Electron-Rich Metal Cations Enable Synthesis of High Molecular Weight, Linear Functional Polyethylenes
Electron-Rich Metal Cations Enable Synthesis of High Molecular Weight, Linear Functional Polyethylenes
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富电子金属阳离子能够合成高分子量、线性官能化聚乙烯
DOI：
10.1021/jacs.8b04712
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发表时间：
2018
影响因子：
15
通讯作者：
Carrow, Brad P.
中科院分区：
文献类型：
--
作者：
Zhang, Wei;Waddell, Peter M.;Tiedemann, Margaret A.;Padilla, Christian E.;Mei, Jiajun;Chen, Liye;Carrow, Brad P.
第10族金属催化剂在非极性与极性烯烃共聚以直接生成功能材料方面显示出很大的前景，但获得高共聚物分子量仍然是该领域实际应用的关键挑战。在确定分子量控制的新策略的背景下，我们报道了一系列高度极化的 P(V)–P(III) 螯合配体，它们在单组分 Pd 聚合催化剂的配位范围内表现出独特的空间填充和静电效应，并对（共）聚合物分子量产生重要影响。单组分阳离子膦二酰胺-膦 (PDAP) Pd 催化剂能够生成 Mwup 约为 1 的线性功能性聚乙烯。 2 × 105g mol–1，明显高于该领域的原型催化剂，并且极性含量高达约。 9摩尔%。这些催化剂的官能团几乎完全沿着主链定位，而不是在链端或支链末端，这模仿了商业线性低密度聚乙烯的微观结构。光谱、X 射线晶体学和计算数据表明 PDAP 与 Pd 的配位表现出阳离子但富含电子的活性物种，这可能与其相对于亲电 α-二亚胺（Brookhart 型）或中性膦磺酸根（Drent 型）配合物等特殊催化剂的互补催化性能相关。尽管催化剂配位球内的空间封闭长期以来一直是催化剂分子量控制的可靠策略，但本研究的数据表明，电子控制应被视为一种补充概念，不易抑制高度空间拥挤的催化剂可能发生的共聚单体链结。
Group 10 metal catalysts have shown much promise for the copolymerization of nonpolar with polar alkenes to directly generate functional materials, but access to high copolymer molecular weights nevertheless remains a key challenge toward practical applications in this field. In the context of identifying new strategies for molecular weight control, we report a series of highly polarized P(V)–P(III) chelating ligands that manifest unique space filling and electrostatic effects within the coordination sphere of single component Pd polymerization catalysts and exert important influences on (co)polymer molecular weights. Single component, cationic phosphonic diamide-phosphine (PDAP) Pd catalysts are competent to generate linear, functional polyethylenes withMwup to ca. 2 × 105g mol–1, significantly higher than prototypical catalysts in this field, and with polar content up to ca. 9 mol %. Functional groups are positioned by these catalysts almost exclusively along the main chain, not at chain ends or ends of branches, which mimics the microstructures of commercial linear low-density polyethylenes. Spectroscopic, X-ray crystallographic, and computational data indicate PDAP coordination to Pd manifests cationic yet electron-rich active species, which may correlate to their complementary catalytic properties versus privileged catalysts such as electrophilic α-diimine (Brookhart-type) or neutral phosphine-sulfonato (Drent-type) complexes. Though steric blocking within the catalyst coordination sphere has long been a reliable strategy for catalyst molecular weight control, data from this study suggest electronic control should be considered as a complementary concept less prone to suppression of comonomer enchainment that can occur with highly sterically congested catalysts.