Evaluation of (η5-C5Me5)Hf(R)2[N(Et)C(Me)N(t-Bu)] (R=Me and i-bu) for the stereospeciric living and degenerative transfer living Ziegler-Natta polymerization of α-olefins

The dialkyl(monocyclopentadienyl)hafnium acetamidinates, Cp*Hf(R)(2)[N(Et)C(Me)N(t-Bu)] [Cp* = eta(5)-C5Me5, R = Me (3a) and i-Bu (3b)], were prepared through either 'one-pot' carbodiimide insertion or two-step salt elimination protocols starting with commercially available Cp*HfCl3. Protonolysis of 3a and 3b with [hNHMe(2)][B(C6F5)(4)] in chlorobenzene produced the cationic complexes, {[Cp*Hf(R)N(Et)C(Me)N(t-Bu)]}[B(C6F5)4] [R = Me (4a) and i-Bu (4b)], which were characterized in solution. Compounds 4a and 4b were evaluated as initiators for the stereospecific living Ziegler-Natta polymerization of 1-hexene. These results reveal that, while an extremely high level of stereoselectivity can be achieved to produce isotactic poly(1-hexene) in a living fashion, the rate constant for polymerization, k(P), using either 4a or 4b, is similar to60 times less than that of the analogous zirconium initiators. Finally, upon substoichiometric activation of 3a with [PhNHMe2][B(C6F5)(4)] in a 2:1 ratio, degenerative transfer living Ziegler-Natta polymerization of 1-hexene can be accomplished to produce atactic poly(1-hexene).