a polyethylene , having 1 to 60 methyl branches and 1 to 60 hexyl or higher branches per 1000 carbon atoms , a g - value of 0 . 5 to 0 . 8 , and a limiting viscosity of 0 . 005 to 20 . 0 dl / g as measured at 140 ° c . in o - dichlorobenzene , and a method for producing the same by polymerizing ethylene using a catalyst system comprising a coordination nickel compound of zero - or two - valent nickel and an aminobisphosphorane represented by a general formula : ## str1 ## where r 1 , r 2 , r 3 and r 4 may be the same as or different from each other and are respectively n - alkyl , isoalkyl , aryl or trialkylsilyl , in the presence of α - olefin .