Exploring the o-aryl halogen effects of α-diimine Pd(ii) catalysts on ethylene (co)polymerization†

Recently, we delved into the impact of o-aryl halogen effects on nickel α-diimine-catalyzed ethylene polymerization and uncovered some intriguing findings. Building on this work, we now further explore the influence of such substitution on the performance of palladium α-diimine catalysts on both the branching density and catalytic activity in ethylene (co)polymerization. To begin, we synthesized and characterized a range of dibenzhydryl α-diimine palladium catalysts, featuring a butyl backbone adorned with diverse o-aryl halogen substituents. These catalysts exhibited high activity (level of 10⁵ g mol⁻¹ Pd h⁻¹) in ethylene polymerization, yielding highly branched (70–83/1000C) polyethylenes with tunable molecular weights (0.9–44.2 kg mol⁻¹). Significantly, the molecular weight of the resultant polyethylene was critically influenced by the size of the o-aryl halogen substituent, wherein larger halogens contributed to a substantial increase. Interestingly, the effect of halogen substitution on both the branching density and catalytic activity was relatively subtle, likely due to the counteracting forces of steric hindrance and electronic effects. In the copolymerization of ethylene with MA, we observed similar trends with respect to the influence of o-aryl halogen substitution on the molecular weight and branching density. Furthermore, we discovered that bulkier halogens hindered MA incorporation, resulting in copolymers with lower MA insertion ratios, yet they surprisingly boosted the copolymerization activity.