Access to High-Molecular-Weight Polyethylenes through High Temperature Ethylene Polymerization Catalysed by Ethylene-Bridged ansa-(3-R-Cyclopentadienyl)(Fluorenyl) Zirconocene Complexes

• Published in: Chinese journal of polymer science 2024, Vol.42 (1), p.42-51
• Main Authors: Li, Bo, Ma, Haiyan, Huang, Jiling
• Format: Article
• Language: English
• Subjects: Aromatic compounds; Characterization and Evaluation of Materials; Chemical synthesis; Chemistry; Chemistry and Materials Science; Condensed Matter Physics; Ethylene; High temperature; Industrial Chemistry/Chemical Engineering; Low molecular weights; Low temperature; Molecular weight; Polyethylene; Polyethylenes; Polymer Sciences; Polymerization; Propylene; Research Article; Temperature dependence; Thermal stability; Zirconium
• ISSN: 0256-7679; 1439-6203
• DOI: 10.1007/s10118-023-3034-z

A series of C 1 -symmetric ethylene-bridged ansa -(3- R -cyclopentadienyl)(fluorenyl) metallocene complexes (Zr: 1–5 ; Hf: 6 ) have been synthesized, characterized and investigated as catalyst precursors for the high temperature ethylene polymerization. Using methylaluminoxane (MAO) as the cocatalyst, zirconium complexes 1–5 bearing a bulky substituent on the 3-position of the cyclopendienyl ring showed high catalytic activities up to 1.48×10 7 g PE ·mol Zr −1 ·h −1 toward the polymerization of ethylene and afforded polyethylenes with high molecular weights (1.49×10 5 −6.31×10 5 g/mol), meanwhile exhibting great thermal stability at high temperatures up to 120 °C together with a long catalytic life time up to 2 h. By adopting low Al/Zr ratios, such as 125, polyethylenes with ultra high molecular weights up to 2.86×10 6 g/mol were obtained. It is worthy of noting that zirconium complexes 1–4 bearing a substituent with an aryl pendant showed temperature-dependent activities, which increased rapidly with the increase of polymerization temperature, thus weak interaction of the pendent aryl group with the cationic active center is proposed to account for the very low activities displayed at low temperatures. In contrast to zirconocene complexes 1–5 , hafnocene complex 6 only displayed very low catalytic activities toward the polymerization of ethylene and afforded polyethylenes with molecular weights ten times smaller than those obtained by zirconocene complexes 1–5 . Zirconocene complexes 1–5 were also able to catalyse the polymerization of propylene at high temperatures, but only afforded waxes with low molecular weights.