Two catalysts, denoted as catalyst 1 [silica/MAO/(nBuCp) 2ZrCl2] and catalyst 2 [silica/nBuSnCl 3/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-1-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, nE MPDSC-GT and n E MPNMR-Flory, as well as the weight-average lamellar thicknesses, Lwav DSC-GT and Lwav SSA 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 LMPDSC-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-α-olefin linear low-density polyethylenes (LLDPEs). © 2013 American Chemical Society.
Bibliographical noteKAUST Repository Item: Exported on 2020-10-01
Acknowledgements: The authors acknowledge the financial support provided by King Abdulaziz City for Science and Technology (KACST) via the Science & Technology Unit at King Fahd University of Petroleum & Minerals (KFUPM) through Project 08-PET90-4 as part of the National Science and Technology Innovation Plan. The technical assistance provided by the Center of Refining & Petrochemicals (CRP), the Center for Engineering Research at Research Institute, the Center of Research Excellence in Petroleum Refining & Petrochemicals (CoRE-PRP), and the Department of Chemical Engineering, KFUPM, Dhahran, Saudi Arabia; the NMR Core Laboratory, King Abdullah University of Science & Technology (KAUST), Thuwal, Saudi Arabia; and the Department of Chemical Engineering, Kasetsart University, Bangkok, Thailand, is also gratefully acknowledged. The technical assistance of Mr. Sagir Adamu is also appreciated.