Zinc-based catalysts

Homopolymerization, epoxides aluminate-Lewis acid catalyst system, 11, 602 via aluminum-porphyrin-Lewis acid catalysts, 11, 599 aluminum-tetradentate ligand catalyst system, 11, 601 anionic polymerization, 11, 598 cationic aluminum catalyst system, 11, 603 cationic polymerization, 11, 598 characteristics, 11, 597 zinc-based catalyst system, 11, 605 Homopolymers, cyclic olefins, 11, 716... 

Taking into account the associated structures of catalysts for the coordination polymerisation of epoxides and considering that the coordinated epoxide is attacked by the nucleophile from the back side [scheme (1)], the catalyst must engage its two metal atoms in order to make the polymerisation possible. In fact, zinc-based catalysts containing associated multinuclear species (—>Zn-O-Zn-O—>), including those with condensed zinc atoms ( Zn O Zn O ), are all characterised by the appearance of active sites with two zinc atoms bridged via a nucleophilic oxygen atom in which the O atom, bound covalently to the octahedral zinc atom, is coordinated to the adjacent tetrahedral Zn atom (Zn O -Zn). 

As regards the dimethylcadmium-(7 )-(—)-3,3-diethyl-1,2-butanediol (1 1) catalyst, it was characterised by a lower stereoelectivity than the diethylzinc-(R)-(—)-3,3-dimethyl- 1,2-butanediol (1 1) catalyst in enantioasymmetric polymerisations of propylene sulphide, cA-2-butene sulphide and cyclohexene sulphide, but the elected chirality was opposite to that found for polymers obtained with the zinc-based catalyst [159]. 

The conversion of methanol to hydrocarbons is the most studied reaction of oxygenates over microporous solids, for both commercial and academic reasons. Methanol can be generated from syngas over copper- and zinc-based catalysts using the ICI process, and syngas can be prepared from methane, which is a readily available resource. Under the correct economic conditions, methanol conversion reactions can provide an important route to higher... 

According to the reaction temperature (Chen et al, 2008), the WGSR falls into two categories high-temperature shift catalyst (HTC) and low-temperature shift catalyst (LTC). The catalyst commonly used in the former is an iron-chromium-based catalyst, whereas a copper-zinc-based catalyst is frequently adopted in the latter. 

Current WGS catalysts suffer from the facT that they are mostly pyrophoric and thus require a defined shutdown cycle to prevent ignition. In addition to that, the copper-zinc-based catalyst also requires reduction prior to use. So the SILP catalysts were tested for their restart behavior and tolerance against condensation. Both rapid shutdown and fast restart are required for hydrogen generation scenarios, but one... 

Zinc alkoxides have also been prepared by reaction of diethylzinc with alcohols. Zn(0-ra-C4H9)2 was studied by Kricheldorf (4). C2H5ZnO(CH2)2Br and Zn(0(CH2)2Br)2 were used by Jerome and co-workers for the controlled ROP of -CL (75). Well-defined single-site zinc-based catalysts, [(BDI-l)ZnO-i-C3H7)]2 (Fig. 14), convert j8-BL and -VL into high molecular weight poly(3-hydroxyalkanoates) (76). Moreover, ROP of (S,S-LA, rac-LA, and meso-IA leads to isotactic, heterotactic, and syndiotactic PIAs, respectively (77-79). 

During research on aluminum catalysts for the stereospecific polymerization of epoxides, it was discovered that the addition of zinc cocatalysts to these systems greatly enhanced catalyst activity. These enhancements prompted a number of studies focusing on the design of zinc-based catalyst systems. 

Cheng, M. Darling, N. A. Lobkovsky, E. B. Coates, G W. Enantiomerically-enriched organic reagents via polymer synthesis Enantioselective copolymerization of cycloalkene oxides and CO2 using homogeneous, zinc-based catalysts. Chem. Commun. 2000, 2007-2008. 

Epoxy/carboxy/acid anhydride systems are also important in OEM clear coats and powder coatings. Catalysts such as tertiary amines, amidines and imidazoles, 1,4-diazabicyclooctane and some alkaline metal-based catalysts such as zinc-based catalysts are commonly used. 

Tanahashi, N. and Doi, Y. (1991) Thermal properties and stereoregularity of poly (3-hydroxybutyrate) prepared from optically active j -butyrolactone with a zinc-based catalyst. Macromolecules, 24, 5732-3. 

The reaction between high acid value acrylics and bisphenol A glycidyl ether epoxy resins is normally accelerated using a zinc based catalyst such as zinc napthenate or zinc octoate. Levels of catalyst vary between 0.5% and 1.5% (calculated as metal based on solid resin). 

CO2 and epoxides are polymerised into polycarbonates in the presence of zinc-based catalysts (e.g. ZnEy combined with water, tert-butylcatechol or pyrogallol (benzene with three -OH groups). For example, the polymerisation of propylene oxide with COj produces poly(propylene carbonate) (PPC), as shown in Figure 9.2. 

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