Silica‐supported (nBuCp)2ZrCl2: effect of catalyst active center distribution on ethylene–1‐hexene copolymerization |
| |
| Authors: | Muhammad Atiqullah Siripon Anantawaraskul Abdul‐Hamid M Emwas Mamdouh A Al‐Harthi Ikram Hussain Anwar Ul‐Hamid Anwar Hossaen |
| |
| Affiliation: | 1. Center for Refining and Petrochemicals, Research Institute, King Fahd University of Petroleum & Minerals, , Dhahran, 31261 Saudi Arabia;2. Center of Research Excellence in Petroleum Refining and Petrochemicals, King Fahd University of Petroleum & Minerals, , Dhahran, 31261 Saudi Arabia;3. Department of Chemical Engineering, Kasetsart University, , Jatujak, Bangkok, 10900 Thailand;4. NMR Core Laboratory, King Abdullah University of Science & Technology, , Thuwal, 23955 Saudi Arabia;5. Department of Chemical Engineering, King Fahd University of Petroleum & Minerals, , Dhahran, 31261 Saudi Arabia;6. Center for Engineering Research, Research Institute, King Fahd University of Petroleum & Minerals, , Dhahran, 31261 Saudi Arabia |
| |
| Abstract: | Metallocenes are a modern innovation in polyolefin catalysis research. Therefore, two supported metallocene catalysts—silica/MAO/(nBuCp)2ZrCl2 (Catalyst 1) and silica/nBuSnCl3/MAO/(nBuCp)2ZrCl2 (Catalyst 2), where MAO is methylaluminoxane—were synthesized, and subsequently used to prepare, without separate feeding of MAO, ethylene–1‐hexene Copolymer 1 and Copolymer 2, respectively. Fouling‐free copolymerization, catalyst kinetic stability and production of free‐flowing polymer particles (replicating the catalyst particle size distribution) confirmed the occurrence of heterogeneous catalysis. The catalyst active center distribution was modeled by deconvoluting the measured molecular weight distribution and copolymer composition distribution. Five different active center types were predicted for each catalyst, which was corroborated by successive self‐nucleation and annealing experiments, as well as by an extended X‐ray absorption fine structure spectroscopy report published in the literature. Hence, metallocenes impregnated particularly on an MAO‐pretreated support may be rightly envisioned to comprise an ensemble of isolated single sites that have varying coordination environments. This study shows how the active center distribution and the design of supported MAO anions affect copolymerization activity, polymerization mechanism and the resulting polymer microstructures. Catalyst 2 showed less copolymerization activity than Catalyst 1. Strong chain transfer and positive co‐monomer effect—both by 1‐hexene—were common. Each copolymer demonstrated vinyl, vinylidene and trans‐vinylene end groups, and compositional heterogeneity. All these findings were explained, as appropriate, considering the modeled active center distribution, MAO cage structure repeat units, proposed catalyst surface chemistry, segregation effects and the literature that concerns and supports this study. While doing so, new insights were obtained. Additionally, future research, along the direction of the present work, is recommended. © 2013 Society of Chemical Industry |
| |
| Keywords: | supported metallocene catalyst ethylene– 1‐hexene copolymerization MWD and CCD deconvolution Schulz– Flory/ Stockmayer active center distribution MAO cage structure segregation effects SSA fractionation end‐group unsaturation |
|
|
