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

Fluorenyl modified N-heterocyclic carbene ligated rare earth metal bis(alkyl) complexes, (Flu-NHC)Ln(CH 2 SiMe 3 ) 2 (Flu-NHC = (C 13 H 8 CH 2 CH 2 (NCHCCHN)C 6 H 2 Me 3 -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 Al i Bu 3 and [Ph 3 C][B(C 6 F 5 ) 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(CH 2 SiMe 3 ) 2 (Ind-NHC = C 9 H 6 CH 2 CH 2 (NCHCCHN)C 6 H 2 Me 3 -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.