Efficient Construction of Macromolecular Loop and Study on Its Dynamics

环状大分子结构由于其闭合的拓扑结构，即没有链端，在基础聚合物化学中得到了越来越多的关注。环状聚合物链端的过量迁移率的缺乏被认为会影响其溶液和本体性质。环状聚合物的典型合成过程是末端官能团聚合物前体在稀释下的闭环反应。然而，开发一种有效的聚合物环化反应仍然是一个持续的挑战。在本研究中，我们通过线型和四臂星形聚苯乙烯的歧化缩合反应，分别合成了环状聚苯乙烯和8-型聚苯乙烯。通过活性阴离子聚合技术制备了聚合物前驱体，并将其用作歧化缩合反应的底物。用核磁共振氢谱、体积排阻色谱(SEC)和临界液相色谱(LCCC)以及MALDI-TOF MS技术对所合成的环状和8字形聚苯乙烯进行了确证。尽管歧化反应是在稀释条件下进行的，但反应浓度应控制在0.2g/L以下，才能得到选择性为80%的环状聚苯乙烯，这是相当好的，但与一些常规方法相当。因此，应继续对有效的环化体系进行进一步的研究，以便为聚合物动力学研究和显微镜分析提供足够数量的环状聚合物样品。

Cyclic macromolecular architectures have gained increasing interests in fundamental polymer chemistry due to its closed loop topology, i.e., absence of chain ends. The lack of the excess mobility conferred by the chain ends of a cyclic polymer is believed to affect its solution and bulk properties. A typical synthetic procedure for cyclic polymers is a ring-closing reaction of an end-functional polymer precursor under dilution. However, a development of an effective polymer cyclization reaction has still been an ongoing challenge. In this study, we have synthesized cyclic and 8-shaped polystyrenes through the metathesis condensation reaction of linear and 4-armed star-shaped polystyrenes having olefinic end groups, respectively, in the presence of Grubbs catalyst. The polymer precursors were prepared by living anionic polymerization technique and were employed as a substrate of the metathesis condensation reaction. The obtained cyclic and 8-shaped polystyrenes were unequivocally characterized by means of 1H NMR, size-exclusion chromatography (SEC) and liquid chromatography at critical condition (LCCC) as well as MALDI-TOF MS techniques. Although the metathesis reaction proceeded even under the diluted condition, the reaction concentration should be below 0.2 g/L to obtain the cyclic polystyrenes in 80% selectivity, which is rather good but comparable to some conventional methods. Thus, the further investigation of an efficient cyclization system should be continued to provide an enough amount of a cyclic polymer sample for a study of polymer dynamics as well as microscopy analysis.