Copolymerization of Ethylene with 1‑Hexene and 1‑Octene Catalyzed by Fluorenyl N‑Heterocyclic Carbene Ligated Rare-Earth Metal Precursors

Rare-earth
metal bis­(alkyl) complexes (Flu–NHC)­Ln­(CH2SiMe3)2 (Ln = Dy (1), Er
(2), Sc (3)) attached by fluorenyl-modified
N-heterocyclic carbene ligands ((Flu H–NHC–H)­Br) have
been synthesized by treatment of (FluH–NHC–H)­Br
with (trimethylsilylmethyl)lithium (LiCH2SiMe3) and rare-earth metal tris­(alkyl)­s (Ln­(CH2SiMe3)3(THF)2) via double-deprotonation reactions
in moderate to high yields. Under mild conditions (40 °C and
normal ethylene pressure), the scandium precursor 3,
upon activation of AliBu3 and
[Ph3C]­[B­(C6F5)4], showed
high activity (4120 kg molSc–1 h–1 atm–1) for the copolymerization
of ethylene and 1-hexene with moderate 1-hexene insertion ratio (20.2%),
although the analogous complexes 1 and 2 were inert. In addition, this system displayed excellent catalytic
performances for the copolymerization of ethylene and a higher α-olefin
1-octene with an activity of up to 3640 kg molSc–1 h–1 atm–1. The content of 1-octene
could be controlled swiftly from 2.1% to 38.7% by varying the 1-octene
feed ratio. Thus the isolated P­(E-co-Oct) polymers varied from opaque
crystalline solids with high melting points, e.g., Tm = 103.6 °C, to transparent elastomers. This represents
the first rare-earth metal based homogeneous catalyst that can initiate
the copolymerization of ethylene and 1-octene, the catalytic performances
of which are comparable with those reported for the most active group
4 metallocene systems.