INFLUENCE OF ELECTRON-DONORS ON THE TACTICITY AND THE COMPOSITION DISTRIBUTION OF PROPYLENE-BUTENE COPOLYMERS PRODUCED BY SUPPORTED ZIEGLER-NATTA CATALYSTS

Four copolymer samples of propylene with a small amount of butene were prepared by different MgCl2-supported Ziegler-Natta catalysts and wer e fractionated by using the preparative temperature rising elution fra ctionation (TREF) technique. Some selected fractions were characterize d by C-13 NMR. The data obtained suggest that the fractionation is mai nly conducted according to tacticity. The influence of electron donors on the tacticity distribution and the composition distribution was in vestigated. It is found that both internal and external donors increas e the isotacticity and narrow the tacticity distribution of copolymers but do not narrow the composition distribution of copolymers, though the butene content is reduced. C-13-NMR results show that electron don ors have a larger effect on the composition of the fractions of highes t and lowest isotacticity, leading to a different dependence of compos ition on elution temperature from that of tacticity. This further conf irms the supposition proposed in our previous work that electron donor s can be present in the environment of both aspecific and isospecific active sites. The differences between internal and external donors wer e also compared. It is observed that internal and external donors chan ge the proportion of fractions differently and that an external donor decreases the butene content of fractions more effectively.