Synthesis and Reactivity of Titanium Amido Complexes with Relevance to Olefin Polymerization

Ziegler-Natta catalysts are well known in the chemical industry. They are responsible for the production of more than half of the HDPE and HDPP worldwide annually. Ziegler-Natta catalysts exist in many forms, both in homogeneous and heterogeneous types as well as both with early and late transition metal basis. While Ziegler-Natta catalysts can exist in various forms, all polymerize via the same general mechanism. Dissecting the known mechanism(s) results in three items that must be in place for polymerization to occur. First, the active metal center must be highly Lewis acidic, enough to allow coordination of approaching olefins. Second, a vacancy in the coordination sphere around the active metal center must exist to allow coordination of the approaching olefin. Thirdly, there must be a metal-alkyl bond, M-R, into which the coordinated olefin can insert.

The Lewis acidity and vacancy in the coordination sphere are obviously necessary for polymerization, but it is the third aspect that is questionable. In organic chemistry Lewis acids are often used to induce carbocationic character on olefinic carbons, which can in turn be utilized in a large variety of transformations. Among those transformations lies the ability to create carbon-to-carbon bonds. Transition metal mediated carboncarbon bond formations are well documented, although they concentrate on sp2-sp2 bond formation.

Synthesis of highly Lewis acidic coordinatively unsaturated titanium-amido systems is reported. These systems include (Ph2N)3TiCl, (Ph2N)3TiMe, (Ph2N)3TiCH2PMe2, [(Ph2N)3Ti•••Me•••BArF 3] and [(Ph2N)3Ti][BArF 4]. Polymerization v studies are reported including three systems: (Ph2N)3TiCl/ MAO, [(Ph2N)3Ti•••Me•••BArF 3], and [(Ph2N)3Ti][BArF 4].