Comparative Study on Kinetics of Ethylene and Propylene Polymerizations with Supported Ziegler–Natta Catalyst: Catalyst Fragmentation Promoted by Polymer Crystalline Lamellae

Abstract

The kinetic behaviors of ethylene and propylene polymerizations with the same MgCl2-supported Ziegler–Natta (Z–N) catalyst containing an internal electron donor were compared. Changes of polymerization activity and active center concentration ([C*]) with time in the first 10 min were determined. Activity of ethylene polymerization was only 25% of that of propylene, and the polymerization rate (Rp) quickly decayed with time (tp) in the former system, in contrast to stable Rp in the latter. The ethylene system showed a very low [C*]/[Ti] ratio (<0.6%), in contrast to a much higher [C*]/[Ti] ratio (1.5%–4.9%) in propylene polymerization. The two systems showed noticeably different morphologies of the nascent polymer/catalyst particles, with the PP/catalyst particles being more compact and homogeneous than the PE/catalyst particles. The different kinetic behaviors of the two systems were explained by faster and more sufficient catalyst fragmentation in propylene polymerization than the ethylene system. The smaller lamellar thickness (<20 nm) in nascent polypropylene compared with the size of nanopores (15–25 nm) in the catalyst was considered the key factor for efficient catalyst fragmentation in propylene polymerization, as the PP lamellae may grow inside the nanopores and break up the catalyst particles.

Effects of titanium dispersion state on distribution and reactivity of active centers in propylene polymerization with MgCl2‐supported Ziegler‐Natta catalysts: A kinetic study based on active center counting

“…According to our previous work, PP lamellae with smaller thickness can enter nanopores in the catalyst particle more easily, and their expansion in the nanopores can promote release of the shielded active site precursors. In contrast, thickness of the PE lamellae is far larger than that of the PP lamellae, resulting in less efficient catalyst disintegration and insufficient release of shielded active sites …”

“…[39] In ethylene polymerization with Cat-1/TEA, the [C*]/[Ti] fraction rose from 25 % at 30 s to 60 % after 600 s polymerization, [32] but [C*]/[Ti] of propylene polymerization with this catalyst rose for more than 3 times in 600 s. The increase of [C*]/[Ti] with time has been explained by gradual exposure of shielded Ti species through disintegration of the catalyst particle by hydraulic forces of expanding propagation chains. [32,37,44,45] A possible example of shielded Ti species is shown in Scheme 2a, where the adsorbed TiCl 4 (B) on a MgCl 2 crystallite bridges with a neighboring MgCl 2 crystallite and becomes inaccessible to the cocatalyst. When the growing polymer chains on the nearby active centers expand the gap between the MgCl 2 crystallites, the bridging structure is broken, making the TiCl 4 (B) available to activation.…”

“…In contrast, thickness of the PE lamellae is far larger than that of the PP lamellae, resulting in less efficient catalyst disintegration and insufficient release of shielded active sites. [45] In summary, by comparing the time-dependent changes of active center distribution and polymer chain structure in propylene polymerization with the model catalysts, it becomes possible to make closer insights on the structure and properties of active centers in MgCl 2 -supported ZÀ N catalysts. Comparison of propylene and ethylene polymerization with the same model catalyst has also provided meaningful information for disclosing the catalyst structure and catalytic mechanism.…”

Kinetic and Thermal Study of Ethylene and Propylene Homo Polymerization Catalyzed by ansa-Zirconocene Activated with Alkylaluminum/Borate: Effects of Alkylaluminum on Polymerization Kinetics and Polymer Structure

“…Obtained polymers products were characterized by high temperature (120 °C) nuclear magnetic resonance spectroscopy ( 1 H-NMR and 13 C-NMR) using instrument Varian Mercury-300 spectrometer (Hangzhou, China) operating 75 MHz within acquisition time 3.0 s. To obtai 13 C-NMR spectra for quantitative determination, the estimated amount (2–3 mg) of relaxation agent Cr(acac) 3 was added to the sample tube, and 1,1,2,2-tetrachloroethane-d 2 was used as the solvent for NMR analysis at 120 °C [ 45 , 46 , 47 ].…”

Responses of a Supported Ziegler–Natta Catalyst to Comonomer Feed Ratios in Ethylene–Propylene Copolymerization: Differentiation of Active Centers with Different Catalytic Features

“…27 However, when the E/P ratio was increased further, the number of active centers grew rapidly until 4−5 times higher than that of homopolymerization. 23,32,61 Since the catalyst used in this work has nanopores with an average size of only about 2 nm (Table S11 and Figures S14 and S15), while the lamellar thickness of iPP is far larger than 5 nm, 61 it is difficult for the PP lamellae to grow inside the catalyst's nanopores to expand them for exposing all of the active-center precursors. When ethylene is introduced into the system, the copolymer chains form thinner lamellae (as reflected by their lower melting temperature) that can grow inside smaller nanopores of the catalyst, which cause a larger extent of particle fragmentation.…”