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Branched polyolefins with controllable topology structures were generated from the chain-walking (co)polymerizations of ethylene, 1-pentene (1P) and 2-pentene (2P) using Brookhart-type a-diimine Ni(II)-based catalysts possessing different para-substituted groups, {[(4-R-2-Et-6-Me-C6H2N = C)2Nap]NiBr2, Nap: 1,8-naphthdiyl; R = CHMePh, Ni1; R = Ph, Ni2; R = H, Ni3}. The X-ray diffraction analysis demonstrated that the crystalline structure of Ni1' is in centrosymmetric dimer structure mode with the bimetallic Ni center connected by two bromide bridges. The para-sec-phenethyl moiety in the catalyst Ni1 obviously improved its catalytic performance and thermal stability in the ethylene polymerization. The Ni1/Et2AlCl system showed great catalytic activities (up to 7.73×106 g·mol−1·h−1) and achieved polyethylene (PE) with alkyl chains, including Me, Et, n-Pr, n-Bu, sec-Bu branches and longer chains (Lg). Compared with the 1-pentene polymerization, this catalyst system successfully mediated the polymerization of 2P to give highly branched polymers with approximately 195 branches/1000C possessing Me, Et, and n-Pr branches and a long methylene sequence due to the monomer isomerization. The Et branches derived from 2,3-insertion is slightly less than the sum of Me and n-Pr branches derived from 3,2-insertion, indicating that the 3,2-insertion mode is a slightly favorable pathway in the polymerization of 2P. [ABSTRACT FROM AUTHOR]