Measurement and mathematical modeling of molecular weight and chemical composition distributions of ethylene/-olefin copolymers synthesized with a heterogeneous Ziegler-Natta catalyst

Abstract

The distributions of molecular weight and chemical composition of two linear low‐density polyethylene resins synthesized with Ziegler‐Natta catalysts were analyzed with several microstructural characterization techniques and the results were used to investigate the nature of the active site types present on the catalyst. Deconvolution of the molecular weight distribution measured by gel permeation chromatography and of the chemical composition distribution measured by either crystallization analysis fractionation or temperature rising elution fractionation leads to consistent results that indicate the presence of multiple active site types on the catalyst. This shows that these fractionation techniques, when combined with adequate mathematical models, are powerful tools to understand the behavior of Ziegler‐Natta catalysts and structural properties of the produced resins.

Effect of Operating Conditions on Dynamic Crystallization of Ethylene/1‐Octene Copolymers

“…[ 1,2 ] Furthermore, the CCD can also be used to investigate the nature of multiple-site-type catalysts and the effect a single-site-type catalyst; therefore, the distributions of molecular weight and chemical composition are narrow and unimodal, and MWD polydispersity is close to 2.0. The sample had 1.86 mol% of 1-octene and a number-average molecular weight (M n ) of 31 600 g mol −1 .…”

Simultaneous Deconvolution of Molecular Weight and Chemical Composition Distribution of Ethylene/1‐Olefin Copolymers: Strategy Validation and Comparison

“…Ideally, the deconvolution of any weight distribution function (Equation 5–8) should identify the same number of site types and yield the same set of mass fractions and kinetic parameters for each site type. However, independent deconvolutions of MWD and CCD were reported to lead to a different number of site types and slightly different set of mass fractions and kinetic parameters 19. Differences in site type number are expected, as some site types may produce chains with similar average molecular weights but different comonomer contents, or vice versa .…”

“…The termination criterion is that when the number of site types increase, the value of the objective function increased, stopped decreasing, or did not decrease significantly. This criterion is commonly used for the deconvolution of microstructural properties distributions 9, 11, 12, 19, 20, 23–25 …”

“…The deconvolution of MWD obtained from gel permeation chromatography (GPC)9–18 and CCD obtained from temperature rising elusion fractionation (TREF) or crystallization analysis fractionation (Crystaf)19–22 has been investigated in several previous publications. However, discrepancies between the number of site types and kinetic parameters were observed when the MWD and CCD of the same sample were deconvoluted separately,19 possibly because some site types may produce chains with similar molecular weight averages but different mean comonomer content or vice versa, suggesting that molecular weight and comonomer composition information should be considered at the same time to yield more consistent results. The simultaneous deconvolution of MWD and CCD (MWD + CCD) have been investigated in our research group to address this issue 23–26…”

Exploring Reaction Kinetics of a Multi‐Site Ziegler‐Natta Catalyst Using Deconvolution of Molecular Weight Distributions for Ethylene‐Hexene Copolymers

“…Kissin and others have found that five to seven Flory components are often needed to adequately match the MWD of polyolefins produced using Ziegler‐Natta catalysts 4,10,12,13. However, it is widely believed that there are not this many chemically distinct types of active sites on Ziegler‐Natta catalysts.…”