Steric-Deficient Effect in Ni/Pd(II)-Catalyzed Long-Chain α-Olefin (Co)polymerization: Systematic Branching Control and Synthesis of Functionalized Elastomers

Achieving precise control over both branching density and branch-type distribution remains a central challenge in chain-walking polymerization with late-transition-metal catalysts. Herein, we clarify and apply the steric-deficient effect, in which a relatively open (less sterically hindered) region within an otherwise crowded ligand framework serves as a sensitive site for steric modulation, leading to abrupt changes in polymerization outcomes upon variation of the ortho steric hindrance of the catalyst. Using α-diimine Ni(II) and Pd(II) complexes, we distinguish between modifications at this sterically deficient site and at more congested (nondeficient) positions. Systematic variation of adjacent aryl substituents shows that tuning steric bulk at the deficient site strongly influences molecular weight, branching density, and melting temperature, whereas analogous changes at nondeficient positions have minimal impact. This effect persists in catalysts with modest steric bulk but becomes attenuated in highly crowded systems due to preexisting steric congestion. Although Ni and Pd catalysts exhibit consistent trends in overall branching regulation, they display opposite tendencies in branch-type distribution. This steric-engineering strategy enables access to polyolefins spanning amorphous to semicrystalline materials. Copolymerization of 1-octene with methyl undecenoate further affords functional elastomers with desirable mechanical and surface properties.