Sequence-controlled ethylene/styrene copolymerization catalyzed by scandium complexes

Abstract

Rare earth metal-based complexes can catalyze ethylene/styrene copolymerization to give copolymers with long syndiotactic polystyrene sequences. Here, sequence-controlled ethylene/styrene copolymerization was achieved by regulating the electronic and steric properties of scandium precursors. When a thiophene-fused cyclopentadienyl scandium complex 1, (2,4,5,6-Me₄-4H-cyclopenta[b]thiophenyl)Sc(CH₂SiMe₃)₂, was used, a copolymer with long syndiotactic polystyrene sequences was obtained. The tetrahydrofuran (THF) adduct of complex 1 (1·THF) showed lower activity to give a copolymer with short polystyrene sequences owing to an increase in the steric effects around the metal center. The pyridine adducts 1·Py and 1·Py_F5 were totally inert. Under the same conditions, complex 2 ((2,5-dimethyl-3-phenyl-4-H-cyclopenta[b]thiophenyl)Sc(CH₂SiMe₃)₂), an analog of 1, incorporated styrene discretely into the copolymer to form a “pseudo random” microstructure. Complex 3 bearing a bulky and less electron-donating constrained-geometry configuration ligand ((Flu-CH₂–Py)Sc(CH₂SiMe₃)₂) could catalyze copolymerization in a livingness manner to afford copolymers containing mainly the alternating ethylene/styrene sequence, where a fast and reversible chain-transfer reaction to aluminum alkyl was observed. The influences of structural and electronic factors of the scandium precursors, such as the steric bulkiness of a ligand and incorporation of a Lewis base to the active metal center, on the catalytic activity and sequence distribution of the resulting copolymers were elucidated.