Zhang Wei, Waddell Peter M, Tiedemann Margaret A, Padilla Christian E, Mei Jiajun, Chen Liye, Carrow Brad P
Department of Chemistry , Princeton University , Princeton , New Jersey 08544 , United States.
J Am Chem Soc. 2018 Jul 18;140(28):8841-8850. doi: 10.1021/jacs.8b04712. Epub 2018 Jul 9.
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 with M up to ca. 2 × 10 g mol, 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.
第10族金属催化剂在非极性烯烃与极性烯烃共聚以直接生成功能材料方面展现出了很大的潜力，但要获得高共聚物分子量仍是该领域实际应用面临的关键挑战。在寻找分子量控制新策略的背景下，我们报道了一系列高度极化的P(V)-P(III)螯合配体，它们在单组分钯聚合催化剂的配位球内表现出独特的空间填充和静电效应，并对（共）聚物分子量产生重要影响。单组分阳离子膦酰二胺-膦（PDAP）钯催化剂能够生成线性、功能性聚乙烯，其M高达约2×10⁵ g/mol，显著高于该领域的典型催化剂，极性含量高达约9 mol%。这些催化剂几乎将官能团仅定位在主链上，而非链端或支链末端，这与商业线性低密度聚乙烯的微观结构相似。光谱、X射线晶体学和计算数据表明，PDAP与钯的配位表现出阳离子但富电子的活性物种，这可能与其相对于亲电α-二亚胺（布鲁克哈特型）或中性膦-磺酸盐（德伦特型）配合物等特权催化剂的互补催化性能相关。尽管催化剂配位球内的空间位阻长期以来一直是控制催化剂分子量的可靠策略，但本研究的数据表明，电子控制应被视为一个互补概念，不太容易抑制高度空间拥挤的催化剂可能发生的共聚单体链增长。
