Long Chain Branching in Polyethylene from the Phillips Chromium Catalyst

Abstract

Supported chromium oxide polymerization catalysts can impart significant levels of long chain branching (LCB) to polyethylene made in a slurry process. LCB often dominates the rheological behavior of the polymer, and is thus responsible for the performance of many HDPE grades during commercial molding operations. LCB is difficult to accurately measure but its presence can be inferred from the visco‐elastic character of the polymer, relative to its molecular weight (MW) and MW distribution. Depending on catalyst choice, LCB levels from chromium oxide catalysts can range from near zero, to very high, rivaling the tightly bridged metallocenes. The most common catalyst and reactor variables were investigated with respect to their effect on LCB formation. The simple “macromonomer insertion” view does not in itself account for many of these LCB responses. In fact, no overall correlation was found between LCB and the catalyst's ability to incorporate large comonomers. Instead, variables that influenced LCB most also govern the active site density, including activation temperature, chromium loading, and the presence of cocatalysts or poisons in the reactor. The physical structure of the catalyst support also had a strong influence on the elastic behavior of the polymer, independent of any effect on MW or MW distribution. These and other variables are discussed.