Copolymerization of ethylene with norbornene catalyzed by cationic rare earth metal fluorenyl functionalized N-heterocyclic carbene complexes

Rare earth metal bis(alkyl) complexes attached by fluorenyl modified N-heterocyclic carbene (NHC) (Flu-NHC)Ln(CH₂SiMe₃)₂ (Flu-NHC = (C₁₃H₈CH₂CH₂(NCHCCHN)C₆H₂Me₃-2,4,6); Ln = Sc (2a); Y (2b); Ho (2c); Lu (2d)), (^tBuFlu-NHC)Ln(CH₂SiMe₃)₂ (^tBuFlu-NHC = 2,7-^tBu₂C₁₃H₆CH₂CH₂(NCHCCHN)C₆H₂Me₃-2,4,6; Ln = Sc (1a); Lu (1d)) and attached by indenyl modified N-heterocyclic carbene (Ind-NHC)Ln(CH₂SiMe₃)₂ (Ind-NHC = C₉H₆CH₂CH₂(NCHCCHN)C₆H₂Me₃-2,4,6; Ln = Sc (3a); Lu (3d)), under the activation of AlⁱBu₃ and [Ph₃C][B(C₆F₅)₄], showed varied catalytic activities toward homo- and copolymerization of ethylene and norbornene. Among which the scandium complexes, in spite of ligand type, exhibited medium to high catalytic activity for ethylene polymerization (10⁵ g mol_Sc⁻¹ h⁻¹ atm⁻¹), but all were almost inert to norbornene polymerization. Remarkably, higher activity was found for the copolymerization of ethylene and norbornene when using Sc based catalytic systems, which reached up to 5 × 10⁶ g mol_Sc⁻¹ h⁻¹ atm⁻¹ with 2a. The composition of the isolated copolymer was varying from random to alternating according to the feed ratio of the two monomers (r_E = 4.1, r_NB = 0.013). The molecular structure of complex 1d was characterized by X-ray analysis. The influences of structural factors of complexes and polymerization conditions on both the catalytic activity and the norbornene content in the copolymer were discussed.