Cationic tropidinyl scandium catalyst: A perfectly acceptable substitute for cationic half-sandwich scandium catalysts in cis -1,4-Polymerization of isoprene and copolymerization with norbornene

Different nonmetallocene rare earth metal alkyl complexes such as monotropidinyl (Trop) scandium dialkyl complex (Trop)Sc(CH₂SiMe ₃)₂(THF) (1), ditropidinyl yttrium alkyl complex (Trop)₂Y(CH₂SiMe₃)(THF) (3) as well as binuclear lutetium alkyl complex bearing one tetradentate dianionic 6-N-methyl-1,4-cycloheptadienyl (NMCH) ligand [(NMCH)Lu(CH₂SiMe ₃)(THF)]₂ (2) have been synthesized in high yields via one-pot acid-base reaction by using of the tris(trimethylsilylmethyl) rare earth metal complexes with the readily available natural product tropidine. The polymerization experiments indicate that the monotropidinyl scandium dialkyl complex 1 displays reactivity akin to that of the analogous monocyclopentadienyl scandium dialkyl complexes. In the presence of activator and a small amount of AlMe₃, complex 1 exhibits similar activities (up to 1.6 × 10⁶ g mol_Sc^-1 h^-1) but higher cis-1,4-selectivities (up to 100%) than (C₅H₅)Sc(CH ₂SiMe₃)₂(THF) (cis-1,4-selectivity as 95%) in the isoprene polymerization, yielding the pure cis-1,4-PIPs with moderate molecular weights (M_n = 0.5-11.2 × 10⁴ g/mol) and bimodal molecular weight distributions (M_w/M_n = 1.48-6.07). Moreover, the complex 1/[Ph₃C][B(C₆F ₅)₄/AlⁱBu₃ ternary system also shows good comonomer incorporation ability in the copolymerization of isoprene and norbornene similar to the [C₅Me₄(SiMe₃)] Sc(η³-CH₂CHCH₂)₂/activator binary system, affording the random isoprene/norbornene copolymers with a wide range of isoprene contents around 57-91 mol % containing cis-1,4 configuration up to 88%.