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

Abstract

A series of amidino N-heterocyclic carbene ligand supported rare-earth metal bis(alkyl) complexes (2,6-R¹₂C₆H₃NCR²NCH₂CH₂(NCHCHN(C₆H₂Me₃-2,4,6)C))Ln(CH₂SiMe₃)₂ [R¹ = ⁱPr, R² = Ph, Ln = Lu (1a); R¹ = ⁱPr, R² = Ph, Ln = Sc (1b); R¹ = ⁱPr, R² = ^tBu, Ln = Lu (2a); R¹ = Me, R² = 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 °C). The 3,4-selectivity decreased with increasing the polymerization temperature (25–60 °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.