Reduced Transition Metal Catalysts on Support

The catalysts that belong to this group are efficient in homopolymerizations of ethylene. They can also be used in some copolymerizations of ethylene with a-olefins. Such catalysts are prepared from TiO₂, V₂O₅, Nb₂O₅, Ta₂O₅, CrO₃, Cr₂O₃, and MoO₃.WO₃, NiO, and CoO. The supports are charcoal, silica, alumina, zinc oxide, or alumina-silica. These catalysts require higher temperatures and pressures for activity than do the typical Ziegler–Natta catalysts. The temperatures range from 140 to 230C and pressures from 420 to 1,000 psi. One of the original disclosures of such catalyst preparation was as follows [308]. A transition metal nitrate solution is used to saturate a charcoal bed, previously leached with nitric acid. The charcoal bed is then heated to temperatures high enough to decompose the nitrate to the oxide.

Following the heating, the oxide is reduced with H₂ or NaBH₄. Sometimes, powdered Al₂O₃ or MoO₃-Al₂O₃ is added prior to reduction. For an Al₂O₃-SiO₂ support, a 3% metal nitrate solution is used. The saturated bed is heated to 500C to decompose the nitrate. This is followed by a reduction. Many modifications of the above procedure developed since. Preparations based on molybdenum oxide are called Standard Oil Co. catalysts, while those based on chromium oxide are known as the Phillips Petroleum Co. catalysts. Following is a description of a preparation of a molybdenum-based catalyst. Alumina is saturated with a solution of ammonium molybdate and then subjected to heating in air at 500–600C. The oxide that forms is reduced with hydrogen at 430–480C. Reducing agents like CO, SO, or hydrocarbons are also used. Hydrogen, however, is preferred at pressures of approximately 75 psi. The catalyst may contain between 5 and 25% of the molybdenum compound dispersed on the surface . Atypical chromium-based catalyst is prepared similarly. It is usually supported by a 9:1 of SiO2: Al₂O₃  carrier. Either Cr(NO₃)₃ 9H₂OorCrO₃ solutions in nitric acid are used to impregnate the support. The nitrates are decomposed in air at 400–1,000C [310]. Optimum chromium content in these catalysts is 2–3%. The mechanism of polymerization was initially believed to be as follows . Cr–O–Si bonds are present on the catalyst surface. Contact with ethylene results in an oxidation-reduction reaction and formation of an ethylene-chromium complex. The initiations are accompanied by gains of hydride ions and formations of terminal methyl groups. Polymer growth was pictured as follows:

The termination reactions were believed to result from hydride-ion transfers. Later, however, polyethylene, formed with these catalysts, was found to contain approximately equal numbers of saturated and unsaturated chain ends. This contradicts the above concept. It may also mean that the initiation involves p-complexes that disproportionate to yield coordination of the metal to–CH=CH₂ and to–CH₂–CH₃ groups . Polymerization of propylene with these catalysts yields polymers with very little crystallinity. Higher a-olefins yield completely amorphous polymers. Formation of partially crystalline polymers was reported for vinylcyclohexane, allylcyclohexane, and 4-phenyl-1-butene. Styrene does not polymerize at all. Isoprene forms a trans-1,4-polymer [262].

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مواضيع ذات صلة

Polymerization of Aldehydes

Isomerization Polymerizations with Coordination Catalysts

Post Ziegler and Natta Coordination Polymerization of Olefins

Steric Control in Polymerization of Conjugated Dienes

Steric Control with Homogeneous Catalysts

Homogeneous Ziegler–Natta Catalysts

Steric Control with Heterogeneous Catalysts

Heterogeneous Ziegler–Natta Catalysts

Coordination Polymerization of Olefins

Termination in Anionic Polymerization

Hydrogen Transfer Polymerization

Steric Control in Anionic Polymerization