Living Ziegler−Natta Polymerization by Early Transition Metals: Synthesis and Evaluation of Cationic Zirconium Alkyl Complexes Bearing β-Hydrogens as Models for Propagating Centers

The synthesis and characterization of a series of cationic zirconium and hafnium complexes with alkyl substituents bearing β-hydrogens of general formula {(η5-C5Me5)MR[N(Et)C(Me)N(t-Bu)]}[B(C6F5)4] [M = Zr; R = Et, n-Pr, i-Pr, n-Bu, i-Bu, and 2-ethylbutyl (5a−f) and M = Hf; R = i-Bu and t-Bu (6 and 7, respectively)] is described, including several isotopically labeled derivatives. The ability of these complexes to serve as model complexes for the living Ziegler−Natta polymerization of olefins that can be effected using the initiator 2a (R = Me in 5) has been addressed. The results obtained shed additional light on the steric and electronic factors that can contribute to the living character of a Ziegler−Natta polymerization based on an early transition metal initiator.