(A) study on the mechanism of ethylene copolymerization and the structure of ethylene copolymer over ziegler - natta catalyst

Abstract

The mechanism of ethylene copolymerization and the microstructure of ethylene copolymer over Ziegler-Natta catalyst were investigated through ethylene-propylene copolymerization over the well defined catalysts of $TiCl_30.3AlCl_3$ ($TiCl_3$(AA)) and $TiCl_3$(AA)/$3MgCl_2$/ THF (T3ME and T3MEDX, X=2, 4,8,12) catalytic system with different states of $Ti^{+3}$ species respectively. $TiCl_3$(AA)/$3MgCl_2$/THF(T3ME) catalyst was prepared by reacting $TiCl_3$(AA) and $MgCl_2$ in the solution of tetrahydrofuran (THF), and THF was removed from T3ME by diethylaluminum chloride to make four different catalysts with different THF content (T3MED2, T3MED4, T3MED8, T3MED12). The investigation of $TiCl_3$(AA)/$3MgCl_2$/THF system throgh FT-IR, XRD and ESR indicated that the removal of THF resulted in the change of the state of $Ti^{+3}$ species; isolated $Ti^{+3}$ species for T3ME, T3MED2 and T3MED4 catalysts and multinuclear $Ti^{+3}$ species for T3MED12 and $TiCl_3$(AA) catalysts. The $^{13}C$-NMR study showed that the microstructure of ethylenepropylene copolymers strongly depended upon the nature of $Ti^{+3}$. Multinuclear $Ti^{+3}$ species increased the relative reactivity of propylene in the copolymerization and isolated $Ti^{+3}$ species with vacancies favored the random insertion of propylene in the ethylene main chain. The activity and stereospecificity for homopolymerization depended on the THF content of the catalyst and the molecular weight distribution of copolymer became broader with THF removal. Ethylene-propylene copolymerization mechanism of $TiCl_3$/$MgCl_2$/THF system was investigated by analyzing the microstructure of copolymer. The dyad-triad sequence distributions in ethylene-propylene copolymers were predicted on the assumption that the copolymerization was a first order Markovian process. The sequence distributions in ethylene-propylene copolymer determined by $^{13}C$-NMR were well simulated by a two-site model. The bimodal short chain branching dist...