Polar Group Coordination Reprograms Active Sites in Binuclear Catalysts

Binuclear organometallic catalysts are often invoked to access cooperative reactivity and different structured polymers beyond mononuclear systems, yet multiple active species in symmetric binuclear catalysts have remained poorly investigated. In this work, a binuclear scandium catalyst bearing structurally symmetric alkyl-bridged bis(fluorenyl) was applied to the copolymerization of ethylene (E) and 1-hexene (Hex) to give distinctly bimodal molecular-weight distributions. The isolated two fractionations have markedly different molecular weights and comonomer incorporations, obviously originating from the in situ generated two types of active species with ratios varying with polymerization conditions. Introducing polar monomers or simple polar additives, the binuclear active species behave similarly to apparent single-sited species, affording unimodal copolymers. Experimental studies combined with density functional theory (DFT) calculations revealed that divergent cooperative structures are generated in the chain-initiation step, leading to differentiated propagation energetic profiles. The presence of polar groups disturbs interactions between scandium centers and reaction double bonds of the monomers by reshaping their steric and electron environments, thereby eliminating multisite behavior. These findings elucidate how subtle coordination perturbations govern active-site speciation in binuclear catalysts and highlight polar-group coordination as an effective strategy to regulate cooperative polymerization pathways.