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Comparing Long Chain Branching Mechanisms for Ethylene Polymerization with Metallocene Catalysts: What Simulated Microstructures Can Teach Us
- Author / Creator
- Abdulrahman Albeladi
The versatility of polyethylene stems from the diversity of its microstructure that can be characterized by its molecular weight averages and distribution, long chain branching, and short chain branching. In an ideal scenario, one would want to predict the polymerization parameters needed to produce a polyethylene with the microstructure required to meet specific application demands. Employing various mathematical and statistical models usually gives insights for the good design of polyethylenes.
Long chain branching is one of the important microstructural properties that influences polyethylene processability and mechanical properties. It is widely accepted that long chain branches (LCB) in polyethylene made with coordination catalysts are formed via terminal branching, which is a random intermolecular incorporation pathway for in-situ produced macromonomers. However, some publications in the literature have claimed that this mechanism disagrees with certain experimental observations, especially for slurry and gas phase polymerization processes. As a result, alternative mechanisms have been proposed to reconcile these differences.
We have chosen two alternative mechanisms for LCB formation: 1) carbon-hydrogen bond (C-H) activation, and 2) intramolecular branching pathway, to be compared with the conventional terminal branching mechanism. We developed simulation models for all three mechanisms in two different polymerization systems: semi-batch and continuous polymerizations. The main aim of this approach was to find out how the different long chain branching mechanisms were reflected in the microstructures of the formed polyethylene chains. Several researchers have proposed alternative mechanisms for this process, but
surprisingly none of them asked the question: If this mechanism is valid, how would if affect the microstructure of the formed polyethylene under different polymerization conditions?
Increasing the ethylene concentration in the semi-batch simulations led to a decrease in LCB frequencies for the terminal branching and intramolecular mechanisms, whereas no effect was observed on the C-H activation mechanism, which disagrees with most previous literature data. Another observation of importance was the evolution of LCB parameters with time and conversion, where the intramolecular mechanism showed almost no dependence on time or ethylene conversion. This is also in disagreement with the published experimental data for these systems.
For the simulation of continuous polymerization processes, an important observation was the comparison of the polydispersity (PDI) dependence on LCB parameters. Data reported in the literature showed that simulations based on the terminal branching mechanism follow the data much more adequately, whereas the C-H activation results follows the experimental data below a certain LCB frequency, but then completely deviates from it. The intramolecular simulation results have shown a poorer comparison than terminal branching, but better than C-H activation.
Therefore, our detailed simulations confirm that the terminal branching mechanism is indeed the most likely mechanism for LCB formation with ethylene when polymerized with coordination catalysts.
- Graduation date
- Fall 2018
- Type of Item
- Master of Science
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