Bimetallic synergy in Si-(CH₂)ₙ-Si-bridged binuclear indenyl-pyrrolidinyl titanium catalysts for ethylene/1-octene copolymerization

To investigate the cooperative effect of binuclear metallocene catalysts in olefin polymerization, a series of Si-(CH₂)ₙ-Si bridged binuclear indene pyrrolidine titanium catalysts 2d-7d (Ti₂) were synthesized. The mononuclear complex NC₄H₈CpC₆H₄(Me)₂SiN(CH₃)₃TiCl₂ (Ti₁) was also prepared for control experiments. DFT calculations show that increasing the methylene spacer length (n=2–7) simultaneously enlarges the Ti-Ti distance (8.93–14.65 Å) and reduces the buried volume (%Vbur=16.2–12.4 %). In the presence of [Ph₃C][B(C₆F₅)₄] as a co-catalyst, 2d-7d (Ti₂) showed higher activity and thermal stability. For ethylene/1-octene copolymers, especially 5d (Ti₂) at 120 °C, the activity was 2.65×10⁷ g/(mol·h), the Mw was 66.93×10⁴ g/mol, and the 1-octene insertion rate was 8.4 mol%, which were 1.8 times, 1.9 times, and 1.3 times higher than that of mononuclear 1d (Ti₁), respectively. Copolymers from binuclear catalysts showed reduced crystallinity and melting points due to enhanced branching, alongside improved tensile strength and elastic recovery.