圆偏振光诱导巯基-烯/炔不对称光聚合反应合成光学活性聚合物

The thiol-ene reaction, first introduced in 1905 and accepted as a “click” chemistry reaction given the reaction’s high yield, rapid rate, and thermodynamic driving force, has been used ubiquitously in the synthesis of functional organic compound and polymer. Moreover, its applications in material synthesis, surface modification, polymer functionalization, micellular drug delivery, and high refractive index optical materials have largely arisen due to the broader implementation of click chemistry. However, up to now, there is still no efficient method for triggering enantioselective click reaction to yield chiral molecules or chiral polymers, since D- and L-enantiomers have exactly the same energies and properties. And it is well established that the enantiomeric excess of the chiral compounds or polymers usually cannot arise from the racemic monomers in the absence of any chiral dopant or catalyst. Therefore, it remains a great challenge to realize enantioselective click reaction and synthesize chiral functional molecules or macromolecules from the racemic enantiomers. By using left- and right-handed circularly polarized light (CPL), people have successfully achieved some asymmetric photochemical reactions including photolysis as well as photosynthesis, and modulated the helical structure in azobenzene-containing polymers, chiral coordination polymers, polyfluorene, thioxanthene derivatives, and polydiacetylene. However, attempts to synthesize chiral polymers through enantioselective thiol-ene click chemistry have not been described up to now. In this application, we will develop a novel facial and effective approach to achieve symmetry breaking in thiol-ene based click reaction only using CPL as the external chiral stimuli, and we can envisage that the optical active polymer could be synthesized from racemic or achiral monomers in the absence of any chiral dopant or catalyst. We will study the effect of the molecular structure of thiol and ene groups, the wavelength or the light intensity of the CPL, and the irradiation time on the inducible optical activity of the final polymer, and explore the possible mechanism of the CPL-triggered thiol-ene based asymmetric polymerization. Moreover, super chiral light will be utilized to trigger thiol-ene based asymmetric polymerization reaction, and the inducible optical activity of the final polymer is estimated to be further enhanced. Finally, CPL-triggered thiol-yne based asymmetric polymerization will be also achieved by a similar mechanism. It is anticipated that above CPL-triggered thiol-ene/yne based asymmetric polymerization techniques can be utilized to synthesize optical active polymer with different topological structures and functions, which may have potential applications in biomedical science, asymmetric catalysis and so on. Above studies may pave the novel way for introducing initial chiral biases into key biomolecular building blocks, and will be highly valuable for understanding the origin of homochirality in living matter.

巯基-烯点击化学反应具有适用范围广、反应条件温和、简单高效等特点，已被广泛应用于有机功能小分子和聚合物的可控合成以及材料表面修饰和功能化等领域。目前从外消旋或非手性小分子出发，利用巯基-烯不对称光化学反应选择性合成手性功能小分子和聚合物已成为有重要意义和极富挑战性的工作。本申请拟将圆偏振光辐照技术与巯基-烯点击化学反应相结合，发展从外消旋或非手性单体出发，不加入任何手性添加剂的条件下高效选择性合成光学活性聚合物的新方法：重点关注含巯基和双键的单体分子结构、圆偏振光波长、辐照时间等对聚合产物光学活性的影响，揭示圆偏振光在巯基-烯不对称光聚合反应过程中的诱导作用机制；同时我们拟引入超手性光辐照技术，进一步提高巯基-烯不对称光聚合反应效率；在此基础上，我们拟将上述方法拓展到巯基-炔光聚合反应中，高效选择性合成不同拓扑结构和功能的光学活性聚合物，以期应用于生物医学和不对称催化等领域。

巯基-烯光化学反应作为一类新型的“点击”（click）反应因其反应高效、活性高及反应条件温和等特点，在高分子合成化学等领域得到了广泛应用。巯基-烯光化学反应将光引发与传统“点击”反应的优点充分结合，在特定的区域和功能团间反应，具有高度的选择性。由于手性对映体分子间物理化学性质相近，在不加入手性催化剂的条件下很难控制巯基-烯化学反应的立体选择性。在国家自然科学基金(21774115)的资助下，我们将圆偏振光（CPL）辐照技术与巯基-烯点击化学反应相结合，发展高效选择性合成光学活性聚合物的新方法，揭示圆偏振光在巯基-烯不对称光聚合反应中的诱导作用机制。研究发现在圆偏振光辐照下巯基-烯光化学反应的对映选择性不高(ee值<5%)，这主要是因为外消旋小分子对圆偏振光的选择性吸收差异非常微弱，且在巯基-烯“点击”化学反应的链转移过程中存在外消旋化过程会进一步导致产物ee值降低。在反应过程中加入非手性的氢原子给体(HAD)能有效增强巯基-烯光化学反应的对映选择性。在此基础上我们利用巯基-烯不对称光聚合反应，制备了一系列具有高光学活性的聚硫醚，研究发现在巯基-烯不对称光聚合过程中存在着手性放大机制，随着聚合物链增长，有利于形成特定的手性微环境，促进圆偏振光与单体及中间体自由基之间相互作用，提高巯基-烯不对称光化学反应对映选择性，最终得到的聚合产物ee值可高达20%。除了高光学活性，聚硫醚还具有簇发光和磷光特性，在低温环境和适当混合溶剂中能够发射圆偏振荧光。此外，我们通过巯基-烯及巯基-炔化学反应合成了一系列不同光学活性的线型与超支化聚硫醚，并组装形成囊泡。我们通过实验证明了光学活性囊泡的细胞毒性不仅与聚合物链的拓扑结构有关，还取决于链的立体化学结构和表面电荷分布。上述研究为不同拓扑结构和功能的光学活性聚合物材料的可控制备提供了关键技术，并对探索手性起源问题给予一定的参考和启示。