Gas-Phase Ethylene Polymerization by Single-Site Cr Centers in a Metal-Organic Framework

We report a systematic study on the gas-phase polymerization of ethylene by a metal-organic framework (MOF) catalyst. Cr3+-exchanged MFU-4l (Cr(III)-MFU-4l, MFU-4l = Zn5Cl4(BTDD)(3), H2BTDD = bis(1H-1,2,3,-triazolo[4,5-b],[4',5'-i])dibenzo[1,4]dioxin)) serves as an exemplary system to demonstrate prereaction treatment with alkylaluminum species as a simple method to isolate an active MOF catalyst for liquid-free polymerization of ethylene. AlMe3-treated Cr(III)-MFU-4l subjected to 40 bar of ethylene exhibits a polymerization activity of 52 000 mol(Ethylene)center dot mol(Cr)(-1)center dot h(-1), an order of magnitude higher than that observed in a slurry-phase reaction with Cr(III)-MFU-4l and excess alkylaluminum species. Furthermore, product polyethylene exhibits a low polydispersity index of 1.36 and a free-flowing granular morphology favorable for industrial processing, highlighting the advantages conferred by the single-site MOF catalysts in gas-phase ethylene polymerization.