Are MgCl2-Immobilized Single-Center Catalysts for Polyethylene Really Single-Center? Confirmation and Refutation Using Melt Rheometry

摘要

Investigation of the melt Theological 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 MgCl2-immobilized Ti complexes and having narrow MWD (M_w/M_n 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.