Syndiotactic polymerization of styrene and copolymerization with ethylene catalyzed by chiral half-sandwich rare-earth metal dialkyl complexes

The syndiotactic polymerization of styrene (St) and the copolymerization of St with ethylene (E) were carried out by using a series of chiral half-sandwich rare-earth metal dialkyl complexes (Cp^x*) as the catalysts. The complexes are Ln(CH₂SiMe₃)₂(THF) (1−4: Ln = Sc (1), Ln = Lu (2), Ln = Y (3), Ln = Dy (4)) bearing chiral cyclopentadienyl ligand containing bulky cylcohexane derivatives in the presence of activator and AlⁱBu₃. For the St polymerization, a high activity up to 3.1 × 10⁶ g of polymer mol_Ln ⁻¹·h⁻¹ and a high syndiotactic selectivity more than 99% were achieved. The resulting syndiotactic polystyrenes (sPSs) have the molecular weights (M_n) ranging from 3700 g·mol⁻¹ to 6400 g·mol⁻¹ and the molecular weight distributions (M_w/M_n) from 1.40 to 5.03. As for the copolymerization of St and E, the activity was up to 2.4 × 10⁶ g of copolymer molSc⁻¹·h⁻¹·MPa⁻¹, giving random St-E copolymers containing syndiotactic polystyrene sequences with different St content in the range of 15 mol%−58 mol%. These results demonstrate that the bulky cyclopentadienyl ligands of the chiral half-sandwich rare-earth metal complexes effectively inhibit the continued insertion of St monomers into the (co)polymer chain to some extent in comparison with the known half-sandwich rare-earth metal complexes.