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| - Preparation and Characterization of PolypseudorotaxanesBased on Block-Selected Inclusion Complexation betweenPoly(propylene oxide)-Poly(ethylene oxide)-Poly(propyleneoxide) Triblock Copolymers and α-Cyclodextrin
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| - 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 α-cyclodextrin (α-CD) to formcrystalline inclusion complexes (ICs), although the middle PEO block was flanked by two thicker PPOblocks, and a PPO chain had been previously thought to be impenetrable to α-CD. X-ray diffraction studiesdemonstrated that the IC domains of the polypseudorotaxanes assumed a channel-type structure similarto the necklace-like ICs formed by α-CD and PEO homopolymers. Solid-state CP/MAS 13C NMR studiesshowed that the α-CD molecules in the polypseudorotaxanes adopted a symmetrical conformation due tothe formation of ICs. The compositions and stoichiometry of the polypseudorotaxanes were studied using1H NMR, and a 2:1 (ethylene oxide unit to α-CD) stoichiometry was found for all polypseudorotaxanesalthough the PPO−PEO−PPO triblock copolymers had different compositions and block lengths, suggestingthat only the PEO block was closely included by α-CD molecules, whereas the PPO blocks were uncovered.The hypothesis was further supported by the differential scanning calorimetry (DSC) studies of thepolypseudorotaxanes. The glass transitions of the PPO blocks in the polypseudorotaxanes were clearlyobserved because they were uncovered by α-CD and remained amorphous, whereas the glass-transitiontemperatures increased, because the molecular motion of the PPO blocks was restricted by the hardcrystalline phases of the IC domains formed by α-CD and the PEO blocks. The thermogravimetric analysis(TGA) revealed that the polypseudorotaxanes had better thermal stability than their free components dueto the inclusion complexation. Finally, the kinetics of the threading process of α-CD onto the copolymerswas also studied. The findings reported in this article suggested interesting possibilities in designing othercyclodextrin ICs and polypseudorotaxanes with block structures.
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