Current Vitality of Ziegler’s Monumental Discovery of Zirconium Catalysis in Olefin Polymerization: Metallocene and Nonmetallocene Catalysts via Reductive Dimerization

Although the avalanche of technical advances unleashed by Ziegler’s discovery of the low-pressure polymerization of ethylene is most often associated with the use of titanium procatalysts, it is well to recall that the key initial experiment leading to the formation of polyethylene was achieved in 1953 with zirconium(IV) acetylacetonate.1 Titanium salts maintained their superiority in the heterogeneous Ziegler-Natta catalysts used for olefin polymerization during the next two decades. With the reawakening of interest in homogeneous, single-site metallocene polymerization catalysts in the late 1970s, the zirconocene(IV) derivatives have regained a marked ascendancy, because they undergo deactivating reduction much less readily than titanocene(IV) derivatives.The synthesis of such metallocenes of titanium, zirconium and hafnium generally requires the interaction of the Group 4 MCl₄ with the appropriately substituted cyclopentadienyl salt. In the present work we show how a great variety of bridge-substituted ansa-metallocenes can be prepared by the reductive dimerization of a suitable fulvene by the preformed Group 4 MCl₂:2If R and R′ are methyl, the tetramethylethylene-bridged ansa-metallocene can readily be obtained. If R = H and R′ = alkyl or aryl, varying proportions of racemic and meso isomers can be obtained and separated into individual isomers.This reductive coupling can also be applied to imines and even carbonyl derivatives and thus can lead to nonmetallocene catalyst systems as exemplified by the reductive coupling of benzalaniline:All the foregoing metallocene and nonmetallocene procatalysts, when combined with MAO, are efficient polymerization catalysts for ethylene. The possible stereoselectivity of the racemic isomers for the polymerization of propylene is under investigation.