more than one family of catalysts. 

We use single site catalysts primarily to make copolymers of ethylene and 1-alkenes, vith densities ranging from approximately 0.87 to 0.93 g/cm. More recently some higher density grades have been introduced, but these are relatively uncommon at the moment. 

A small number of companies use metal oxide catalysts, such as the example shown in Fig. 18.6, to make high density polyethylene. The polyethylene made with this catalyst generally has a narrower molecular weight distribution than high density polyethylene made with Ziegler-Natta catalysts. 

We typically crosslink polyethylene using one of two different strategies we can create direct crosslinks betsveen chains via a free radical mechanism or we can create short chemical bridges between chains. 

---

The copolymers of ethylene and propylene (OCVP) are obtained by coordination catalysis using a derivative of vanadium and a derivative of an aluminum alkyl. Molar compositions of ethylene and propylene are usually on the order 45 and 55%. 

P.A. Chaloner, Handbook of Coordination Catalysis in Organic Chemistry, Butterworths, London, 1986, p. 39. 

G. Lefebvre and Y. Chauvin Dimerisation and codimerisation of olefinic compounds by coordination catalysis, pp. 108-203. 

We make polyethylene resins using two basic types of chain growth reaction free radical polymerization and coordination catalysis. We use free radical polymerization to make low density polyethylene, ethylene-vinyl ester copolymers, and the ethylene-acrylic acid copolymer precursors for ethylene ionomers. We employ coordination catalysts to make high density polyethylene, linear low density polyethylene, and very low density polyethylene. 

Polystyrene is unusual among commodity polymers in that we can prepare it in a variety of forms by a diversity of polymerization methods in several types of reaction vessel. j Polystyrene may be atactic, isotactic, or syndiotactic. Polymerization methods include free radical, cationic, anionic, and coordination catalysis. Manufacturing processes include bulk, solution, suspension, and emulsion polymerization. We manufacture random copolymers ... 

Although Ziegler-type catalysts have been widely investigated for the homogeneous hydrogenation of polymers, their catalytic mechanism remains unknown. One possible reason for this may be the complexity of the coordination catalysis and the instability of the catalysts. Metallocene catalysts are highly sensitive to impurities, and consequently it is very difficult to obtain reproducible experimental data providing reliable kinetic and mechanistic information. 

The synthetic aspects of coordination catalysis still receive far more attention than mechanistic aspects, and precious little can be said with assurance concerning the mechanisms of the foregoing polymerization reactions. Almost the opposite is true of the free radical initiation of vinyl polymerization by metal carbonyls. Bamford et al. (10,11) have studied the kinetics of several such reactions and arrived at plausible interpretations of their results. The initiation of methyl methacrylate polymerization by tetracobalt dodecacarbonyl in the presence of carbon tetrachloride was studied in detail and the results were rationalized in terms of the following mechanism ... 

Bioinorganic chemistry, insofar as it involves catalytic transformations, is largely the chemistry of metalloenzymes (i.e., metalloproteins). Although the complex three-dimensional structure of the protein ligands obviously plays a vital role in determining the activity and stability of metalloenzymes, at the heart of the matter their reactions involve coordination catalysis by metal ions. Thus, at the molecular level we are dealing with homogeneous catalysis in aqueous media. 

The majority of biocatalytic processes fall into the category of coordination catalysis, although there are a few that involve organometallic catalysis e.g., reactions mediated by nickel-dependent (see earlier discussion and Chapter 9) and vitamin-B-12-dependent enzymes (see Chapter 13). 

Zhiquan Shen and Jun Ouyang, Rare earth coordination catalysis in stereospecific polymerization 395 Errata 429... 

Shen Z, Ouyang J Coordination catalysis in stereospecific polymerization 1987 [42]... 

As mentioned above, catalytic oxidation of olefins via coordination catalysis with an intermediate such as LnM (olefin) 02 seemed an attractive possibility, and Collman s group (45) tentatively invoked such catalysis in the 02-oxidation of cyclohexene to mainly 2-cyclo-hexene-1-one promoted by IrI(CO)(PPh3)2, a complex known to form a dioxygen adduct. Soon afterwards (4, 46, 47) such oxidations involving d8 systems generally were shown to exhibit the characteristics of a radical chain process, initiated by decomposition of hydroperoxides via a Haber-Weiss mechanism, for example Reactions 10 and 11. Such oxidations catalyzed by transition-metal salts such as... 

Coordination catalysis via alkyl hydroperoxides is well documented (4, 31). Selective oxidations of olefins to epoxides (Reaction 16), using especially Group IV, V, and VI transition-metal complexes, can occur possibly via oxygen-transfer processes of the type... 

The indole oxidation has been shown to proceed via the hydroperoxide intermediate 9 (126), but whether this is formed via coordination catalysis, for example, as suggested in Reaction 41 for a phenol substrate (10— 12,13,14) (124), or via Haber-Weiss initiation, poses the same problem encountered in the organometallic type systems. A reactivity trend observed for Reaction 40 using tetraphenyl-porphyrin complexes (Co(II) Cu(II) Ni(II)) is reasonable in that the Co(II) system is known to give 1 1 02-adducts (at least, at low temperatures) but the reactivity trend also was observed for the catalyzed decomposition rate of 9. It is interesting to note that in Reac-... 

---