Effect of alkylaluminum cocatalyst on ethylene/1-hexene copolymerization and active center distribution of MgCl2-supported Ziegler-Natta catalyst

Abstract

Ethylene-1-hexene copolymerization was catalyzed by TiCl4/diester/MgCl2 type Z-N catalyst activated by triethylaluminium (TEA), triisobutylaluminium (TIBA) or TEA/TIBA mixtures. Copolymerization activity decreased with increase of TIBA content in the cocatalyst. Each copolymer sample was fractionated into boiling n-heptane soluble and insoluble fractions. 1-Hexene content of the soluble fraction was 2 ∼ 6 times larger than that of the insoluble fraction. With introduction of TIBA in the cocatalyst, chemical composition distribution of the copolymer was broadened, attributable to increase of 1-hexene in the soluble fraction. Total number of active centers markedly decreased with increasing TIBA in cocatalyst, and active center number in the soluble fraction decreased more than that in the insoluble fraction. Rate constant of ethylene insertion (k pE) in the soluble fraction was much smaller than that of the insoluble fraction. Rate constant of 1-hexene insertion (k pH) of the soluble fraction became far larger than that of the insoluble fraction when TEA/TIBA ratio was lower than 80/20. In the presence of TIBA, only a part of active center precursors with stronger ability of incorporating 1-hexene were effectively activated in the soluble fraction. For the active centers in insoluble fraction, cocatalyst structure exerted more complicated effects on their activation. Active center models were proposed to explain the observed cocatalyst effects.