Stereoselective Helix-Sense-Selective Cationic Polymerization of N-Vinylcarbazole Using Chiral Lewis Acid Catalysis.
Stereoselective Helix-Sense-Selective Cationic Polymerization of N-Vinylcarbazole Using Chiral Lewis Acid Catalysis.
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DOI：
10.1021/jacs.2c02738
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发表时间：
2022-05
影响因子：
15
通讯作者：
Cole C Sorensen;Frank A. Leibfarth
中科院分区：
文献类型：
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作者：
Cole C Sorensen;Frank A. Leibfarth
具有确定主链阻旋异构体构象的螺旋聚合物在高价值应用如非线性光学和手性分离中是重要的材料。目前，不存在用于前手性乙烯基单体的阳离子螺旋-有义-选择性聚合的方法，这限制了获得许多潜在有价值的光学活性螺旋聚合物。在这里，我们展示了第一个立体选择性阳离子螺旋-有义选择性聚合的前手性乙烯基单体，它提供了访问的光学活性螺旋的聚（N-乙烯基咔唑）。手性双（恶唑啉）-钪刘易斯酸作为手性抗衡离子与N-乙烯基咔唑形成高度全同立构（高达94%内消旋三单元组）的聚合物。机理研究揭示了负责构象独立控制的独特现象（即，螺旋度）和构型（即，立构规整度）立体化学。聚合物螺旋度强烈影响的立体选择性的第一个单体的传播，而聚合物的立构规整度是由不断增长的聚合物链端的构象控制。总的来说，这种方法通过螺旋有义选择性聚合扩展了可访问的螺旋聚合物的套件，并提供了如何独立地控制聚合物立构规整度和螺旋度的机理见解。
Helical polymers with a defined main-chain atropoisomeric conformation are important materials in high value applications such as nonlinear optics and chiral separations. Currently, no methods exist for the cationic helix-sense-selective polymerization of prochiral vinyl monomers, which limits access to a number of potentially valuable optically active helical polymers. Here, we demonstrate the first stereoselective cationic helix-sense-selective polymerization of a prochiral vinyl monomer, which provides access to optically active helices of poly(N-vinylcarbazole). Chiral bis(oxazoline)-scandium Lewis acids serve as chiral counterions to polymerize N-vinylcarbazole into highly isotactic (up to 94% meso triads) polymers. Mechanistic investigations uncovered the distinct phenomenon that are responsible for independent control of conformational (i.e., helicity) and configurational (i.e., tacticity) stereochemistry. Polymer helicity was strongly influenced by the stereoselectivity of the first monomer propagation, whereas polymer tacticity was dictated by the thermodynamically controlled conformation of the growing polymer chain end. Overall, this method expands the suite of accessible helical polymers through helix-sense-selective polymerization and provides mechanistic insight into how polymer tacticity and helicity can be controlled independently.