EFFECTS OF ELECTRON DONORS ON ZIEGLER-NATTA CATALYZED ETHYLENE POLYMERIZATION AND COPOLYMERIZATION WITH 1-HEXENE BY QUANTUM CHEMICAL CALCULATIONS

Abstract

The investigation of ethylene insertion reaction pathway using Ziegler-Natta catalyst (ZNc) with the presence and absence of electron donors (EDs) systems on the (110) MgCl2 surface of the ethylene homopolymerization and ethylene/1-hexene copolymerization was carried out within the density functional theory (DFT) calculations. Firstly, the ethylene homopolymerization where EDs generated from mixing Mg(OEt)2 + SiCl4 + TiCl4 are Si(OEt)mCln (m + n = 4) in the catalytic system was considered. The coadsorption of four different EDs on the Mg atoms of catalyst surface was also investigated by the adsorption energy (Eads). The calculation shows the strongest coadsorption of Si(OEt)4 on (110) MgCl2 surface with the preferential bidentate mode. The potential energy surface (PES) maps indicated that the reaction mechanism of ethylene insertion to Ti-C bond at active center is pseudo-concerted mechanism. We found that the EDs are not tended to decrease the (intrinsic) activation energy (Ea) but they stabilize the electron donor complexes and π-complexes by decreasing the apparent activation energy (Eaa). The obtained Eaa values found in the order of absence-ED > SiOEtCl3 > Si(OEt)2Cl2 > Si(OEt)3Cl > Si(OEt)4, respectively. Therefore, presence EDs in ZNc system can be lowered the Eaa where Si(OEt)4 is found to be the most suitable ED for ethylene homopolymerization. Secondly, the ethylene insertion pathway to the Ti-C bond at the Ti-HE (H=hexane, E=ethylene) growing chain with presence and absence of EDs systems in the ethylene/1-hexene copolymerization was investigated. Based on experimental data, the presence of Si(OEt)2(OtBu)(OiPr) donor molecule is the most active catalyst in which the strongest coadsorption was found. The optimized geometry was found in the preferential bidentate binding mode on (110) MgCl2 surface. The calculation here revealed that the C-C double bond of dynamic ethylene molecule can formed the stable π-complexes either orthogonal or parallel orientation to the Ti-C bond. Finally, the Si(OEt)2(OtBu)(OiPr) complex is the most active catalyst system due to the lowest Ea found from insertion of dynamic ethylene to either back and front side to the Ti-C bond at active center.