Optimization of Starting Material Composition in the Synthesis of Highly Active Magnesium Ethoxide-Supported Ziegler–Natta Catalysts for Propylene Polymerization

Abstract

We aim to deepen our understanding of the structure-performance relationship in Ziegler–Natta catalysts (ZNCs) for propylene polymerization. To this end, three different Mg(OEt)₂ supports were synthesized by reacting magnesium powder with varying amounts of MnCl₂ during the magnesium-ethanol reaction stage. Despite similar XRD patterns among all obtained Mg(OEt)₂ samples, at an optimal MnCl₂/Mg ratio of 2 g/g, well-spherically shaped Mg(OEt)₂ particles with plate-like building units were formed. The prepared support precursors were then used in ZNC synthesis with different TiCl₄/Mg(OEt)₂ ratios and internal donor types, and their behavior in propylene polymerization was thoroughly explored. The overall results indicated that the optimal Ti/Mg ratio is 8 mol/mol. Additionally, phthalate as an internal donor demonstrated higher activity compared to diethyl ether or succinate-based internal donors. The kinetic curve of propylene uptake versus time in polymerization experiments with varying external donor content was mathematically modeled to evaluate changes in rate constants. The calculated rate constants showed that as the external donor content increases, k_trH, k_d, k_iED and k_tED all enhance, while k_p decreases significantly due to the poisoning of active sites by the external donor. These findings highlight the critical role of the type and amount of catalyst components in achieving optimal performance.