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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
Journal of the American Chemical Society ( IF 14.4 ) Pub Date : 2020-04-01 , DOI: 10.1021/jacs.0c02045 Quan H. Tran 1 , Maurice Brookhart 1 , Olafs Daugulis 1
Journal of the American Chemical Society ( IF 14.4 ) Pub Date : 2020-04-01 , DOI: 10.1021/jacs.0c02045 Quan H. Tran 1 , Maurice Brookhart 1 , Olafs Daugulis 1
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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.
中文翻译：
新型中性镍钯夹心催化剂：通过高度可控的聚合和链传播机理研究合成超高分子量聚乙烯 (UHMWPE)
已经制备了新的中性镍和钯乙烯聚合催化剂，其中包含阴离子 (N,O) 螯合配体。两个“夹心”方向的 3,5-二氯芳基部分提供了广泛的轴向屏蔽。相对于镍催化剂中的迁移插入，这种屏蔽导致链转移速度异常缓慢，因此观察到乙烯的高度受控聚合，导致轻微支化的超高分子量聚乙烯，Mn 值高达 4.1 × 106 g/摩尔。类似的钯催化剂为电子不对称中性钯催化剂中链增长的详细机理研究提供了手段。甲基乙烯复合物的两种异构体都可以在低温下生成和观察，从而仅通过 DFT 研究或通过模型研究的检查，就可以实验阐明在其他电子不对称系统中探测到的链增长的机制细节。在这些复合物中迁移插入的障碍是大约。19.2 大卡/摩尔。在过量乙烯存在下甲基乙烯异构体平衡的研究表明异构化速率取决于乙烯浓度。这是第一个直接证明这些烷基乙烯中间体的异构化是由乙烯催化的。此外，异构体平衡比迁移插入快得多，因此无法确定单个异构体插入的障碍。
更新日期：2020-04-01
中文翻译：
新型中性镍钯夹心催化剂：通过高度可控的聚合和链传播机理研究合成超高分子量聚乙烯 (UHMWPE)
已经制备了新的中性镍和钯乙烯聚合催化剂，其中包含阴离子 (N,O) 螯合配体。两个“夹心”方向的 3,5-二氯芳基部分提供了广泛的轴向屏蔽。相对于镍催化剂中的迁移插入，这种屏蔽导致链转移速度异常缓慢，因此观察到乙烯的高度受控聚合，导致轻微支化的超高分子量聚乙烯，Mn 值高达 4.1 × 106 g/摩尔。类似的钯催化剂为电子不对称中性钯催化剂中链增长的详细机理研究提供了手段。甲基乙烯复合物的两种异构体都可以在低温下生成和观察，从而仅通过 DFT 研究或通过模型研究的检查，就可以实验阐明在其他电子不对称系统中探测到的链增长的机制细节。在这些复合物中迁移插入的障碍是大约。19.2 大卡/摩尔。在过量乙烯存在下甲基乙烯异构体平衡的研究表明异构化速率取决于乙烯浓度。这是第一个直接证明这些烷基乙烯中间体的异构化是由乙烯催化的。此外，异构体平衡比迁移插入快得多，因此无法确定单个异构体插入的障碍。
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