Copolymerization of ethylene and alpha-olefins with combined metallocene catalysts. I. A formal criterion for molecular weight bimodality

Metallocene and other single-site catalysts can be combined to produce poly olefins with broadened distributions of molecular weight, chemical composit ion, and long-chain branching. These resins are finding increasing applicat ions because of their enhanced properties compared to ones made with conven tional Ziegler-Natta catalysts. Resins with bimodal molecular weight distri butions (MWDs) have especially attractive mechanical and rheological proper ties. Although the use of these resins is expected to increase, there are v ery few studies available to quantify MWD bimodality or to decide a priori which combinations of metallocene catalysts will lead to the formation of p olyolefins with bimodal MWDs. In this article, a necessary condition for th e production of polymer with bimodal MWD using two single-site-type catalys ts is derived. Additionally, a bimodality index is defined to quantify MWD bimodality. (C) 2000 John Wiley & Sons, Inc. J Polym Sci A: Poly Chem 38: 1 408-1416, 2000.