Preparation and Characterization of Polypseudorotaxanes Based on Block-Selected Inclusion Complexation between Poly(propylene oxide)-Poly(ethylene oxide)-Poly(propylene oxide) Triblock Copolymers and α-Cyclodextrin

Abstract

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.