Ziegler-Natta polymerization supported systems

Examples of synergistic effects are now very numerous in catalysis. We shall restrict ourselves to metallic oxide-type catalysts for selective (amm)oxidation and oxidative dehydrogenation of hydrocarbons, and to supported metals, in the case of the three-way catalysts for abatement of automotive pollutants. A complementary example can be found with Ziegler-Natta polymerization of ethylene on transition metal chlorides [1]. To our opinion, an actual synergistic effect can be claimed only when the following conditions are filled (i), when the catalytic system is, thermodynamically speaking, biphasic (or multiphasic), (ii), when the catalytic properties are drastically enhanced for a particular composition, while they are (comparatively) poor for each single component. Therefore, neither promotors in solid solution in the main phase nor solid solutions themselves are directly concerned. Multicomponent catalysts, as the well known multimetallic molybdates used in ammoxidation of propene to acrylonitrile [2, 3], and supported oxide-type catalysts [4-10], provide the most numerous cases to be considered. Supported monolayer catalysts now widely used in selective oxidation can be considered as the limit of a two-phase system. 

Oxides of a variety of metals on finely divided inert support materials initiate polymerization of ethylene and other vinyl monomers by a mechanism that is assumed to be similar to that of heterogeneous Ziegler-Natta polymerization that is, initiation probably occurs at active sites on the catalyst surface [2j. Unlike the traditional Ziegler-Natta two-component catalyst systems, the supported metal-oxide catalysts are essentially one-component systems. Among the metals that have been investigated for these catalyst... 

PREPARATIVE TECHNIQUES Ziegler-Natta polymerization with titanium halide/ aluminum cdkyl catalyst and, optionally, ether, ester, or silane activator. Catalyst may be deposited on a magnesium chloride support. Slurry and gas phase processes are used. Catcdyst systems based on metallocenes are under development. Typical comonomers are ethylene and 1-butene. 

For Ziegler-Natta polymerization of alkenes, second-order deactivation decay explains much better experimental data than the first-order decay. Such behavior in supported systems was attributed to simultaneous deactivation of adjacent catalytic species on the surface. In homogeneous polymerization with metallocene catalysts (Fig. 9.53), a metallocene complex Cp2ZrCl2 reacts first (kj) with a cocatalyst MAO ([—O—Al—CH (CH3-) ). Subsequently, alkenes molecules are inserted (kpi) into the Zr-C bond of the formed metaUocenium ion Cp2Zr -CH3. A polymer molecule grows in length by numerous insertion reactions (kpi). 

In the following review we will focus on two classes of systems dispersed metal particles on oxide supports as used for a large variety of catalytic reactions and a model Ziegler-Natta catalyst for low pressure olefin polymerization. The discussion of the first system will focus on the characterization of the environment of deposited metal atoms. To this end, we will discuss the prospects of metal carbonyls, which may be formed during the reaction of metal deposits with a CO gas phase, as probes for mapping the environment of deposited metal atoms [15-19]. 

Ziegler-Natta catalysts are not active at all in polymerization of disubstituted acetylenes.415 Mo- and W-based systems (for alkynes with small substituents) and Nb- and Ta-based catalysts (for alkynes with bulky groups), in turn, are very effective catalysts used to convert disubstituted acetylenes into polymers with very high molecular weight.414 415 A polymerization mechanism similar to that of metathesis polymerization of cycloalkenes are supported by most experimental observations.414 423 424... 

HDPE is produced mainly by a suspension (slurry) process in various types of reactors and with various polymerization procedures. In these processes, a supported Ziegler-Natta catalyst system or a Phillips catalyst in a solvent is used. Because the temperature (80-100°C) is lower than the melting point of the polyethylene (140°C), the polymer produced is separated as a solid. This process is highly versatile and can be used to produce many kinds of polyethylenes. 

Polymerization with Complex Catalysts. High density polyethylene reached a domestic production of 1.25 billion pounds in 1968. It is made either with a stereospecific Ziegler-Natta catalyst or on a supported chromium oxide catalyst. The latter forms a complex with the silica-alumina and is activated by treatment with air and steam at elevated temperature. The mechanism is such that electrons are donated to the catalyst in order to be returned under polymerizational-promoting conditions, consequently lowering the energy of the system ... 

In addition to titanium-based Ziegler-Natta catalysts, vanadium-based systems have also been developed for PE and ethylene-based co-polymers, particularly ethylene-propylene-diene rubbers (EPDM). Homogeneous (soluble) vanadium catalysts produce relatively narrow molecular mass distribution PE, whereas supported V catalysts give broad molecular mass distribution.422 Polymerization activity is strongly enhanced by the use of a halogenated hydrocarbon as promoter in combination with a vanadium catalyst and aluminum alkyl co-catalyst.422,423... 

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