A Second Look at Modeling the Multiplicity of Active Site Types of Ziegler-Natta Catalysts with Flory's and Stockmayer's Distributions

Abstract

ABSTRACTThe presence of multiple site types on the surface of heterogeneous Ziegler-Natta catalysts is generally used to explain the production of polyolefins with broad and sometimes multimodal distributions of molecular weight (MWD) and chemical composition (CCD). These distributions can be modelled as a superposition of several Flory's and Stockmayer's distributions for each site type. However, several individual distributions are generally needed to properly describe MWD and CCD, making it difficult to assign these distributions to chemically distinct catalyst sites. In the model proposed herein, Flory's and Stockmayer's distributions are used to explain the underlying polymerization mechanism on each site type, but catalyst site-surface interactions are considered to broaden these distributions. As a result, fewer Flory's and Stockmayer's distributions are required to describe broad and multimodal MWDs and CCDs.