ANALYSIS AND CONTROL OF THE MOLECULAR-WEIGHT AND CHEMICAL-COMPOSITIONDISTRIBUTIONS OF POLYOLEFINS MADE WITH METALLOCENE AND ZIEGLER-NATTA CATALYSTS

The final properties of polyolefins are closely related to their distr ibutions of molecular weight (MWD) and chemical composition(GCD). Poly olefins produced with conventional heterogeneous Ziegler-Natta catalys ts have broad MWDs and CCDs caused by the presence of several active-s ite types on these catalysts. These distributions can be adequately de scribed as a weighted superposition of narrower distributions valid fo r each catalytic site type. With the advent of metallocene catalysts, it is now possible to synthesize polyolefins with narrow MWDs and CCDs , allowing a much better control over the MWDs and CCDs of these polym ers. The selective combination of different metallocene types on a giv en support should permit the production of polyolefins with designed M WDs and CCDs from the knowledge of the MWDs and CCDs of polyolefins ma de with the individual metallocenes. This paper reviews some recent te chniques used to analyze the MWDs and CCDs of polyolefins made with mu ltiple-site-type catalysts and shows how to control the MWD of polyeth ylene made with a two-site-type silica-supported metallocene catalyst by varying the ethylene pressure during polymerization.