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（NHOs）作为聚合反应的新型有机催化剂/有机引发剂，尤其是那些单体（丙烯酸酯和环氧化物），它们仍然极具挑战性，无法通过无金属方法进行聚合。令人惊讶的是，尽管NHOs已被证明是优秀的亲核试剂和电子密度供体，但迄今为止它们尚未应用于聚合催化。NHOs保留了相关n -杂环碳烯（NHCs）的结构多样性，同时提供了完全不同的活性，即作为潜在的，可调的和完全有机衍生的碳离子。因此，本文所述的研究旨在探索nho的催化潜力，并绘制出它们对不同单体的反应性曲线。将制备和表征一系列不同构成的nho。要研究的结构基序应该包括五元和六元的NHOs，它们具有不同的空间需求和电子性质的n -烷基和n -芳基取代基。这些NHOs应作为易受亲核攻击的单体聚合的催化剂进行研究，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，或环氧化物单体，如环氧丙烷（PO）。筛选使用不同NHO的合适聚合条件（体积、溶剂、温度、引发剂负载）将使NHO性质和NHO活性相互关联，从而允许合理设计优化的催化剂。聚合机理，本质上被认为是两性离子的，应该通过动力学实验和聚合物表征来阐明。两性离子生长预计将保留在离子聚合中发现的所有积极特征（控制分子量，易于引入端基），但同时提供了以一种容易的方式合成环状聚合物结构的可能性，否则在无金属的方式中很难实现这一特征。

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.