TABLE OF CONTENTS The term Ziegler-Natta catalysts refers to a wide variety of polymerization initiators generally formed from mixtures of transition metal salts of Group IV to VIII metals and base metal alkyls of Group II or III metals [21, 227, 228]. It arose from the spectacular discovery of Ziegler et al. [229] that mixtures of titanium tetrachloride and aluminum alkyls polymerize ethylene at low pressures and temperatures; and from the equally spectacular discovery by Natta [230] that the Ziegler catalysts can stereospecifically polymerize monoolefins to produce tactic, crystalline polymers. As can be imagined, these systems can involve many combinations of catalyst components, not all of which are catalytically active or stereospecific. However, we shall be concerned here only with polymerizations involving the commercial elastomers, principally polyisoprene, polybutadiene [231 233], and the ethylene-propylene copolymers [234 238].
The mechanism of polymerization of alkenes using Ziegler-Natta-type catalysts is described as a coordination [239] or insertion [240] polymerization process. The coordination terminology assumes that the growing polymer chain is bonded to a transition metal atom and that insertion of the monomer into the carbon-metal bond is preceded by, and presumably activated by, the coordination of the monomer with the transition metal center. Since coordination of the monomer may or may not be a specific feature of these polymerizations, the insertion terminology focuses on the proposal that these reactions involve a stepwise insertion of the monomer into the bond between the transition metal atom and...
