Zinc Aryl/Alkyl β‑diketiminates: Balancing Accessibility and Stability for High-Activity Ring-Opening Polymerization of <i>rac</i>-Lactide

The development of nontoxic initiators which display both high activity and stability at low catalyst concentrations can be challenging, as these desired attributes often involve antagonistic design elements. In this study, we report a family of N-aryl/N′-alkyl zinc β-diketiminate catalysts, Zn­(Ar/AlkL)­[N­(SiMe3)2] and [Zn­(Dipp/BnL)­(OiPr)]2, which can overcome historic challenges encountered with traditional zinc systems in the ring-opening polymerization (ROP) of rac-lactide and l-lactide. The disymmetric 2,6-diisopropylphenyl (Dipp)/benzyl (Bn) derivative, Zn­(Dipp/BnL)­[N­(SiMe3)2], can access polylactic acid (PLA) with a wide range of molecular weights in the presence of a chain transfer agent (e.g., isopropanol; Mn(PLA) = 7–521 kg/mol, D̵ = 1.03–1.07). ROP proceeds rapidly at RT with high turnover frequencies (TOFs = 13,950 h–1 at 0.1 mol % [Zn]; [lactide] = 1.0 M) and low catalyst loadings (down to 0.02 mol %) and enables the synthesis of narrow dispersity ultrahigh molecular weight PLA (UHMW-PLA). Our detailed mechanistic studies identify a unifying and quantitative structure–function relationship for a diverse range of Zn­(R1/R2L)­[N­(SiMe3)2]. This takes the form of a volcano plot, which links the catalyst’s reaction site steric profile (Vb,RS) with catalyst activity in the ROP of rac-lactide [log­(kp)]. Our results establish the utility of disymmetric designs in accessing catalysts with high activity and stability at low concentrations and the prevalence of Sabatier’s principle and volcano plots in the design of homogeneous catalysts for ROP.开发在低催化剂负载下同时兼具高活性与稳定性的无毒引发剂颇具挑战，因为这些理想性能往往对应相互拮抗的设计要素。本研究报道了一类N-芳基/N'-烷基β-二亚胺基锌催化剂（β-diketiminate）：Zn(Ar/AlkL)[N(SiMe3)2]与[Zn(Dipp/BnL)(OiPr)]₂，其可克服传统锌基体系在外消旋丙交酯（rac-lactide）和左旋丙交酯（l-lactide）开环聚合（ring-opening polymerization, ROP）中面临的长期难题。其中不对称的2,6-二异丙基苯基（2,6-diisopropylphenyl, Dipp）/苄基（benzyl, Bn）衍生物Zn(Dipp/BnL)[N(SiMe3)2]，可在链转移剂（如异丙醇）存在下制备分子量范围跨度极广的聚乳酸（polylactic acid, PLA）：数均分子量Mn(PLA) = 7–521 kg/mol，分散度Đ = 1.03–1.07。该开环聚合在室温下快速进行，周转频率（turnover frequencies, TOFs）极高（当锌摩尔负载量为0.1 mol %、丙交酯浓度为1.0 M时，TOFs=13950 h⁻¹），且催化剂负载量可低至0.02 mol %，同时可合成窄分散度超高分子量聚乳酸（ultrahigh molecular weight PLA, UHMW-PLA）。我们的详细机理研究为一系列Zn(R1/R2L)[N(SiMe3)2]型催化剂确立了统一且定量的构效关系：该关系以火山图形式呈现，将催化剂反应位点的空间位阻参数(Vb,RS)与外消旋丙交酯开环聚合中的催化活性[log(kp)]相关联。本研究结果证实了不对称设计在开发低负载下兼具高活性与稳定性催化剂方面的应用价值，同时揭示了萨巴捷原理（Sabatier's principle）与火山图在开环聚合均相催化剂设计中的普遍适用性。