Are MgCl2-immobilized single-center catalysts for polyethylene really single-center? confirmation and refutation using melt rheometry

Investigation of the melt rheological properties of polyethylenes prepared with chromium, titanium, vanadium, and zirconium catalysts immobilized on MgCl2 supports has confirmed narrow (Schulz-Flory) molecular weight distribution, indicative of single-center catalysis, for Cr-, V-, and Zr-based systems, but not for Ti. In the case of polymers prepared with MgCl 2-immobilized Ti complexes and having narrow MWD (M w/Mn 2-3) according to GPC, deviation from a Schulz-Flory distribution was evident from a decrease in storage modulus with decreasing angular frequencies, whereas polyethylenes prepared with analogous vanadium complexes exhibited a constant (plateau) modulus over a wide frequency range. The presence of the constant plateau modulus in high molecular weight polyethylenes is characteristic for narrow MWD, so that melt rheology provides a valuable tool to prove, or disprove, the presence of a Schulz-Flory distribution in cases where GPC does not provide a definitive answer. © 2007 American Chemical Society.