Effects of Molecular Dispersion State of Surface Ti Species on Ethylene-Propylene Copolymerization with TiCl_3-based Ziegler-Natta Model Catalyst

The dispersion state of surface Ti species is one of the most important factors for polymerization properties of MgCl_2-supported heterogeneous Ziegler-Natta catalysts. In this paper, ethylene-propylene copolymerization was conducted using a novel Ziegler-Natta model catalyst having “isolated” and “clustered” TiCl_3 molecules on MgCl_2. At a lower ethylene/propylene feed ratio, the dispersion state of the Ti species was found to stronglyaffect both the composition and sequence distribution of copolymers. The “isolated” Tispecies gave a relatively random copolymer and clustering of Ti species obviously enhanced the blockiness. The addition of an external donor basically deteriorated the random copolymerization character of the “isolated Ti species, but the degree of the deterioration depended on the kind of donors. On the contrary, the balance between themonomer feed and consumption dominated the copolymerization performance at a higher ethylene/propylene feed ratio to obscure the effects of the dispersion state