Pan Ching W, Xia Katherine, Parker Samantha A, Tillman Eric S
Department of Chemistry and Biochemistry, Santa Clara University, 500 El Camino Real, Santa Clara, CA 95053, USA.
Polymers (Basel). 2018 Jul 31;10(8):844. doi: 10.3390/polym10080844.
Cyclic polymers were produced by end-to-end coupling of telechelic linear polymers under dilute conditions in THF, using intramolecular atom transfer radical coupling or click chemistry. In addition to the expected shift to longer elution times on gel permeation chromatography (GPC) indicative of the formation of cyclic product, lower molecular weight species were consistently observed upon analysis of the unpurified cyclization reaction mixture. By systematically removing or altering single reaction components in the highly dilute cyclization reaction, it was found that THF itself was responsible for the low-molecular-weight material, forming oligomeric chains of poly(THF) regardless of the other reaction components. When the reactions were performed at higher concentrations and for shorter time intervals, conducive to intermolecular chain-end-joining reactions, the low-molecular-weight peaks were absent. Isolation of the material and analysis by ¹H NMR confirmed that poly(THF) was being formed in the highly dilute conditions required for cyclization by end-to-end coupling. When a series of mock cyclization reactions were performed with molar ratios of the reactants held constant, but concentrations changed, it was found that lower concentrations of reactants led to higher amounts of poly(THF) side product.
环状聚合物是通过遥爪线性聚合物在四氢呋喃（THF）稀溶液中，利用分子内原子转移自由基偶联或点击化学进行端对端偶联制备的。除了在凝胶渗透色谱（GPC）上预期出现的洗脱时间向更长方向的偏移，这表明环状产物的形成外，在分析未纯化的环化反应混合物时，始终观察到较低分子量的物质。通过系统地去除或改变高度稀释的环化反应中的单一反应组分，发现THF本身是低分子量物质的成因，无论其他反应组分如何，都会形成聚（THF）的低聚链。当反应在较高浓度和较短时间间隔下进行时，有利于分子间链端连接反应，此时不存在低分子量峰。对该物质进行分离并用¹H NMR分析证实，在端对端偶联环化所需高度稀释的条件下会形成聚（THF）。当以固定的反应物摩尔比进行一系列模拟环化反应，但改变浓度时，发现较低的反应物浓度会导致更多的聚（THF）副产物。
