Ethylene-propylene copolymers were synthesized using a TiCl4／MgCl2／SiO2／diester type supported Ziegler-Natta catalyst with or without the presence of hydrogen in a slurry polymerization process·The copolymer was inhomogeneous in composition and can be fractionated into two parts

fractions soluble and insoluble in n-octane at room temperature

respectively．The soluble part is a nearly random copolymer

while the insoluble part is a kind of segmented copolymer

as revealed by 13C—NMR and DSC analyses．Adding H2in the reaction system caused an increase of copolymer yield and a decrease of the insoluble part

but the propylene content and average length of propylene segments of the insoluble part were markedly increased

meanwhile the average length of ethylene segments of the insoluble part was markedly decreased．Copolymer sequence distribution of the soluble part was only slightly changed by H2．The segmented copolymer fraction present in this ethylene-propylene copolynmr has the similar structure as the segmented copolymer fractions found in polypropylene／poly(ethylene—-copropylene)reactor alloy. Mechanism of the hydrogen effect on copolymer structure is discussed．