Effects of Supported (nBuCp)2ZrCl2 Catalyst Active-Center Distribution on Ethylene鈥?-Hexene Copolymer Backbone Heterogeneity and Thermal Behaviors

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• 作者：Muhammad Atiqullah ; Siripon Anantawaraskul ; Abdul-Hamid M. Emwas ; Mamdouh A. Al-Harthi ; Ikram Hussain ; Anwar Ul-Hamid ; Anwar Hossaen
• 刊名：Industrial & Engineering Chemistry Research
• 出版年：2013
• 出版时间：July 10, 2013
• 年：2013
• 卷：52
• 期：27
• 页码：9359-9373
• 全文大小：640K
• 年卷期：v.52,no.27(July 10, 2013)
• ISSN：1520-5045

文摘

Two catalysts, denoted as catalyst 1 [silica/MAO/(ⁿBuCp)₂ZrCl₂] and catalyst 2 [silica/ⁿBuSnCl₃/MAO/(ⁿBuCp)₂ZrCl₂] were synthesized and subsequently used to prepare, without separate feeding of methylaluminoxane (MAO), ethylene homopolymer 1 and homopolymer 2, respectively, and ethylene鈥?-hexene copolymer 1 and copolymer 2, respectively. Gel permeation chromatography (GPC), Crystaf, differential scanning calorimetry (DSC) [conventional and successive self-nucleation and annealing (SSA)], and ¹³C nuclear magnetic resonance (NMR) polymer characterization results were used, as appropriate, to model the catalyst active-center distribution, ethylene sequence (equilibrium crystal) distribution, and lamellar thickness distribution (both continuous and discrete). Five different types of active centers were predicted in each catalyst, as corroborated by the SSA experiments and complemented by an extended X-ray absorption fine structure (EXAFS) report published in the literature. ¹³C NMR spectroscopy also supported this active-center multiplicity. Models combined with experiments effectively illustrated how and why the active-center distribution and the variance in the design of the supported MAO anion, having different electronic and steric effects and coordination environments, influence the concerned copolymerization mechanism and polymer properties, including inter- and intrachain compositional heterogeneity and thermal behaviors. Copolymerization occurred according to the first-order Markovian terminal model, producing fairly random copolymers with minor skewedness toward blocky character. For each copolymer, the theoretical most probable ethylene sequences, n_E聽MPDSC-GT and n_{E聽MPNMR-Flory}, as well as the weight-average lamellar thicknesses, L_{wav聽DSC鈥揋T} and L_{wav聽SSA聽DSC}, were found to be comparable. To the best of our knowledge, such a match has not previously been reported. The percentage crystallinities of the homo- and copolymers increased linearly as a function of L_MPDSC-GT. This indicates that the homo- and copolymer chains folded excluding the butyl branch. The results of the present study will contribute to developing future supported metallocene catalysts that will be useful in the synthesis of new grades of ethylene鈭捨?olefin linear low-density polyethylenes (LLDPEs).