Effects of Internal Electron Donor on Distribution and Reactivity of Active Centers in Ethylene/1-Hexene Copolymerization with MgCl2-Supported Ziegler-Natta Catalyst

Abstract

Ethylene/1-hexene copolymerization with two MgCl₂-supported Ziegler-Natta catalysts containing no internal electron donor or diethylphthalate (DEP) is conducted for different polymerization time . Effects of DEP on active center distribution are studied by fractionating each copolymer sample into boiling n-heptane soluble (C7-sol) and insoluble (C7-ins) fractions, and counting the number of active centers in the copolymer fractions . The main effect of introducing DEP in the catalyst are reduction in the Ti content and significant increase in the proportion of active centers producing C7-ins fraction. The propagation rate constants of ethylene insertion (k_pE) and 1-hexene insertion (k_pH) are respectively estimated by linear fitting/extrapolating the change of apparent propagation rate constants (k_pi)_a with polymer yield according to a simplified multi-grain particle model. In both catalysts, k_pE in the C7-ins fraction is 9–12 times larger than that in the C7-sol fraction, and k_pH in the C7-ins fraction is 3–4 times larger than that in the C7-sol fraction. The two groups of active centers have distinctly different catalytic properties. Introducing DEP reduced the k_pE and k_pH values and the extent of diffusion limitation . In summary, addition of electron donor in MgCl₂-supported Z-N catalyst significantly changed the active center distribution and catalytic properties of its two groups of active centers.