Highly 3,4-Selective Living Polymerization of Isoprene with Rare Earth Metal Fluorenyl N-Heterocyclic Carbene Precursors

Abstract

Fluorenyl modified N-heterocyclic carbene ligated rare earth metal bis(alkyl) complexes, (Flu-NHC)Ln(CH2SiMe3)2 (Flu-NHC = (C13H8CH2CH2(NCHCCHN)C6H2Me3-2,4,6); Ln = Sc (1a); Ln = Y (1b); Ln = Ho (1c); Ln = Lu (1d)), were synthesized and fully characterized by NMR and X-ray diffraction analyses. Complexes 1b−d with the activation of AliBu3 and [Ph3C][B(C6F5)4] exhibited high activity, medium syndio- but remarkably high 3,4-regio-selectivity, and the unprecedented livingness for the polymerization of isoprene. Such distinguished catalytic performances could be maintained under various monomer-to-initiator ratios (500−5000) and broad polymerization temperatures (25−80 °C). The resultant polymers are crystalline, having syndiotactically enriched (racemic enchainment triad rr = 50%, pentad rrrr = 30%) 3,4-regulated (99%) microstructure and high glass-transition temperatures (40−49 °C). In contrast, complex 1a was almost inert, while complexes bearing indenyl modified N-heterocyclic carbene moiety, (Ind-NHC)Ln(CH2SiMe3)2 (Ind-NHC = C9H6CH2CH2(NCHCCHN)C6H2Me3-2,4,6; Ln = Sc (2a); Ln = Y (2b); Ln = Ho (2c); Ln = Lu (2d)), also showed low activity or selectivity. These differences in catalytic performance could be attributed mainly to the electronics and spacial sterics of the metal center of these precursors.