Development of thermally stable nickel-based catalysts is highly desirable for industrial gas-phase olefin polymerizations. On the basis of the strategy of promoting the thermostability of nickel catalyst by the ligand backbone, we herein reported novel dibenzobarrelene-derived a-diimine nickel precatalysts for ethylene polymerization. Increasing the steric bulk on the ligand backbone was expected to inhibit the N-aryl rotation of the a-diimine ligands by the repulsive interactions, thus enhancing thermal stability (100 degrees C) and living fashion a temperatures up to 80 degrees C. Bulk ligand backbone also improved tolerance of nickel catalyst toward polar groups, and the alpha-diimine nickel catalyst containing a 2,6-Bu-t-dibenzobarrelene backbone catalyzed living copolymerization of ethylene and methyl 10-undecenoate.