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Introduction. Polymer hydrogels are attractive for different biomedical applications because of their favorable biocompatibility. 1 Hydrogels, for example, play an important role in controlled drug delivery because of their pertinence in delivering delicate bioactive agents such as proteins. Recently, physical hydrogels formed with temperature-or pH-sensitive copolymers or based on stereocomplexation of enantiomeric polymers have attracted much attention and been studied potentially as injectable drug delivery matrix. 2 Cyclodextrins (CDs) are a series of cyclic oligosaccharides composed of 6, 7, and 8 D(+)-glucose units linked by R-1,4-linkages and named R-, β-, and γ-CD, respectively. They have been extensively studied in supramolecular chemistry as host molecules capable of including guest molecules ranging from small organic/inorganic compounds to polymers. 3 It was reported that linear polymers such as poly(ethylene oxide) (PEO) can penetrate the inner cavity of cyclodextrins to form inclusion complexes with necklace-like supramolecular structure. 4 Previously, we studied the sol-gel transition during inclusion complexation between R-CD and high molecular weight PEOs. 5 Pluronics are the triblock copolymers of poly(ethylene oxide)-poly(propylene oxide)poly(ethylene oxide) (PEO-PPO-PEO), which were extensively studied as a nonionic surfactant. 6 Here we have found the gel formation of pluronic and R-CD when PEO blocks of the copolymers form inclusion complexes with R-CD in aqueous solution. This is the first report that a block copolymer forms hydrogels with a cyclodextrin.Results and Discussion. Table 1 shows the Pluronic PEO-PPO-PEO triblock copolymers used in this study and the results of the gelation of the Pluronic copolymers with and without R-CD in aqueous solution. It was reported that aqueous solutions of certain Pluronics form gels at high concentrations and elevated temperatures. 7 We tested the gel formation of Pluronics at concentrations of 40 and 13 wt %, respectively, at 22 °C. Of all the copolymers we tested, Pluronic copolymers 5, 6, and 9 formed hydrogels at 40 wt %, while none of the copolymers gelled at 13 wt % concentration. However, aqueous solutions containing 13 wt % of Pluronic copolymers 4, 5, 6, 7, 8, 9, or 10 and 9.7 wt % of R-CD turned into viscous gels at 22 °C (Table 1). In other words, R-CD can aid the gel formation at room temperature for Pluronics with 25 wt % or more of PEO segment even at a low polymer concentration. Interest-

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.