Dual Polymerization Pathway for Polyolefin-Polar Block Copolymer Synthesis via MILRad: Mechanism and Scope
Dual Polymerization Pathway for Polyolefin-Polar Block Copolymer Synthesis via MILRad: Mechanism and Scope
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DOI：
10.1021/jacs.0c10588
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发表时间：
2020-12-23
影响因子：
15
通讯作者：
Harth, Eva
中科院分区：
文献类型：
--
作者：
Dau, Huong;Keyes, Anthony;Harth, Eva
本工作探讨的机制，使阳离子二亚胺钯（II）配合物结合配位插入和自由基聚合，形成聚烯烃极性嵌段共聚物。最初的要求包括将单个丙烯酸酯单体插入到Pd（II）-聚烯烃中间体中，其通过链行走机制产生稳定的聚合物螯合物。这种化学稳定的螯合物也被发现是光化学惰性的，并发现了一种独特的机制，允许自由基聚合。螯合物通过辅助配体的速率决定性打开，随后是额外的链行走，允许金属迁移到丙烯酸酯部分的α-碳。最终，建立了蓝光触发的Pd-C键从该α-碳均裂形成碳中心大自由基物种所需的分子参数。该中间体被理解为引发丙烯酸单体的自由基聚合，从而促进从单个Pd（II）络合物合成嵌段共聚物。通过详尽的分析方法分离并全面表征了关键中间体，该方法详细说明了机理，同时确认了聚烯烃极性嵌段的结构完整性。链行走结合蓝光照射的功能作为从配位插入到自由基聚合的机械开关。在这些发现的基础上，已经证明了用烯烃（乙烯和1-己烯）和丙烯酸类（丙烯酸甲酯、丙烯酸乙酯、丙烯酸正丁酯和甲基丙烯酸甲酯）的稳健的二嵌段和三嵌段共聚物合成，所述烯烃产生无定形或结晶嵌段，所述丙烯酸类具有宽的分子量范围和组成，产生AB二嵌段和BAB三嵌段。
This work explores the mechanism whereby a cationic diimine Pd(II) complex combines coordination insertion and radical polymerization to form polyolefin-polar block copolymers. The initial requirement involves the insertion of a single acrylate monomer into the Pd(II)-polyolefin intermediates, which generate a stable polymeric chelate through a chain-walking mechanism. This thermodynamically stable chelate was also found to be photochemically inactive, and a unique mechanism was discovered which allows for radical polymerization. Rate-determining opening of the chelate by an ancillary ligand followed by additional chain walking allows the metal to migrate to the alpha-carbon of the acrylate moiety. Ultimately, the molecular parameters necessary for blue-light-triggered Pd-C bond homolysis from this alpha-carbon to form a carbon-centered macroradical species were established. This intermediate is understood to initiate free radical polymerization of acrylic monomers, thereby facilitating block copolymer synthesis from a single Pd(II) complex. Key intermediates were isolated and comprehensively characterized through exhaustive analytical methods which detail the mechanism while confirming the structural integrity of the polyolefin-polar blocks. Chain walking combined with blue-light irradiation functions as the mechanistic switch from coordination insertion to radical polymerization. On the basis of these discoveries, robust di- and triblock copolymer syntheses have been demonstrated with olefins (ethylene and 1-hexene) which produce amorphous or crystalline blocks and acrylics (methyl acrylate, ethyl acrylate, n-butyl acrylate, and methyl methacrylate) in broad molecular weight ranges and compositions, yielding AB diblocks and BAB triblocks.