Introducing N-Heterocyclic Olefins as a Novel Class of Organocatalysts for Polymerization

本研究的最终目标是建立N-杂环烯烃（NHO）作为一类新型的有机催化剂/有机引发剂的聚合反应，最值得注意的是那些单体（丙烯酸酯和环氧化物），仍然是极具挑战性的聚合无金属的方法。令人惊讶的是，虽然NHO已被证明是优良的亲核试剂和电子密度供体，但迄今为止它们尚未被应用于聚合催化。 NHO保留了相关的N-杂环卡宾（NHC）的结构多样性，同时提供完全不同的活性，即充当潜在的、可调节的和完全有机衍生的碳负离子。因此，本文所述的研究旨在探索NHO的催化潜力，并绘制出它们对非常不同的单体的反应性曲线。一系列不同构成的NHO将被准备和表征。待研究的结构基序应包括五元和六元NHO，其具有不同空间需求和电子性质的N-烷基和N-芳基取代基。应研究这些NHO作为易受亲核攻击的单体聚合的催化剂，例如，环氧化物、丙烯酸单体和环酯单体。应特别关注合适的有机催化剂的范围仍然非常有限的情况，最值得注意的是丙烯酸酯，包括甲基丙烯酸甲酯（MMA），商业上最重要的丙烯酸酯，或环氧单体，如环氧丙烷（PO）。采用不同的NHO筛选合适的聚合条件（本体、溶剂、温度、引发剂负载）将使得能够关联NHO性质和NHO活性，从而允许优化催化剂的合理设计。聚合机理，这被认为是两性离子的性质，应阐明动力学实验和聚合物表征。预期两性离子生长保留在离子聚合中发现的所有积极特征（控制分子量，容易引入端基），但同时提供了以容易的方式合成环状聚合物结构的可能性，这是以无金属的方式难以实现的特征。

The ultimate goal of this research is to establish N-heterocyclic olefins (NHOs) as a novel class of organocatalysts/organoinitiators for polymerization reactions, most notably for those monomers (acrylates and epoxides) that remain extremely challenging to polymerise by metal-free methods. Surprisingly, though NHOs have been shown to be excellent nucleophiles and electron density donors, they have not been applied for polymerization catalysis so far. NHOs retain the structural diversity of the related N-heterocyclic carbenes (NHCs) while offering a completely different activity, which is to act as latent, tuneable and completely organo-derived carbanions. The study described herein is therefore aimed at exploring the catalytic potential of NHOs and map out their reactivity profile regarding very different monomers. A range of differently constituted NHOs will be prepared and characterized. The structural motifs that are to be investigated shall include five- and six-membered NHOs fitted with N-alkyl and N-aryl substituents of varying steric demands and electronic properties. These NHOs shall be investigated as catalysts for polymerization of monomers that are susceptible to nucleophilic attack, e.g., epoxides, acrylic monomers and cyclic ester monomers. Special focus shall be put on situations where the range of suitable organocatalysts is still very limited, most notably for acrylates including methyl methacrylate (MMA), the commercially most important acrylate, or epoxide monomers like propylene oxide (PO). Screening suitable polymerization conditions (bulk, solvents, temperature, initiator loading) employing the different NHOs will enable correlation of NHO properties and NHO activity, allowing for the rational design of optimized catalysts. The polymerization mechanism, which is thought to be zwitterionic in nature, shall be elucidated by kinetic experiments and polymer characterization. The zwitterionic growth is expected to retain all the positive features that are found in (an)ionic polymerization (control over molecular weight, facile introduction of end groups), but at the same time offers the possibility to synthesise cyclic polymer architectures in a facile manner, a feature that is otherwise difficult to achieve in a metal-free way.