Control of a Living Radical Polymerization of Methacrylates by Light
Control of a Living Radical Polymerization of Methacrylates by Light
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DOI：
10.1002/anie.201203639
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发表时间：
2012-01-01
影响因子：
16.6
通讯作者：
Hawker, Craig J.
中科院分区：
文献类型：
--
作者：
Fors, Brett P.;Hawker, Craig J.
在聚合物合成中，精确控制分子量和分子量分布以及增益序列和结构控制的能力是相当重要的，并且极大地影响了科学和技术的进步。[1]事实上，可控活性聚合方法的发展已经深刻地改变了聚合物研究的策略，如氮氧介导的自由基聚合（NMP），[2]原子转移自由基聚合（ATRP），[3]和可逆加成断裂链转移聚合（RAFT），[4]允许轻松合成结构和功能多样的定义明确的聚合物。最近，人们一直在努力通过开发利用外部刺激调节活化和失活步骤的策略来显著增加活性自由基聚合的范围。[5-7]可以说，控制起始和生长步骤的最成功的策略是Matyjaszewski及其同事最近的工作，他们利用电化学的独特方面来控制ATRP中激活剂与失活剂的比例。[5]通过选择性靶向氧化还原活性催化物质，可以通过调节参数如施加的电流、电势和通过的总电荷来“打开”和“关闭”聚合反应。与传统的自由基聚合一样，最强大和最广泛使用的调节形式是通过光聚合，这是学术界和工业界普遍采用的方法。[8]因此，开发光控活性自由基聚合的能力将是一个重大突破。有趣的是，开发活性自由基聚合的最早尝试之一涉及在UV照射下使用二硫代氨基甲酸酯的引发剂聚合。[9]然而，该方法本质上是有限的，并且控制差，得到宽的分子量分布。随后，已经开发了ATRP、[7] NMP、[10-13]和RAFT [14-17]聚合的光引发，尽管在所有情况下，仅引发步骤是光控制的，并且所有后续生长步骤不能被光调节。因此，开发一种高响应性的光控活性自由基过程，提供对链生长过程的控制，对活性聚合的未来来说既是一个重大的机遇，也是一个挑战。解决这一挑战的关键是Macmillan，[18] Yoon，[19]斯蒂芬森，[19] Stephenson，[1 [20]和其他人[21]，他们利用光氧化还原催化剂的力量进行可见光介导的有机转化。[22-25]我们设想这些光氧化还原催化剂的独特性质将允许开发高度响应的光控活性自由基聚合。我们提出的这一过程的机制如方案1所示。fac-[Ir（ppy）3]（1，图1），一种化合物，
The ability to precisely control molecular weight and molecular weight distributions, as well as gain sequence and architecture control in polymer synthesis is of considerable importance and has greatly impacted the advancement of science and technology.[1] Indeed, the development of controlled living polymerization methods has profoundly changed polymer research with strategies, such as nitroxidemediated radical polymerization (NMP),[2] atom transfer radical polymerization (ATRP),[3] and reversible addition fragmentation chain transfer polymerization (RAFT),[4] allowing the facile synthesis of well-defined polymers that are diverse in both their structure and function. Recently there has been an effort to dramatically increase the scope of living radical polymerization through the development of strategies to regulate the activation and deactivation steps by using an external stimulus.[5–7] Arguably, the most successful strategy that controls both the initiation and growth steps has been the recent work of Matyjaszewski and co-workers who exploited the unique aspects of electrochemistry to control the ratio of activator to deactivator in ATRP.[5] By selective targeting of redox-active catalytic species, the polymerization reaction could be turned “on” and “off” by adjusting parameters such as applied current, potential, and total charge passed. As with traditional radical polymerization, the most robust and widely used form of regulation is through photopolymerization, which is a pervasive procedure in both academia and industry.[8] The ability to develop a photocontrolled living radical polymerization would, therefore, represent a significant breakthrough. Interestingly, one of the earliest attempts to develop a living radical polymerization involved iniferter polymerization using a dithiocarbamate under UV irradiation.[9] However, the procedure was intrinsically limited and poor control and broad molecular weight distributions were obtained. Subsequently, photoinitiation of ATRP,[7] NMP,[10–13] and RAFT [14–17] polymerizations have been developed, though in all cases only the initiation step was photocontrolled and all subsequent growth steps could not be photoregulated. As a result, the development of a highly responsive photocontrolled living radical procedure, which affords control over the chain growth process, is both a major opportunity as well as challenge for the future of living polymerizations.The key to addressing this challenge was recent work by the research groups of Macmillan,[18] Yoon,[19] Stephenson,[20] and others [21] who have exploited the power of photoredox catalysts for organic transformations that are mediated by visible light.[22–25] We envisaged that the unique properties of these photoredox catalysts would allow for the development of a highly responsive photocontrolled living radical polymerization. Our proposed mechanism for this process is shown in Scheme 1. The fac-[Ir (ppy) 3](1, Figure 1), a com-