Highly 3,4-selective living polymerization of 2-phenyl-1,3-butadiene with amidino N-heterocyclic carbene ligated rare-earth metal bis(alkyl) complexes

A series of amidino N-heterocyclic carbene ligand supported rare-earth metal bis(alkyl) complexes (2,6-(R2C6H3N)-C-1=(CRNCH2CH2)-N-2(NCHCHN(C6H2Me3-2,4,6)C))Ln(CH2SiMe3)(2) [R-1 = Pr-i, R-2 = Ph, Ln = Lu (1a); R-1 = Pr-i, R-2 = Ph, Ln = Sc (1b); R-1 = iPr, R-2 = Bu-t, Ln = Lu (2a); R-1 = Me, R-2 = Ph, Ln = Lu (3a)] were synthesized and characterized by NMR spectroscopy. On activation with [(Ph3C)][(B(C6F5)(4))], complexes 1a, 2a and 3a exhibited high activities and high 3,4-selectivities (96.9%) for the polymerization of 2-phenyl-1,3-butadiene in a living mode to afford new plastic polydienes with high glass-transition temperatures (44.3-56.1 degrees C). The 3,4-selectivity decreased with increasing the polymerization temperature (25-60 degrees C), but increased with polarity of polymerization solvent in the order of chlorobenzene > toluene > cyclohexane. In contrast, complex 1b was almost inert for the polymerization of 2-phenyl-1,3-butadiene owing to its steric bulkiness.