This work attempts to prepare ethylene-a-olefin copolymer with relatively narrow composition distribution by modification of TiCl4/MgCl2 type supported Ziegler-Natta catalyst. A TiCl4/ID/MgCl2 type catalyst, where ID is a diester, was treated at room temperature with 2,6-'Pr2C6H3-OH at phenol: Ti molar ratio of 1:1 and the modified catalyst was analyzed for contents of titanium and aryloxy group. It was found that about one third of the titanium atoms in the modified catalyst (M-cat) was connected with an aryloxy group, but its Ti content was decreased to 1.44 wt% from 2.36 wt% of the original catalyst (O-cat). Ethylene-1-hexene copolymerization was conducted in n-heptane with M-cat or O-cat as catalyst and triisobutyl aluminium or methylaluminoxane as cocatalyst. The M-cat showed similar activity as the O-cat in catalyzing the copolymerization, but presented a weaker comonomer activation effect than the latter. The 1-hexene content of copolymer synthesized by M-cat was lower than that of the product of O-cat at the same conditions, but had a markedly narrower composition distribution. Each copolymer sample was fractionated into two fractions by extraction with boiling n-heptane, and the soluble part contains much more 1-hexene units than the insoluble part. The ratio of I-hexene content in the soluble fraction to that of the insoluble fraction was higher than 18 in copolymer prepared with O-cat, but this ratio was reduced to 3.7 when M-cat was used as the catalyst. On the other hand, the soluble part of copolymer prepared with M-cat showed a more random sequence distribution than that of the O-cat. The distributions of active centers in both the M-cat and O-cat were studied by deconvolution of the polymer's molecular weight distribution curves with multiple Flory most-probable distribution function. Both catalysts showed a broad molecular weight distribution that can be ascribed to 5 similar to 6 types of active centers, but the introduction of aryloxy group in the catalyst caused marked changes in the active center distribution. Possible mechanism of the catalyst modification was proposed.
