Effects of comonomer on active center distribution of TCl4/MgCl2–AlEt3 catalyst in ethylene/1-hexene copolymerization

J. Polym. Sci. Part A: Polym. Chem.

et al. 2016

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A series of ethylene, propylene homopolymerizations, and ethylene/propylene copolymerization catalyzed with rac‐Et(Ind)2ZrCl2/modified methylaluminoxane (MMAO) were conducted under the same conditions for different duration ranging from 2.5 to 30 min, and quenched with 2‐thiophenecarbonyl chloride to label a 2‐thiophenecarbonyl on each propagation chain end. The change of active center ratio ([C*]/[Zr]) with polymerization time in each polymerization system was determined. Changes of polymerization rate, molecular weight, isotacticity (for propylene homopolymerization) and copolymer composition with time were also studied. [C*]/[Zr] strongly depended on type of monomer, with the propylene homopolymerization system presented much lower [C*]/[Zr] (ca. 25%) than the ethylene homopolymerization and ethylene–propylene copolymerization systems. In the copolymerization system, [C*]/[Zr] increased continuously in the reaction process until a maximum value of 98.7% was reached, which was much higher than the maximum [C*]/[Zr] of ethylene homopolymerization (ca. 70%). The chain propagation rate constant (kp) of propylene polymerization is very close to that of ethylene polymerization, but the propylene insertion rate constant is much smaller than the ethylene insertion rate constant in the copolymerization system, meaning that the active centers in the homopolymerization system are different from those in the copolymerization system. Ethylene insertion rate constant in the copolymerization system was much higher than that in the ethylene homopolymerization in the first 10 min of reaction. A mechanistic model was proposed to explain the observed activation of ethylene polymerization by propylene addition. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 867–875

Section: Introductionmentioning

confidence: 71%

Kinetics and mechanism of metallocene‐catalyzed olefin polymerization: Comparison of ethylene, propylene homopolymerizations, and their copolymerization

J. Polym. Sci. Part A: Polym. Chem.

et al. 2016

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“…In ethylene polymerization with MgCl 2 ‐supported Ziegler–Natta catalysts, the activity can be markedly enhanced by adding a small amount of α‐olefin comonomer like propylene, 1‐butene, or 1‐hexene . In order to synthesize PE at higher polymerization activity, small amount of propylene was added to the ethylene polymerization system after the prepolymerization step, and ethylene–propylene copolymerization was conducted for 30 min.…”

Section: Resultsmentioning

confidence: 92%

Particle morphology and morphogenesis of nascent polyethylene produced with a spherical MgCl₂‐supported Ziegler–Natta catalyst in slurry process

J of Applied Polymer Sci

Yang

et al. 2017

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Ethylene polymerizations were conducted in slurry process with a spherical MgCl 2 -supported Ziegler-Natta catalyst activated by triethylaluminum, and the morphology of nascent polymer particles was observed by scanning electron microscope. Three kinds of typical microscale morphologies: nodular and rope-like structures on the external particle surface, and nodular structures in the bulk of the particle were observed. These structures are composed of polyethylene lamellae and amorphous phase filling the space between the lamellae. There is close relation between the polymerization activity and microporosity of the particles. When the activity was higher than 1500 g PE/g Cat, large number of tiny pores appeared in the particles, and rope-like structures appeared on their surface. Formation of the morphologies can be reasonably explained by models based on multiscale solid structure of the catalyst particle and dynamic interactions among the growing polymer phases.

“…This strong activation effect could be attributed to the so-called comonomer effect, which has been reported in literatures before. [30][31][32][33][34][35] The SEM pictures of typical PE particles are shown in Figure 4. It is seen that size of the biggest particle was about 1.5 mm [ Figure 4(a)], which was five times bigger than that of the biggest PE particles produced without prepolymerization.…”

Section: Article Wileyonlinelibrarycom/appmentioning

confidence: 99%

“…In our previous works, increasing the concentration of a-olefin comonomer was found to continuously increase the activity of ethylene/a-olefin copolymerization with a supported Z-N catalyst. 34,35 On the other hand, increasing the comonomer content in ethylene/a-olefin copolymer will reduce its crystallinity, 35 thus changing the physical and morphological characteristics of the polymer particles. In this section, we studied the effects of adding designed amount of propylene into the reactor after the prepolymerization step just before introducing ethylene.…”

Section: Effect Of Propylene As Comonomer On the Polymerizationmentioning

confidence: 99%

Millimeter‐size polyethylene hollow spheres synthesized with MgCl₂‐supportedZiegler‐Natta catalyst

J of Applied Polymer Sci

et al. 2015

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Polyethylene hollow spheres with diameters of 0.4–2 mm were synthesized by a two‐step slurry polymerization in a single reactor with a spherical MgCl2‐supported Ziegler‐Natta catalyst activated by triethylaluminum, in which the first step was prepolymerization with 0.1 MPa propylene and the second step was ethylene polymerization under 0.6 MPa. The prepolymerization step was found necessary for the formation of hollow spherical particles with regular shape (perfectly spherical shape). The effects of adding small amount of propylene (propylene/ethylene < 0.1 mol/mol) in the reactor after the prepolymerization step were investigated. Average size of the polymer particles was increased, and the polymerization rate was markedly enhanced by the added propylene. Development of the particle morphology with polymerization time was also studied. The polymer particles formed by less than 20 min of ethylene polymerization showed hollow spherical morphology with thin shell layer. Most of the particles had ratio of shell thickness/particle radius smaller than 0.5. By prolonging the ethylene polymerization, the shell thickness/particle radius ratio gradually approached 1, and the central void tended to disappear. Central void in polymer particles formed from smaller catalyst particles disappeared after shorter time of polymerization than those formed from bigger catalyst particles. The shell layer of the hollow particles contained large number of macro‐, meso‐ and micro‐pores. The mesopore size distributions of four typical samples were analyzed by nitrogen adsorption–desorption experiments. A simplified multigrain model was proposed to explain the morphogenesis of the hollow spherical particles. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43207.