A comprehensive ¹³C nuclear magnetic resonance (NMR) approach for characterizing the location of chain ends of polyethers and polyesters, at the crystallite surface or in the amorphous layers, is presented. The OH chain ends of polyoxymethylene are labeled with ¹³COO-acetyl groups and their dynamics probed by ¹³C NMR with chemical shift anisotropy (CSA) recoupling. At least three-quarters of the chain ends are not mobile dangling cilia but are immobilized, exhibiting a powder pattern characteristic of the crystalline environment and fast CSA dephasing. The location and clustering of the immobilized chain ends are analyzed by spin diffusion. Fast ¹H spin diffusion from the amorphous regions shows confinement of chain ends to the crystallite surface, corroborated by fast ¹³C spin exchange between chain ends. These observations confirm the principle of avoidance of density anomalies, which requires that chains terminate at the crystallite surface to stay out of the crowded interfacial layer.

提出了一种全面的¹³ C核磁共振（NMR）方法，用于表征微晶表面或无定形层中聚醚和聚酯的链端位置。聚甲醛的OH链末端标记有^13个COO-乙酰基，并通过¹³ C NMR与化学位移各向异性（CSA）再偶联探测其动力学。至少四分之三的链末端不是活动的悬垂纤毛，而是固定的，表现出结晶环境和快速CSA脱相的粉末状特征。通过自旋扩散分析固定链末端的位置和聚集。快¹H从无定形区域的自旋扩散显示出链端限制在微晶表面，这通过链端之间的快速¹³ C自旋交换得到证实。这些观察结果证实了避免密度异常的原理，该原理要求链终止于微晶表面以保持在拥挤的界面层之外。