Effect of Polymerization Time and Pressure on the Molecular Weight and Molecular Weight Distribution of Polyethylene

Molecular weight and molecular weight distribution (MWD) of polyethylenes (PE) are important in determining their physical, mechanical, and rheological properties of end-products. In principle, molecular weight controls the mechanical properties of polymers, while MWD mainly affects the rheological properties. Therefore, it is necessary to control molecular weight and MWD to optimize the property and processibility of PE. The conventional Ziegler-Natta catalyst is still the main industrial catalyst for the production of PE. Hence, the control of MWD of PE produced using Ziegler-Natta catalyst has always been one of the most worthy research targets for industry and academy. The influence of the polymerization time and pressure on the molecular weight, MWD and relative MWD of PE in the ethylene polymerization with a MgCl2/SiO2/TiCl4/AlEt3 catalyst system in the slurry phase was studied by using gel permeation chromatography (GPC). It has been found that with the lapse of time increases the molecular weight and MWD of PE. However, the increase of the polymerization pressure increases the molecular weight and decreases the MWD of PE.