Electronic Effect Rate Enhancement in the Stereoselective Living Coordinative Polymerization of α‑Olefins by α,α,α-Trifluoroacetamidinate-Modified Group 4 Metal CPAM^CF3 Initiators

The C₁-symmetric cyclopentadienyl, α,α,α-trifluoroacetamidinate (CPAM^CF3) group 4 metal dimethyl complexes, Cp*­[N­(Et)­C­(CF₃)­N­(^tBu)]­M­(Me)₂ (Cp* = η⁵-C₅Me₅) for M = Zr (3) and Hf (4), serve as initiators for the stereoselective (isotactic) living coordination polymerization (LCP) of α-olefins upon in situ “activation” using one equiv of the borate co-initiator, [PhNMe₂H]­[B­(C₆F₅)₄]. For the LCP of 1-hexene using 4, a six-fold enhancement in the rate of polymerization, R_p, is observed relative to the LCP of this same α-olefin when the nonfluorinated CPAM^CH3 structural analogue, Cp*­[N­(Et)­C­(CH₃)­N­(^tBu)­Hf­(Me)₂] (2), is employed as the preinitiator. For the LCP of propene, an eight-fold increase in R_p using the CPAM^CF3 preinitiator 4 now permits production of practical quantities of highly stereo- and regioregular isotactic polypropene (iPP) under reaction conditions that are not amenable for use with the corresponding CPAM^CH3 preinitiator 2. These results provide strong support for a unique synergistic coupling of steric and electronic effects of the CPAM^CF3 ligand environment.