Sterically demanding Ni-II alpha-diimine precatalysts were synthesized utilizing 2,6-bis(diphenylmethyl)-4-methyl aniline. When activated with methylaluminoxane, the catalyst NiBr2(ArN=C(Me)-C(Me)=NAr) (Ar = 2,6 bis(diphenylmethyl)-4-methylbenzene) was highly active, produced well-defined polyethylene at temperatures up to 100 degrees C (M-w/M-n = 1.09-1.46), and demonstrated remarkable thermal stability at temperatures appropriate for industrially used gas-phase polymerizations (80-100 degrees C).