hereinafter , the present invention will be described in detail by way of the following examples , which are not intended to limit the scope of the present invention . styrene and 1 , 3 - butadiene were copolymerized using n - butyl lithium as an initiator in four steps . polymerization was performed in a stainless steel reactor equipped with a 10l jacket in the nitrogen atmosphere . during the polymerization reaction , the anhydrous reaction mixture was continuously stirred . the polymerization terminator was water and carbon dioxide . a mixture of irganox 1010 0 . 2 phm and tris ( nonylphenyl ) phosphite 0 . 4 phm was added to the copolymer product as a stabilizer , and the solvent was vaporized / removed using a vacuum oven or a devolatilizing extruder . the solid polymer removed of the solvent was measured in regard to different properties . molecular weight information was determined using gpc ( gel permeation chromatography ). for gpc , a waters - 2690 body ( including pump , injector and column box ), a waters 410 differential refractometer as a detector , and hr5e - hr4 - hr4 - hr2 ( waters ) as serial columns were used . tetrahydrofuran was employed as a solvent for gpc to measure the molecular weight of the polymer with a flux of 0 . 3 ml / min at 41 ° c . for 60 minutes per sample . samples for measurement of mechanical properties were fabricated by injection molding . the measurement standards are presented in table 2 . the type and the added amount of the individual ingredient in the respective steps are presented in table 3 . in table 3 , the symbol “( l and s )” represents the addition of the initiator and styrene , and the symbol “( b and s )” represents the addition of 1 , 3 - butadiene and styrene . the symbol “ phm ” represents the part by weight of a specific ingredient based on 100 parts by weight of all monomers . as illustrated in table 3 , the products were prepared by addition of two charges of styrene and the initiator and polymerization ( steps ( 1 ) and ( 3 )) and addition of two charge of 1 , 3 - butadiene and styrene and polymerization ( step ( 2 ) and ( 4 )) in examples 1 to 3 . namely , the product was a bimodal composition of a hepta - block copolymer having two tapered blocks and a tetra - block copolymer having one tapered block . more specifically , the chain structure of the bimodal block copolymer thus obtained was a bimodal mixture composed of a hepta - block copolymer of vinyl aromatic block ( ps1 )- conjugated diene block ( pb1 )- tapered block ( t1 )- vinyl aromatic block ( ps2 - ps3 )- conjugated diene block ( pb2 )- tapered block ( t2 )- vinyl aromatic block ( ps4 ), and a tetra - block copolymer of vinyl aromatic block ( ps3 )- conjugated diene block ( pb2 )- tapered block ( t2 )- vinyl aromatic block ( ps4 ). namely , it is the bimodal block copolymer comprising hepta - block copolymer composed of ( ps1 )-( pb1 )-( t1 )-( ps2 - ps3 )-( pb2 )-( t2 )-( ps4 ) and tetra - block copolymer composed of ( ps3 )-( pb2 )-( t2 )-( ps4 ). the content of the hepta - block copolymer in the bimodal linear copolymer was 72 % in example 1 , 70 % in example 2 , and 72 % in example 3 , respectively . the molecular weight of the hepta - block copolymer and the tetra - block copolymer obtained in the respective examples were analyzed by gpc . a block copolymer having one tapered block in the middle of the polymer chain was prepared from styrene and butadiene by one charge of an initiator . namely , the preparation method of the block copolymer was performed in three steps : ( 1 ) adding styrene and n - butyl lithium , ( 2 ) adding 1 , 3 - butadiene and styrene and ( 3 ) adding styrene . the type and the added amount of the individual ingredient in the respective steps are presented in table 4 . in table 4 , the symbol “( l and s )” represents the addition of the initiator and styrene and the symbol “( b and s )” represents the addition of 1 , 3 - butadiene and styrene . the symbol “ phm ” represents the part by weight of a specific ingredient based on 100 parts by weight of all monomers . the block copolymer chain thus obtained has a tetra - block structure of vinyl aromatic block ( ps1 )- conjugated diene block ( pb )- tapered block ( t )- vinyl aromatic block ( ps2 - ps3 ), i . e ., a monomodal structure of ( ps1 )-( pb )-( t )-( ps2 - ps3 ), which was analyzed by gpc . a block copolymer having one tapered block at the end of the polymer chain was prepared from styrene and butadiene by three charges of an initiator . namely , the preparation method of the block copolymer was performed in five steps : ( 1 ) adding n - butyl lithium and styrene , ( 2 ) further adding n - butyl lithium and styrene , ( 3 ) adding 1 , 3 - butadiene , ( 4 ) further adding n - butyl lithium and styrene , and ( 5 ) adding 1 , 3 - butadiene and styrene . the type and the added amount of the individual ingredient in the respective steps are presented in table 5 . in table 5 , the symbol “( l and s )” represents the addition of the initiator and styrene and the symbol “( b and s )” represents the addition of 1 , 3 - butadiene and styrene . the symbol “ phm ” represents the part by weight of a specific ingredient based on 100 parts by weight of all monomers . the block copolymer chain thus obtained has a trimodal composition composed of a hexa - block copolymer of vinyl aromatic block ( ps1 - ps2 )- conjugated diene block ( pb1 )- vinyl aromatic block ( ps3 )- conjugated diene block ( pb2 )- tapered block ( t )- vinyl aromatic block ( ps4 ); a hexa - block copolymer of vinyl aromatic block ( ps3 )- conjugated diene block ( pb2 )- tapered block ( t )- vinyl aromatic block ( ps4 ); and a tetra - block copolymer of vinyl aromatic block ( ps3 )- conjugated diene block ( pb2 )- tapered block ( t )- vinyl aromatic block ( ps4 ), i . e ., it is a trimodal copolymer comprising two hexa - block copolymers of ( ps1 - ps2 )-( pb 1 )-( ps3 )-( pb2 )-( t )-( ps4 ) and ( ps2 )-( pb 1 )-( ps3 )-( pb2 )-( t )-( ps4 ); and one tetra - block copolymer of ( ps3 )-( pb2 )-( t )-( ps4 ). this was analyzed by gpc . this example compares the properties of the bimodal linear copolymer resin according to the present invention with those of the monomodal linear copolymer resin having one tapered block in the middle of the polymer chain and the trimodal linear copolymer having one tapered block at the end of the polymer chain according to the comparative examples . the bimodal linear copolymer resins of the present invention were prepared according to examples 1 to 3 , and the comparative copolymer resins were prepared according to comparative examples 1 and 2 . the comparison data of properties are presented in table 6 . as can be seen from table 6 , the bimodal linear block copolymers obtained according to the present invention are excellent in rigidity , toughness and flexibility with general properties balanced . but the monomodal block copolymer of comparative example 1 is inferior in strength and toughness and the trimodal block copolymer is inferior in flexibility . namely , the bimodal linear block copolymers of the present invention is readily prepared by a simple method as comparative example 1 , overcoming the limitation in property of the monomodal block copolymer , and acquires more balanced and excellent properties than the trimodal block copolymer of comparative example 2 without using a complex preparation method . as described above , the block copolymer resin of the present invention can be easily controlled in regard to the composition of high molecular weight and low molecular weight copolymers whenever needed and thereby designed with variable mechanical properties such as strength , flexibility , toughness and rigidity . in addition , the block copolymer resin of the present invention readily acquires diversity of properties by varying the molecular weight and the content of the polyvinyl aromatic block and balance of different properties by simplifying its preparation method . while this invention has been described in connection with what is presently considered to be the most practical and preferred embodiment , it is to be understood that the invention is not limited to the disclosed embodiments , but , on the contrary , is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims .