Simultaneous Deconvolution of the Bivariate Distribution of Molecular Weight and Chemical Composition of Polyolefins Made with Ziegler‐Natta Catalysts

Abstract

Polyolefins made with Ziegler-Natta catalysts have non-uniform distributions of molecular weight (MWD) and chemical composition (CCD). The MWD is usually measured by high-temperature gel permeation chromatography (GPC) and the CCD by either temperature rising elution fractionation (TREF) or crystallization analysis fractionation (CRYSTAF). A mathematical model is needed to quantify the information provided by these analytical techniques and to relate it to the presence of multiple site types on Ziegler-Natta catalysts. We developed a robust computer algorithm to deconvolute the MWD and CCD of polyolefins simultaneously using Flory's most probable distribution and the cumulative CCD component of Stockmayer's distribution, which includes the soluble fraction commonly present in linear low-density polyethylene (LLDPE) resins and have applied this procedure for the first time to several industrial LLDPE resins. The deconvolution results are reproducible and consistent with theoretical expectations.

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

“…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 …”

“…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…”

Origin of Chemical Composition of Ethylene/Propylene Copolymer Produced with Ziegler–Natta Catalyst

“…Moreover, their versatility and cost efficiency in existing plants keep the competitiveness of Ziegler–Natta catalysts even for ethylene‐based copolymer of certain grades. The multi‐site nature of Ziegler–Natta catalysts is essential for broad molecular weight distribution, while it simultaneously results in relatively broad chemical composition distribution . Copolymer produced with Ziegler–Natta catalysts is known to have a block‐wise chemical structure, in contrast to the random nature of metallocene‐based copolymer .…”

“…Moreover, states of active sites vary in the course of polymerization due to interaction/reactions with alkylaluminum activator and donors . Most of previous researches have assumed that states of active sites mainly affect the chemical composition and its distribution of produced copolymer . Kakugo et al studied the influence of the amount of an external donor on the chemical composition distribution of ethylene/propylene copolymer .…”

Simultaneous Deconvolution of MWD and CCD of Ethylene/1‐Olefin Copolymers Using Genetic Algorithm

“…However, the discrepancy in the estimated number of active site types and mass fractions of polymers produced on each active site type has been reported when MWD and CCD of the same sample are deconvoluted separately . Recently, the simultaneous deconvolution of MWD and CCD has been reported to be the effective approach to yield consistent results …”