Effects of comonomer on active center distribution of TCl4/MgCl2-AlEt3 catalyst in ethylene/1-hexene copolymerization

Ethylene/1-hexene copolymerization with MgCl2/TiCl4-AlEt3 catalyst has been conducted under different initial 1-hexene concentration (0-0.5 mol/L). Number of polymerization active centers was determined by quenching the reaction with 2-thiophenecarbonyl chloride and measuring sulfur content of the quenched polymer. Each copolymer sample was fractionated into boiling n-heptane soluble and insoluble fractions, and active centers in these fractions were also counted. The rate constants of ethylene and 1-hexene insertion in the active centers were calculated, respectively. Molecular weight distribution (MWD) curves of the polymers were deconvoluted with 4-5 Flory components, and changes of activity of the Flory components with [1-hexene] were analyzed. The polymerization activity and the number of active centers were significantly enhanced by increasing [1-hexene]. Large number of active centers were revived by small amount of 1-hexene. With the increase of [1-hexene], the number of active centers producing polymer chains with lower molecular weight and higher 1-hexene content was increased more than those producing polymer chains with higher molecular weight and lower 1-hexene content, and the MWD curve continuously inclined to the low molecular weight side. The active centers with higher 1-hexene incorporation rate have relatively smaller rate constant of ethylene insertion. When [1-hexene] was increased, the rate constant of ethylene insertion was only slightly changed, but the rate constant of 1-hexene insertion was markedly lowered, meaning that the active centers revived by 1-hexene have relatively lower ability of incorporating 1-hexene. (C) 2015 Elsevier B.V. All rights reserved.