Effects of ethylene as comonomer on the active center distribution of 1-hexene polymerization with MgCl2-supported Ziegler–Natta catalysts

1-Hexene homopolymerization and 1-hexene–ethylene copolymerization with TiCl4/MgCl2–Al(C2H5)3 catalyst were compared to investigate the effect of ethylene on the distribution of active centers. The polymerizations were quenched with cinnamoyl chloride, and the number of active centers ([C*]/[Ti]) was determined by measuring the cinnamoyl group labeled on the propagation chains. Both polymer samples were fractionated into 9–10 fractions according to molecular weight, and [C*]/[Ti] in each fraction was also determined. Adding small amount of ethylene in 1-hexene polymerization markedly increased the number of active centers that produce low molecular weight polymer. This phenomenon agrees with the mechanism suggesting the presence of Ti−CH(CH3)(CH2)3CH3 type dormant sites and their activation by ethylene. The kp value of the newly emerging active centers in the copolymerization system is much lower than that of the homopolymerization system. In the copolymerization system, the active centers producing polymer chains of the second highest molecular weight and isotacticity show the highest kp value, while those producing polymer of the highest molecular weight and isotacticity show only the second highest kp. On the other hand, the active centers producing polymer with lower–middle chain length show the lowest ethylene incorporation rate. These results disclose differences of catalytic properties between the multiple active center types and correlations between their different kinetic parameters, which may lead to new understanding of the active centers and polymerization mechanism.

Graphical abstractThe effects of ethylene on the active center distribution of 1-hexene polymerization with TiCl4/MgCl2−Al(C2H5)3 catalyst were studied by counting the number of active centers in polymer fractions of different molecular weight. The results disclose differences of catalytic properties between the multiple active centers and correlations between their kinetic parameters.Figure optionsDownload full-size imageDownload high-quality image (90 K)Download as PowerPoint slideHighlights► 1-Hexene is polymerized with TiCl4/MgCl2–Al(C2H5)3 catalyst. ► Active center distribution is studied by counting active centers in polymer fractions. ► Effects of adding ethylene on the active center distribution are studied. ► Ethylene increases active centers in the low molecular weight fractions. ► Ethylene increases kp value of the medium–high molecular weight fractions.