Flexible α-Diimine Ni(II) and Pd(II) Catalysts Featuring Backbone and Axial Cycloalkyl Substituents in Ethylene (Co)polymerization

Abstract

Recently, axial flexible substituents have found widespread application in ethylene (co)polymerization catalyzed by late transition metals, yielding impressive results. In this research, we designed and synthesized a novel class of flexible α-diimine Ni(II) and Pd(II) catalysts, distinguished by their flexible backbones and axial substituents that both incorporate cycloalkyl moieties. During nickel-catalyzed ethylene polymerization, these flexible nickel catalysts demonstrated high activity (well above 10⁶ g/(mol Ni·h)) and thermal stability, producing polyethylenes with very high molecular weights (up to 1022 kg/mol) and branching densities (up to 103/1000C). Interestingly, the catalyst reported in this study exhibits higher activity compared to nickel catalysts with classic and rigid backbones. The resultant polyethylene materials exhibited outstanding mechanical properties and elastic recovery (with a strain recovery (SR) of up to 79%), qualifying them as high-performance thermoplastic elastomers. In contrast, during palladium-catalyzed ethylene polymerization, these flexible palladium catalysts showed moderate activity (level of 10⁵ g/(mol Pd·h)) and generated polyethylenes with high branching densities (up to 99/1000C) and molecular weights (up to 203 kg/mol). In the case of palladium-catalyzed copolymerization of ethylene with methyl acrylate (MA), the copolymerization activity was notably reduced compared to homopolymerization, resulting in E-MA copolymers with lower molecular weights and higher branching densities. However, under the experimental copolymerization conditions, we successfully obtained copolymers with significant incorporation (1.53–4.54 mol%) of MA. It is worth noting that the cyclohexyl group displayed superior chain transfer inhibition in both nickel and palladium systems compared to the cyclopentyl group. Nevertheless, a notable difference was observed in their influence on branching density regulation: in the nickel system, the cyclohexyl group facilitated the formation of polyethylenes with higher branching, while in the palladium system, it had the opposite effect when compared to the cyclopentyl group.