Electronic Effect Rate Enhancement in the Stereoselective Living Coordinative Polymerization of α‑Olefins by α,α,α-Trifluoroacetamidinate-Modified Group 4 Metal CPAMCF3 Initiators
journal contributionposted on 2018-12-24, 17:55 authored by Rick R. Thompson, Peter Y. Zavalij, Lawrence R. Sita
The C1-symmetric cyclopentadienyl, α,α,α-trifluoroacetamidinate (CPAMCF3) 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, Rp, is observed relative to the LCP of this same α-olefin when the nonfluorinated CPAMCH3 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 Rp using the CPAMCF3 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 CPAMCH3 preinitiator 2. These results provide strong support for a unique synergistic coupling of steric and electronic effects of the CPAMCF3 ligand environment.