New Neutral Nickel and Palladium Sandwich Catalysts: Synthesis of Ultra-High Molecular Weight Polyethylene (UHMWPE) via Highly Controlled Polymerization and Mechanistic Studies of Chain Propagation

New neutral nickel and palladium ethylene polymerization catalysts have been prepared that incorporate an anionic (N,O) chelating ligand. Extensive axial shielding is provided by two 3,5-dichloroaryl moieties in a “sandwich” orientation. Such shielding results in an exceptionally slow rate of chain transfer relative to migratory insertion in the nickel catalyst, and thus highly controlled polymerization of ethylene is observed, leading to lightly branched ultra-high molecular weight polyethylene with Mn values up to 4.1 × 106 g/mol. The analogous palladium catalysts provide the means for a detailed mechanistic study of chain propagation in an electronically asymmetric neutral palladium catalyst. Both isomers of the methyl ethylene complex can be generated and observed at low temperatures allowing experimental elucidation of mechanistic details of chain propagation probed in other electronically asymmetric systems only through DFT studies or by examination of model studies. The barrier to migratory insertion in these complexes is ca. 19.2 kcal/mol. Investigation of the equilibration of the methyl ethylene isomers in the presence of excess ethylene showed the isomerization rate is dependent on ethylene concentration. This is the first direct proof that isomerization in these alkyl ethylene intermediates is catalyzed by ethylene. Furthermore, isomer equilibration is much faster than migratory insertion so that the barriers for insertion of individual isomers cannot be determined.

已成功制备出一类搭载阴离子型（N,O）螯合配体的中性镍与钯基乙烯聚合催化剂。该催化剂的轴向位阻屏蔽由两个3,5-二氯芳基基团以“三明治”构型提供。针对镍催化剂体系，该位阻屏蔽使得链转移速率相较于迁移插入速率显著减慢，因此可实现高度可控的乙烯聚合，最终得到支化度极低的超高分子量聚乙烯，其数均分子量（Mn）最高可达4.1×10^6 g/mol。同系列的钯基催化剂则为电子不对称型中性钯催化剂的链增长机理研究提供了详实的实验途径：在低温条件下可生成并观测到甲基乙烯基配合物的两种异构体，这使得无需仅依靠密度泛函理论（DFT）研究或模型体系验证，即可通过实验阐明其他电子不对称体系中链增长的机理细节。该类配合物的迁移插入能垒约为19.2 kcal/mol。对过量乙烯氛围下甲基乙烯基异构体的平衡过程进行研究后发现，异构化速率与乙烯浓度呈相关性。这是首次直接证明，此类烷基乙烯基中间体的异构化反应可由乙烯催化。此外，异构体的平衡过程速率远高于迁移插入速率，因此无法测定单一异构体的插入能垒。