Steric Influences on Chain Microstructure in Palladium-Catalyzed α-Olefin (Co)polymerization: Unveiling the Steric-Deficient Effect.

This study addresses the challenge of controlling branching density and branch-type distribution in late-transition-metal-catalyzed chain walking polymerizations. We explored α-diimine Pd(II) complexes with incrementally increased -aryl sterics for long-chain α-olefin (co)polymerization. - catalysts, which feature gradually increased -aryl sterics and at least one small CH substituent, exhibited similar 2,1-insertion fractions (44-50%), polymer branching densities (55-63/1000C), and melting temperatures (26-28 °C). In contrast, with bulky -aryl sterics covering all sides demonstrated a significant increase in 2,1-insertion fractions up to 82%, leading to "PE-like" polymers with high melting temperatures ( > 111 °C). This abrupt change in polymerization behavior, termed the "steric-deficient effect", contrasts with the gradual changes observed in similar Ni(II) systems that we reported previously. Furthermore, due to the rapid chain walking ability of Pd(II) catalysts in long-chain α-olefin (co)polymerization, these catalysts favor the production of polyolefins with higher proportions of methyl branches compared to those produced by Ni(II) catalysts. Particularly, these Pd(II) catalysts are capable of synthesizing functionalized semicrystalline copolymers by copolymerizing 1-octene with a variety of polar comonomers, thereby significantly altering the surface properties of the materials.