Unexpected Effect of Catalyst’s Structural Symmetry on the Branching Microstructure of Polyethylene in Late Transition Metal Polymerization Catalysis

In
this study, several α-diimine nickel complexes were prepared
by merging a bulky dibenzobarrelene backbone and bulky dibenzhydrylanilines.
The single-crystal X-ray diffraction analysis confirmed that the complex Ni1 exists in an anti-conformation, possessing a quasi-centrosymmetry.
These nickel complexes displayed high activities (above 106 g mol–1·h–1) in ethylene
polymerization, even at 90 °C, providing polyethylenes with very
high molecular weights (well above 500 kg/mol). Unexpectedly, slightly
branched semi-crystalline polyethylenes (11-34/1000C) with high melting
points were obtained with these quasi-centrosymmetric nickel catalysts,
contrary to those common bulky α-diimine nickel complexes that
often render highly branched polyethylenes. The resultant slightly
branched semi-crystalline polyethylenes have good mechanical properties,
possessing moderate to high stress at break values and high strain
at break values. Ethylene–methyl undecenoate copolymerization
using these nickel complexes also gave access to slightly branched
high-molecular-weight polar functionalized copolymers. The experimental
and density functional theory calculation results suggested that the
distribution of the steric hindrance in the as-prepared nickel complexes,
namely, the catalyst’s structural symmetry, might be more favorable
to the chain propagation relative to the branch formation, which was
not observed in α-diimine nickel polymerization systems before.

在本研究中，通过将庞大的二苯并巴雷林主骨架与庞大的二苯基乙基胺基团相结合，成功制备了多种α-二亚胺镍络合物。单晶X射线衍射分析证实，该络合物Ni1以反式构象存在，并具有准中心对称性。这些镍络合物在乙烯聚合反应中表现出极高的活性（超过10^6 g mol–1·h–1），即便在90 °C的高温条件下，也能提供具有非常高的分子量（远超500 kg/mol）的聚乙烯。令人意想不到的是，使用这些准中心对称的镍催化剂，获得了具有较高熔点的微分支半结晶聚乙烯（11-34/1000C），与通常导致高度分支聚乙烯的庞大α-二亚胺镍络合物形成鲜明对比。所得的微分支半结晶聚乙烯具有良好的机械性能，具有中等到高的抗断裂应力和高断裂伸长率。此外，利用这些镍络合物进行乙烯-甲基十一烯酸酯共聚合反应，也实现了微分支高相对分子质量极性功能化共聚物的制备。实验和密度泛函理论计算结果表明，所制备的镍络合物中空间位阻的分布，即催化剂的结构对称性，可能更有利于链增长相对于支链形成，这在之前的α-二亚胺镍聚合体系中未曾观察到。