Stereospecific Living Radical Polymerization: Dual Control of Chain Length and Tacticity for Precision Polymer Synthesis
Stereospecific Living Radical Polymerization: Dual Control of Chain Length and Tacticity for Precision Polymer Synthesis
复制标题
DOI：
10.1002/chin.201011274
复制
发表时间：
2010-03
期刊：
影响因子：
--
通讯作者：
K. Satoh;M. Kamigaito
中科院分区：
文献类型：
--
作者：
K. Satoh;M. Kamigaito
因为“自由基”这个词会让人联想到一些活跃的东西，所以与离子聚合相比，将自由基聚合控制在一个可控的反应中是很困难的，在离子聚合中，生长的物种的反应活性和选择性可以通过与反离子的电子相互作用来调节。尽管有这些臭名昭著的形象，但自由基聚合是工业和实验室规模的化学生产中最有利的化学反应之一，因为它可以将各种乙烯基单体转化为高分子量的聚合物材料，而无需对市售单体和溶剂进行大量纯化，甚至在大量水和极性化合物存在的情况下。10,11自由基聚合的这种高通用性、生产率和耐受性可以通过中性生长物质来实现，这种中性生长物质对各种乙烯基化合物的碳-碳双键具有高度反应性，在反应过程中牺牲了化学选择性、区域选择性和立体选择性。在自由基聚合中，生长的自由基（~ ~ ~ C•）与反应介质中显著存在的单体的CdC双键迅速反应，从而有效地生成聚合物。然而，它也与另一个生长中的自由基链端（~ ~ ~ C•）发生极快的反应，其浓度很低，导致终止或相对缓慢地从聚合物或溶剂中的CH键中提取氢，以诱导链转移反应。虽然这些对链传播不要求的反应的内容取决于绝对反应速率常数和对应物的浓度，但这种化学选择性的缺陷减少了聚合过程中分子量和链端基的控制。此外，特别是对于像醋酸乙烯这样的非共轭单体，由于碳自由基对单体中乙烯基的两个碳原子的区域选择性不完善，形成了高活性的不稳定生长自由基，因此会发生一定程度的头对头繁殖。此外，自由基反应通常诱导非立体选择性繁殖，从单取代乙烯基单体几乎只能得到无规聚合物，因为在平面状sp2-碳自由基物种周围缺乏不对称环境。尽管这些困难可能阻碍了合成聚合物化学家开发可控的自由基聚合，但从各种具有极性官能团的乙烯基单体中获得可控聚合物的更高要求以及对相关研究领域（包括其应用）的潜在强烈影响已经推动他们挑战这一主题。由于前人的重大努力，一些自由基聚合控制现在已经成为可能。特别是，自20世纪90年代以来，可控/活性自由基聚合的巨大发展不仅使各种乙烯基聚合物的分子量得以控制，甚至在水溶液中也得以控制，而且还合成了精细控制结构的聚合物，如嵌段、端功能化、接枝、星形和更复杂的结构聚合物。43-49由于自由基聚合与其他组分的相容性，这些发展已进一步继续并被应用于相关研究领域，涉及通过与天然或生物聚合物、无机物或陶瓷、金属等缀合的新材料。50-59
Because the word “radical” evokes something active, taming a radical polymerization into a controlled reaction has been difficult compared with ionic counterparts, in which the reactivity and selectivity of the growing species can be regulated by the electronic interaction with the counterion. 1-9 Despite such notorious images, radical polymerization is one of the most favorable chemical reactions employed in both industry-and laboratory-scale chemical productions, because it can convert a wide variety of vinyl monomers into high molecular weight polymeric materials without extensive purification of commercially available monomers and solvents or even in the presence of large amounts of water and polar compounds. 10, 11 Such high versatility, productivity, and tolerance of the radical polymerization can be attained by the neutral growing species, which is highly reactive toward the carbon-carbon double bonds of various vinyl compounds, at the expense of the chemo-, regio-, and stereoselectivities during reaction.In radical polymerizations, the growing radical species (∼∼∼ C•) rapidly reacts with the CdC double bonds of the monomers, which significantly exist in the reaction media, to yield the polymers efficiently. However, it also reacts extremely rapidly with another growing radical chain end (∼∼∼ C•), which is present at a very low concentration, to result in termination or abstracts hydrogen relatively slowly from CH bonds in the polymers or solvents to induce chaintransfer reaction. Although the contents of these undemanding reactions for the chain propagation depend on both the absolute reaction rate constant and the concentration of the counterparts, such defects in the chemoselectivity reduce the control of the molecular weights and chain-end groups in the polymerizations. In addition, especially for unconjugated monomers like vinyl acetate, which forms a highly reactive unstabilized growing radical species, some degree of headto-head propagation occurs due to the imperfect regioselectivity of the carbon radical species toward the two carbon atoms of the vinyl group in the monomer. Furthermore, a radical reaction generally induces nonstereoselective propagation to give almost only atactic polymers from monosubstituted vinyl monomers due to the lack of an asymmetric environment around a planar-like sp2-carbon radical species. Although these difficulties might have discouraged synthetic polymer chemists from developing controlled radical polymerizations, much higher demands for obtaining wellcontrolled polymers from various vinyl monomers with polar functional groups and possible strong impacts of the developments on the related research areas including their applications have been pushing them to challenge the topics. Due to the significant efforts of predecessors, some radical polymerization controls have now become possible. Especially, tremendous developments in the controlled/living radical polymerizations from the 1990s have enabled not only thecontrolofmolecularweightsforvariousvinylpolymers10-36 even in aqueous solvents37-42 but also the synthesis of the finely controlled structures of polymers, such as block, endfunctionalized, graft, star, and more complicated architecture polymers. 43-49 These developments have been further continuing and being applied to related research areas involving novel materials via conjugation with natural or biopolymers, inorganics or ceramics, metals, etc., due to the compatibility of the radical polymerization with other components. 50-59