Steric Influences\non Chain Microstructure in Palladium-Catalyzed\nα‑Olefin (Co)polymerization: Unveiling the Steric-Deficient\nEffect

This\nstudy addresses the challenge of controlling branching\ndensity\nand branch-type distribution in late-transition-metal-catalyzed chain\nwalking polymerizations. We explored α-diimine Pd(II) complexes\nwith incrementally increased <i>ortho</i>-aryl sterics for\nlong-chain α-olefin (co)polymerization. <b>Pd0</b>–<b>Pd3</b> catalysts, which feature gradually increased <i>ortho</i>-aryl sterics and at least one small CH<sub>3</sub> substituent,\nexhibited similar 2,1-insertion fractions (44–50%), polymer\nbranching densities (55–63/1000C), and melting temperatures\n(26–28 °C). In contrast, <b>Pd4</b> with bulky <i>ortho</i>-aryl sterics covering all sides demonstrated a significant\nincrease in 2,1-insertion fractions up to 82%, leading to “PE-like”\npolymers with high melting temperatures (<i>T</i><sub>m</sub> > 111 °C). This abrupt change in polymerization behavior,\ntermed\nthe “steric-deficient effect”, contrasts with the gradual\nchanges observed in similar Ni(II) systems that we reported previously.\nFurthermore, due to the rapid chain walking ability of Pd(II) catalysts\nin long-chain α-olefin (co)polymerization, these catalysts favor\nthe production of polyolefins with higher proportions of methyl branches\ncompared to those produced by Ni(II) catalysts. Particularly, these\nPd(II) catalysts are capable of synthesizing functionalized semicrystalline\ncopolymers by copolymerizing 1-octene with a variety of polar comonomers,\nthereby significantly altering the surface properties of the materials.