Lewis base, magnesium chloride catalysts

Supported precursors for Ziegler-Natta catalysts may be obtained, depending on the kind of support, in two ways by treatment of the support containing surface hydroxyl groups with a transition metal compound with chemical covalent bond formation, and by the treatment of a magnesium alkoxide or magnesium chloride support with a Lewis base and transition metal compound with coordination bond formation. 

Internal Lewis bases (ILB) and external Lewis bases (ELB) are both necessary with the supported Ziegler-Natta catalysts to achieve the requirements of high activity and stereospecificity. The ILB is not directly involved in the active center. During the catalyst preparation the ILB masks the magnesium chloride surface, blocking the acidic defects. The catalyst is then formed by a selective replacement of the weakest bonded ILB by the titanium tetrachloride, giving thus access to low acidic active centers. The scheme can be considered as an active center selection. High activity is observed when a dynamic selection process results in an accumulation of trapped defects. 

Catalytic activity can be considerably increased if the magnesium chloride is first combined with a suitable Lewis base, such as an ester, acid, alcohol, or amine, before reaction with an excess of titanium tetrachloride and subsequent activation with triethyl aluminum. Polyethylene produced with these catalysts has a lower molecular weight than that from the titanium trichloride catalysts mentioned earlier and less hydrogen is needed for molecular weight control. 

The magnesium supports used for polyethylene catalysts could be modified for use in polypropylene production, and magnesium chloride proved to be the most suitable when used with a Lewis base election donor. Milled magnesium chloride was known to have the same layer stracmre as a- and y-titanium trichloride with the quadrivalent titanium ion (0.068 nm diameter), being about the same size as the divalent magnesium ion (0.066 nm diameter). In 1968, Montedison and Mitsui Petrochemical Industries both disclosed the production of a highly active, very steieospecific catalyst that contained about 3% titanium on a magnesium chloride support, promoted with a Lewis base, such as ethyl benzoate. The polymer produced contained less than 1 ppm titanium with an isotactic index of more than 90%, which was improvement on the product made with previous catalysts. 

Dehydrochlorination of chlorinated derivatives such as 1,1,2-trichloroethane may be carried out with a variety of catalytic materials, including Lewis acids such as aluminum chloride. Refluxing 1,1,2-trichlorethane with aqueous calcium hydroxide or sodium hydroxide produces 1,1-dichloroethylene in good yields (22), although other bases such as magnesium hydroxide have been reported (23). Dehydrochlorination of the 1,1,1-trichloroethane isomer with catalytic amounts of a Lewis acid also yields 1,1-dichloroethylene. Other methods to dehydrochlorinate 1,1,1-trichloroethane include thermal dehydrochlorination (24) and by gas-phase reaction over an alumina catalyst or siUca catalyst (25). 

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