Li Jun, Ni Xiping, Zhou Zhihan, Leong Kam W
Institute of Materials Research and Engineering (IMRE), 3 Research Link, Singapore 117602, Republic of Singapore.
J Am Chem Soc. 2003 Feb 19;125(7):1788-95. doi: 10.1021/ja026623p.
A series of new polypseudorotaxanes were synthesized in high yields when the middle poly(ethylene oxide) (PEO) block of poly(propylene oxide)-poly(ethylene oxide)-poly(propylene oxide) (PPO-PEO-PPO) triblock copolymers was selectively recognized and included by alpha-cyclodextrin (alpha-CD) to form crystalline inclusion complexes (ICs), although the middle PEO block was flanked by two thicker PPO blocks, and a PPO chain had been previously thought to be impenetrable to alpha-CD. X-ray diffraction studies demonstrated that the IC domains of the polypseudorotaxanes assumed a channel-type structure similar to the necklace-like ICs formed by alpha-CD and PEO homopolymers. Solid-state CP/MAS (13)C NMR studies showed that the alpha-CD molecules in the polypseudorotaxanes adopted a symmetrical conformation due to the formation of ICs. The compositions and stoichiometry of the polypseudorotaxanes were studied using (1)H NMR, and a 2:1 (ethylene oxide unit to alpha-CD) stoichiometry was found for all polypseudorotaxanes although the PPO-PEO-PPO triblock copolymers had different compositions and block lengths, suggesting that only the PEO block was closely included by alpha-CD molecules, whereas the PPO blocks were uncovered. The hypothesis was further supported by the differential scanning calorimetry (DSC) studies of the polypseudorotaxanes. The glass transitions of the PPO blocks in the polypseudorotaxanes were clearly observed because they were uncovered by alpha-CD and remained amorphous, whereas the glass-transition temperatures increased, because the molecular motion of the PPO blocks was restricted by the hard crystalline phases of the IC domains formed by alpha-CD and the PEO blocks. The thermogravimetric analysis (TGA) revealed that the polypseudorotaxanes had better thermal stability than their free components due to the inclusion complexation. Finally, the kinetics of the threading process of alpha-CD onto the copolymers was also studied. The findings reported in this article suggested interesting possibilities in designing other cyclodextrin ICs and polypseudorotaxanes with block structures.
当聚环氧丙烷-聚环氧乙烷-聚环氧丙烷（PPO-PEO-PPO）三嵌段共聚物的中间聚环氧乙烷（PEO）嵌段被α-环糊精（α-CD）选择性识别并包含形成结晶包合物（ICs）时，一系列新型聚准轮烷得以高产率合成，尽管中间的PEO嵌段两侧是两个较厚的PPO嵌段，且此前认为PPO链对α-CD是不可渗透的。X射线衍射研究表明，聚准轮烷的IC域呈现出与α-CD和PEO均聚物形成的项链状ICs相似的通道型结构。固态CP/MAS (13)C NMR研究表明，由于形成了ICs，聚准轮烷中的α-CD分子采取了对称构象。使用(1)H NMR研究了聚准轮烷的组成和化学计量，发现所有聚准轮烷的化学计量均为2:1（环氧乙烷单元与α-CD），尽管PPO-PEO-PPO三嵌段共聚物具有不同的组成和嵌段长度，这表明只有PEO嵌段被α-CD分子紧密包含，而PPO嵌段未被覆盖。聚准轮烷的差示扫描量热法（DSC）研究进一步支持了这一假设。聚准轮烷中PPO嵌段的玻璃化转变清晰可见，因为它们未被α-CD覆盖且保持无定形，而玻璃化转变温度升高，是因为PPO嵌段的分子运动受到α-CD和PEO嵌段形成的IC域的硬结晶相的限制。热重分析（TGA）表明，由于包合络合作用，聚准轮烷比其游离组分具有更好的热稳定性。最后，还研究了α-CD在共聚物上的穿线过程动力学。本文报道的研究结果为设计其他具有嵌段结构的环糊精ICs和聚准轮烷提供了有趣的可能性。
