Ethylene copolymer having multiple pitch molecular weight distribution and the method of preparing the same

A method for producing an ethylene copolymer having a multimodal molecular weight distribution, the method being carried out by a multistage solution polymerization using at least three polymerization reactors adiabatically operated under stirring, and comprising the steps of:

• (a) polymerizing ethylene with at least one C4-C10 α-olefin in a first reactor using a metallocene catalyst represented by the following formula 1, which comprises a cyclopentadienyl group containing ligand around a transition metal and aryloxide ligand or ligands substituted with aryl derivatives at the ortho-positions, the ligands not being bridged, thus producing a first copolymer solution in which a polymer fraction has a density of 0.860-0.920 g/cm3 and a melt index (MI) of 0.001-1.0 g/10 min.
• (b) passing the first copolymer solution through a second reactor, which contains the same metallocene catalyst as used in step (a), ethylene and at least one C4-C10 α-olefin, at a temperature higher than the reaction temperature of step (a), thus producing a second copolymer solution in which a polymer fraction has a density of 0.900-0.940 g/cm3 and a melt index (MI) of 0.1-10.0 g/10 min;
• and
• (c) passing the second copolymer solution through a third reactor, which contains a Ziegler-Natta catalyst different from the catalyst used in step (b), ethylene and at least one C4-C10 α-olefin, at a temperature higher than the reaction temperature of step (b), thus producing a third copolymer solution:
• wherein M is a group-IV transition metal in the periodic table;
• Cp is cyclopentadienyl or a derivative thereof, which can form an η5-bond with the central metal;
• R1, R2, R3, R4, R5, R6, R7, R8 and R9 on the aryloxide ligands are each independently a hydrogen atom, a halogen atom, a C1-C20 linear or nonlinear alkyl group optionally substituted with at least one halogen atom, a silyl group containing a C1-C20 linear or nonlinear alkyl group optionally substituted with at least one halogen atom, a C6-C30 aryl group optionally substituted with at least one halogen atom, an C7-C30 arylalkyl group optionally substituted with at least one halogen atom, an alkoxy group having a C1-C20 linear or nonlinear alkyl group optionally substituted with at least one halogen atom, a siloxy group substituted with C3-C20 alkyl or C6-C20 aryl, an amido or phosphido group having a C1-C20 hydrocarbon group, or a mercapto or nitro group substituted with C1-C20 alkyl, wherein these substituents may also optionally bind to each other to form a ring;
• and X is selected from the group consisting of a halogen atom, a C1-C20 alkyl group other than a Cp derivative, a C7-C30 arylalkyl group, an alkoxy group having a C1-C20 alkyl group, a siloxy group substituted with C3-C20 alkyl, and an amido group having a C1-C20 hydrocarbon group;
• and n is an integer between 1 and 3.