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Subject - Inorganic Chemistry
In general, all known Ziegler-Natta catalysts that promote and activate polymerization reactions are combinations of transition metals of groups I to VIII with organometallic compounds of groups I to III. They were first developed by Karl Ziegler (1953, for ethylene) and Giulio Natta (1954, for propylene). Mainly, they are being used for the stereospecific polymerization of olefins under normal pressure. However, they are not catalysts in the classical sense because they are not recovered from the reaction unchanged. The catalyst remains in the polymer and has to be deactivated.
In most cases, the organometallic component is an aluminum trialkyl (e.g. triethylaluminum) compound that is combined with the salt of a transition metal, mostly (titanium tetrachloride). Other halogenides, alcoholates and acetyl acetonates of transition metals of groups IV, V and VI are also widely used. The catalytically active center is generated by a reaction of the salt of the transition metal with the organometallic compound substituting the chloride ligand with an ethyl group. Over several intermediate steps, ethylene is inserted into the transition metal-ethylene bond initiating the growth of the polymer chain which is propagated by inserting additional ethylene units.
A large number of Ziegler-Natta catalysts with defined characteristics have been developed for specific applications, e.g. + for the polymerization of ethylene, + for the stereospecific polymerization of propylene and + for the copolymerization of ethylene, propylene and a diene to yield EPDM rubber.