| Copolymers of ethylene with a-olefins (propylene,1-butene,1-hexene etc.) are important polymer materials with wide applications. At present, more than 80% of these copolymers are produced by MgCl2-supported Ziegler-Natta catalysts. As an important feature, Ziegler-Natta catalysts produce ethylene-a-olefin copolymer with very broad chemical composition distribution (CCD), which is caused by multiple active centers in the catalyst. In ethylene/a-olefin copolymerization with Ziegler-Natta catalysts, enhancement of reaction activity by the a-olefin comonomer (so-called "comonomer effect") and electron donor have long been reported and studied. Though several theories have been developed to rationalize phenomena, there is still not a generally accepted explanation or mechanistic model. In this work, MgCl2-supported Ziegler-Natta catalysts containing no internal donor or diethyl phthalate (DEP) and controlled Ti contents were prepared. Their microkinetics of ethylene/1-hexene copolymerization and the active center distribution, characterize labeled propagation chain ends after quenching the catalysis systems were studied. Based on the results of kinetic study and active center characterization on different systems, the structural features, surface locations and catalysis mechanism were clarified.Internal donor effects on ethylene/1-hexene copolymerization reactions using two MgCl2-supported Ziegler-Natta catalysts containing no internal donor or DEP were carried out over a broad range of co-monomer concentration from 0 to 0.5 mol/L. It was found that no matter which catalyst was used, activity of catalyst and content of boiling n-heptane soluble fraction of polymers increased with co-monomer concentration rising. Introducing DEP enhanced activity of catalyst and content of boiling n-heptane insoluble fraction. Tm and ΔHm of the products were all reduced when the co-monomer concentration increased, and were no significant differences between the polymers catalyzed by two above-mentioned catalysts. Number of polymerization active centers was determined by quenching the reaction with 2-thiophenecarbonyl chloride and measuring sulfur content of the quenched polymer. Each copolymer sample was fractionated into boiling n-heptane soluble and insoluble fractions, and active centers in these fractions were also counted. The rate constants of ethylene and 1-hexene insertion in the active centers were calculated, respectively. Molecular weight distribution (MWD) curves of the polymers were deconvoluted with 4-5 Flory components, and changes of activity of the Flory components with [1-hexene] were analyzed. Large number of active centers were revived by small amount of 1-hexene. With the increase of [1-hexene], the number of active centers producing polymer chains with lower molecular weight and higher 1-hexene content was increased more than those producing polymer chains with higher molecular weight and lower 1-hexene content, and the MWD curve continuously inclined to the low molecular weight side. The active centers with higher 1-hexene incorporation rate have relatively smaller rate constant of ethylene insertion. When [1-hexene] was increased, the rate constant of ethylene insertion was only slightly changed, but the rate constant of 1-hexene insertion was markedly lowered when there was no internal donor, meaning that the active centers revived by 1-hexene have relatively lower ability of incorporating 1-hexene. Introducing internal electron donor in the catalyst enhanced not only the number of active centers but also the chain propagation rate constant. The rate constant of ethylene insertion and the rate constant of 1-hexene insertion were all markedly lowered when introducing DEP. The internal donor weakened the comonomer effects to some extent, and changed the distribution of comonomer effects among different types of active centers.Internal donor effects on ethylene/1-hexene copolymerization reactions using two MgCl2-supported Ziegler-Natta catalysts containing no internal donor or DEP were also carried out over different polymerization time using the same methods. It was found that the activity of catalyst increased markedly in a very short period of time introducing the internal electron donor. The internal donor weakened the comonomer effects to some extent, and reducing the ratio of boiling n-heptane soluble fractions. Because of the internal donor, the active centers were more stable. In the initial stage reaction, DEP really affected the CA*. Ti(II) which were redused from high valence of Ti had very high complexing capacities with DEP in later stage reaction. It made the decrease of [C*]. Meanwhile, the rate constant of ethylene insertion and the rate constant of 1-hexene insertion all increased. It suggested that the deactivation happened to the active species with lower values of kp.By the end of the article, comonomer effects in copolymerization of ethylene and 1-hexene with three MgCl2-supported Ziegler-Natta catalysts with controlled Ti contents were investigated. It was found that the efficiency of catalyst increased when Ti content of catalyst increased. The copolymerization activity was higher than that of homopolymerization because of comonomer effect when Ti content of the catalyst was higher. It must be due to experimental mistakes when Ti content of the catalyst was very low that the copolymerization activity was lower than that of homopolymerization.The results obtained in this work can be new evidences for understanding the mechanism of the comonomer effect, and can provide clues for figuring out structure of different types of active centers. |
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