Synthesis and characterization of matched double-chain and loose-fit single-chain stranded γ-CD-based polypseudorotaxane containing block copolymers

By carefully controlling the feeding ratio and slow feeding the reactants, 2-bromoisobutyryl end-capped tetraethylene glycol azobenzene-4,4'-dicarboxylic ester (Br-EG₄-AZO-EG₄-Br, 1) was synthesized as an initiator. With n(γ-CD)/n(Br-EG₄-AZO-EG₄-Br) = 4/1, the resulting initiator 1 was self-assembled with ?-cyclodextrin (γ-CD) in water to create first a double-chain stranded polypseudorotaxane (iPPR) as supramolecular initiator, which was then used to initiate, in situ, atom transfer radical polymerization (ATRP) of N-isopropylacrylamide (NIPAAm) to prepare polypseudorotaxane containing block copolymers (PPRs). It was found that the added γ-CDs accommodate both an azobenzene and a folded tetraethylene glycol chain in the block copolymers, presented as a matched double-chain stranded inclusion complexing structure. However, in the case of self-assembling with PNIPAAm-EG₄-AZO-EG₄-PNIPAAm prepared from ATRP of NIPAAm, γ-CD was randomly distributed along the whole copolymer chain to give access to a series of loose-fit single-chain stranded inclusion complexing structured polypseudorotaxanes (aPPRs). Compared with Br-EG₄-AZO-EG₄-Br and aPPRs, the characteristic peak of ultraviolet (UV) absorption due to trans-azobenzene in PPRs was obviously red-shifted, but the unique cis-isomerization of this group appeared not leading to the threaded γ-CDs sliding off the polymeric chain. All these characterizations suggested that the matched double chain stranded inclusion complexing structure was constructed by an azobenzene and a folded tetraethylene glycol chain jointing to enter the cavity of γ-CD, showing not only a relatively higher stability, but also a reversible transformation into the double tetraethylene glycol chain stranded inclusion complexing structure in the process of cis-isomerization of the azobenzene.