Role of Radical Species in Salicylaldiminato Ni(II) Mediated Polymer Chain Growth: A Case Study for the Migratory Insertion Polymerization of Ethylene in the Presence of Methyl Methacrylate.
Role of Radical Species in Salicylaldiminato Ni(II) Mediated Polymer Chain Growth: A Case Study for the Migratory Insertion Polymerization of Ethylene in the Presence of Methyl Methacrylate.
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DOI：
10.1021/jacs.5b08612
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发表时间：
2015-11
影响因子：
15
通讯作者：
Franz Ölscher;Inigo Göttker-Schnetmann;V. Monteil;S. Mecking
中科院分区：
文献类型：
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作者：
Franz Ölscher;Inigo Göttker-Schnetmann;V. Monteil;S. Mecking
迄今为止，关于中性 Ni(II) 插入聚合催化剂对甲基丙烯酸甲酯 (MMA) 的行为还存在不确定且部分矛盾的情况。我们通过综合机械 NMR 和 EPR 研究、关键 Ni(I) 中间体的分离、乙烯和 MMA 的压力反应器研究以及详细的聚合物分析，阐明了这个问题。揭示了插入和自由基聚合的连锁机制图。两种聚合同时进行（25 bar 乙烯，纯 MMA，70 °C）；然而，链增长循环是彼此独立的，因此仅获得均聚PE和均聚PMMA的物理混合物。 Ni-C 键断裂被排除为自由基源。相反，不稳定芳基膦配体中的均裂 P-C 键断裂以及低价 Ni(0/I) 物质与特定碘取代的 N^O (Ar-I) 配体的反应被证明可以引发自由基 MMA 聚合。催化剂前体或活性中间体的几种还原消除分解途径被证明可形成低价镍物质。其中一个途径是通过中间体 (N^O)Ni(I) 形成的双分子还原偶联。通过用有机自由基源捕获、形成插入聚合活性[(N^O)Ni(II)-R]物种并延长乙烯聚合活性，可以防止这些中间体Ni(I)物种最终分解。
To date, an inconclusive and partially contradictive picture exists on the behavior of neutral Ni(II) insertion polymerization catalysts toward methyl methacrylate (MMA). We shed light on this issue by a combination of comprehensive mechanistic NMR and EPR studies, isolation of a key Ni(I) intermediate, and pressure reactor studies with ethylene and MMA, followed by detailed polymer analysis. An interlocking mechanistic picture of an insertion and a free radical polymerization is revealed. Both polymerizations run simultaneously (25 bar ethylene, neat MMA, 70 °C); however, the chain growth cycles are independent of each other, and therefore exclusively a physical mixture of homo-PE and homo-PMMA is obtained. A Ni-C bond cleavage was excluded as a free radical source. Rather a homolytic P-C bond cleavage in the labile aryl phosphine ligand and the reaction of low-valent Ni(0/I) species with specific iodo substituted N^O (Ar-I) ligands were shown to initiate radical MMA polymerizations. Several reductive elimination decomposition pathways of catalyst precursor or active intermediates were shown to form low-valent Ni species. One of those pathways is a bimolecular reductive coupling via intermediate (N^O)Ni(I) formation. These intermediate Ni(I) species can be prevented from ultimate decomposition by capturing with organic radical sources, forming insertion polymerization active [(N^O)Ni(II)-R] species and prolonging the ethylene polymerization activity.