Metal-Free Ring-Opening Metathesis Polymerization: From Concept to Creation
Metal-Free Ring-Opening Metathesis Polymerization: From Concept to Creation
复制标题
无金属开环复分解聚合：从概念到创造
DOI：
10.1021/acs.accounts.0c00427
复制
发表时间：
2020
影响因子：
18.3
通讯作者：
Boydston, Andrew J.
中科院分区：
文献类型：
--
作者：
Lu, Pengtao;Kensy, Victoria K.;Tritt, Rachel L.;Seidenkranz, Daniel T.;Boydston, Andrew J.
开环复分解聚合（ROMP）源于过渡金属基烯烃的复分解，已经发展成为学术界和工业上制造功能高分子材料最流行的技术之一。ROMP的最初发现和进展是使用不明确的金属盐混合物来引发聚合。目前最常用的引发剂是定义明确的金属-烷基烯配合物，这些配合物使人们对聚合的机理有了很好的了解，并提高了引发剂的活性、稳定性和官能团耐受性。ROMP的发展无疑是以金属为中心的，通往荣誉的道路主要是在钌、钼和钨基系统中铺设的。我们从ROMP的起点出发的部分灵感来自于最近在自由基介导聚合方面的突破，即利用它们的机制来开发无金属的反应条件。发明一种传统ROMP的无金属补充材料，从本质上讲，需要摆脱几十年来无机和有机金属的发展，但有希望跨越新的合成能力和好奇心。在这种动机的驱动下，以及社区对开发“更环保”的可控聚合的渴望，我们的团队率先寻找并发现了一种完全有机的替代传统金属介导的ROMP。在这篇文章中，我们回顾了我们最近在开发无金属开环复分解聚合（MF-ROMP）方面所做的努力，这是受到先前在电和光介导有机转化方面的报道的启发。这项工作开始于探索烯醚的直接氧化和随后的自由基阳离子引发ROMP的倾向。为了克服电化学条件的限制，接下来研究了光氧化介导的方法，利用光激发的吡啶盐氧化烯醇醚。通过该系统，我们证明了生产ROMP产品和暂时控制聚合的能力。进一步研究了反应的不同方面，包括单体范围、官能团耐受性、改变光催化剂性质的影响以及控制分子量的能力。MF-ROMP的独特机制，以及相对容易合成烯醇醚引发剂，使得制备许多难以通过传统金属介导途径获得的聚合物材料成为可能。在本报告的最后，我们对这一新兴领域的未来机遇进行了展望。
ConspectusRing-opening metathesis polymerization (ROMP), which is derived from transition-metal-based olefin metathesis, has evolved into one of the most prevalent technologies for making functional polymeric materials in academia and in industry. The initial discovery of and advances in ROMP used ill-defined mixtures of metal salts to initiate polymerization. The initiators most commonly used today, developed with tremendous efforts, are well-defined metal–alkylidene complexes that have enabled a good mechanistic understanding of the polymerization as well as improvement of the initiators’ activity, stability, and functional group tolerance.The evolution of ROMP has been decidedly metal-centric, with the path to accolades being paved primarily in ruthenium-, molybdenum-, and tungsten-based systems. Our departure from the ROMP trailhead was inspired in part by recent breakthroughs in radical-mediated polymerizations, whereby their mechanisms were leveraged to develop metal-free reaction conditions. Inventing a metal-free complement to traditional ROMP would essentially involve stepping away from decades of inorganic and organometallic developments, but with the promise of crossing new synthetic capabilities and curiosities.Driven by this motivation, as well as a community-inspired desire to develop “greener” controlled polymerizations, our team pioneered the search for, and discovery of, a wholly organic alternative to traditional metal-mediated ROMP. In this Account, we review our recent efforts to develop metal-free ring-opening metathesis polymerization (MF-ROMP), which is inspired by previous reports in electro- and photo-mediated organic transformations.This work began with an exploration of the direct oxidation of enol ethers and the propensity of the ensuing radical cations to initiate ROMP. To overcome limitations of the electrochemical conditions, a photoredox-mediated method was investigated next, using photoexcited pyrylium salts to oxidize the enol ethers. With this system, we demonstrated the ability to produce ROMP products and temporally control the polymerization.Further investigations into different aspects of the reaction included monomer scope, functional group tolerance, the impact of changing photocatalyst properties, and the ability to control molecular weight. The unique mechanism of MF-ROMP, along with the relative ease of synthesizing enol ether initiators, enabled the preparation of numerous polymeric materials that are hard to access through traditional metal-mediated pathways. At the end of this Account, we provide a perspective on future opportunities in this emerging area.