Employing Moment Equations Model to Study the Effect of Different Active Centers on Homopolymerization Kinetics of Ethylene
Ethylene was homopolymerized over Ziegler-Natta catalyst and the homopolymerization was modeled using moment equations. Mechanism was modeled according to five different reaction centers of catalyst. For each center, there are different reaction rate coefficients; therefore the final product of each center would be expected to be different. Modeling results showed good conformity to the experimental results. According to the results obtained, the molecular weight distribution of each active center follows a Schultz-Flory distribution. However, the molecular weight distribution of polymer produced is much broader than a Schultz-Flory distribution. Besides, the order of polymerization with regards to monomer concentration is different for each center and it is higher than unity. Moreover, the catalyst active centers deteriorate in the presence of hydrogen and consequently catalyst yield drops. Nevertheless, polymerization kinetics is not affected much by hydrogen. Hydrogen also reduces polymer molecular weight since it is a strong transfer agent in olefin polymerizations. Notwithstanding, it does not affect polydispersity index. Finally, by increasing the cocatalyst concentration the activity of active centers is not changed, while it lessens the molecular weight as a transfer agent.
کلیدواژه ها [English]
Polyethylene, Ziegler-Natta catalyst, modeling, moment equations, homopolymerization