Effects of ethylene as comonomer on the active center distribution of 1-hexene polymerization with MgCl 2-supported Ziegler–Natta catalysts

Abstract

1-Hexene homopolymerization and 1-hexene–ethylene copolymerization with TiCl 4/MgCl 2–Al(C 2H 5) 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(CH 3)(CH 2) 3CH 3 type dormant sites and their activation by ethylene. The k p 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 k p value, while those producing polymer of the highest molecular weight and isotacticity show only the second highest k p. 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.