Highly 3,4-Selective Living Polymerization of Isoprene and Copolymerization with ε‑Caprolactone by an Amidino N‑Heterocyclic Carbene Ligated Lutetium Bis(alkyl) Complex

The
amidino-modified N-heterocyclic carbene ligated lutetium bis­(alkyl)
complex 1, (Am-NHC)­Lu­(CH2SiMe3)2, was synthesized by treatment of (AmH-NHC-H)Br ((2,6-iPrC6H3NC­(C6H5)­NHCH2CH2(NCHCHN­(C6H2Me3-2,4,6)­CH)­Br) with ((trimethylsilyl)­methyl)­lithium
(LiCH2SiMe3) and lutetium tris­(alkyls) (Lu­(CH2SiMe3)3(THF)2) via double-deprotonation
reactions and characterized by NMR spectroscopy and X-ray diffraction
analysis. Under activation of an organoborate, complex 1 exhibited distinguished catalytic performance for the polymerization
of isoprene with respect to high activity, 3,4-regioselectivity (99.3%),
and livingness mode. In contrast to the systems reported to date,
this system seemed not to be affected obviously by the polymerization
temperature (0–80 °C), solvents, monomer-to-initiator
ratios (500–5000), and type of organoborate. The resultant
polymers have high glass-transition temperatures (38–48 °C)
and moderate syndiotacticity (racemic enchainment triad rr 45%, pentad rrrr 20%). In addition, the living
lutetium–polyisoprene active species could further initiate
the ring-opening polymerization of ε-caprolactone to give selectively
the poly­(3,4-isoprene)-b-polycaprolactone block copolymers
with controllable molecular weight (from 4.9 × 104 to 10.2 × 104) and narrow polydispersity.