Key Words:ETHYLENE TRI-/TETRAMERIZATION；TRIMERIZATION CATALYSTS；PHILLIPS CATALYST；CR-PNP;POLYMERIZATION;OLIGOMERIZATION;COORDINATION;ACTIVATION;MECHANISM;SITES
Abstract:The diverse properties and applications of polyethylenes depend on their molecular weights, molecular weight distributions, and chain topology. Considering the importance of chromium complexes incatalytic ethylene polymerization and oligomerization, we have synthesized aseries of Cr complexes (Cr1-Cr6) bearing a SNN-tridentate ligand. In thepresence of MAO as cocatalyst, complexesCr1-Cr6exhibited moderate toextremely high activity (up to 2.4x107g(PE) mol-1(Cr) h-1) towardethylene polymerization. The influences of the ligand and of ious reactionparameters, including the nature and amount of the cocatalyst and the reactiontemperature and pressure, were systematically investigated withCr1. It wasfound that lower reaction temperatures (50 degrees C) and ethylene pressure (5 atm)and larger MAO/Cr ratios (1500) favored bimodal distributions with thedominant high-Mwfraction. In contrast, higher reaction temperatures (>= 80 degrees C) and ethylene pressure (40 atm) and lower MAO/Cr ratios (<= 500) almost exclusively led to the production of low-Mwpolyethylene waxes with monomodal and narrow distributions. Based on DFT calculations and UV-vis-NIR spectroscopy, twotypes of active species generated byCr1and MAO were proposed to be responsible for the production of bimodal polyethylene. Bytuning the structures of the Cr complexes in theCr1-Cr6/MAO systems and the reaction conditions, polyethylenes with molecular weights ranging from low-Mwwaxes to UHMWPE and monomodal or bimodal distributions were readily synthesized
Volume:
Issue:7
Translation or Not:no