CO/styrene copolymerization by alpha-diimine palladium catalysts is a promising method for direct synthesis of polyketones. The effect of the catalyst backbone structure on CO/styrene copolymerization has been studied with the aim of developing robust alpha-diimine palladium catalysts able to improve the polymerization productivity and controllability. Dibenzobarrelene derived alpha-diimine palladium catalysts without o-aryl substituents were designed and synthesized for CO/styrene alternating copolymerization. Introduction of the rigid and bulky dibenzobarrelene backbone enhanced the thermal stability and the productivity of palladium catalyst. The dibenzobarrelene ligand backbone also improved the polymerization controllability, and living CO/styrene copolymerizations were achieved at 15 degrees C using alpha-diimine palladium catalysts in CH2Cl2. The steric hindrance of backbone and the pi-pi stacking between the dibenzobarrelene backbone and the aniline played crucial roles in stereocontrol and productivity.