Systematic Investigations of Ligand Steric Effects on α‑Diimine Palladium Catalyzed Olefin Polymerization and Copolymerization

In the Brookhart type α-diimine palladium catalyst system, it is highly challenging to tune the polymer branching densities through ligand modifications or polymerization conditions. In this contribution, we describe the synthesis and characterization of a series of α-diimine ligands and the corresponding palladium catalysts bearing both the dibenzhydryl moiety and with systematically varied ligand sterics. In ethylene polymerization, it is possible to tune the catalytic activities ((0.77–8.85) × 105 g/(mol Pd·h)), polymer molecular weights (Mn: (0.2–164.7) × 104), branching densities (25–116/1000C), and polymer melting temperatures (amorphous to 98 °C) over a very wide range. In ethylene–methyl acrylate (E–MA) copolymerization, it is possible to tune the catalytic activities ((0.3–8.8) × 103 g/(mol Pd·h)), copolymer molecular weights (1.1 × 103–79.8 × 103), branching densities (30–119/1000C), and MA incorporation ratio (0.4–13.8%) over a very wide range. The molecular weights and branching densities could also be tuned in α-olefin polymerization. The tuning in polymer microstructures leads to significant tuning in polyethylene mechanical properties and the surface properties of the E–MA copolymer.在Brookhart型α-二亚胺（α-diimine）钯催化剂体系中，通过配体修饰或聚合条件调控聚合物支化密度极具挑战。本工作报道了一系列兼具二苯甲基（dibenzhydryl）结构单元、且配体空间位阻可系统调控的α-二亚胺配体及其对应钯催化剂的合成与表征。在乙烯聚合反应中，可在极宽范围内调控催化活性((0.77~8.85)×10⁵ g/(mol Pd·h))、聚合物数均分子量（Mn：(0.2~164.7)×10⁴）、支化密度（25~116/1000C）以及聚合物熔融温度（无定形态至98℃）。在乙烯-丙烯酸甲酯（E-MA）共聚反应中，可在极宽范围内调控催化活性((0.3~8.8)×10³ g/(mol Pd·h))、共聚产物分子量（1.1×10³~79.8×10³）、支化密度（30~119/1000C）以及丙烯酸甲酯（MA）插入率（0.4%~13.8%）。在α-烯烃（α-olefin）聚合反应中，同样可调控聚合物的分子量与支化密度。聚合物微观结构的调控可显著改变聚乙烯的力学性能以及E-MA共聚产物的表面性能。