Threading α-cyclodextrin through poly [(R,S)-3-hydroxybutyrate] in poly[(R,S)-3-hydroxybutyrate]-poly(ethylene glycol)-poly[(R,S)-3-hydroxybutyrate] triblock copolymers:: Formation of block-selected polypseudorotaxanes

A series of polypseudorotaxanes were synthesized from alpha-cyclodextrin (alpha-CD) and poly[(R,S)3-hydroxybutyrate]-poly(ethylene glycol) -poly [(R,S)-3 -hydroxybutyrate] (PHB-PEG-PHB) triblock copolymers with flanking PHB blocks of different lengths and the middle PEG block of M-n 3000 Da. Formation of inclusion complexes was confirmed by X-ray diffraction data, with a-CD adopting channel-type crystalline structures. The H-1 NMR spectroscopy and thermogravimetric analysis results confirmed the presence of both host and guest molecules, and the compositions determined thereof from the two techniques were in good agreement. In the presence of excess alpha-CD, complexation stoichiometries between ethylene oxide units and alpha-CD for all polypseudorotaxanes were near the theoretical value of 2 despite the different lengths of PHB chains of the copolymers. Together with differential scanning calorimetry measurements where crystallization of the middle PEG block of the copolymers was completely absent while the glass transition of atactic PHB was detected, alpha-CD was thought to selectively cover the middle PEG block leaving telechelic PHB uncovered. The hypothesis was further substantiated by kinetic measurements; precipitation due to aggregation of the stable polypseudorotaxanes was slower with longer PHB chains. These findings demonstrated the successful threading of a-CD over the atactic PHB chain, which was previously thought to be impossible due to the mismatch in cross-sectional area. The study has highlighted the importance of block-selected molecular recognition of alpha-CD on PEG in the formation of stable polypseudorotaxanes of a block copolymer. The above revelations have interesting implications pertaining to design and synthesis of functional materials based on polypseudorotaxanes.