Preparation Of Single-chain Stranded Poly(pseudo)rotaxanes Based On γ-Cyclodextrins

Poly(pseudo)rotaxanes self-assembled from the inclusion complexation of cyclodextrins(CDs) with polymer chains possess the characteristics of supramolecular non-covalent interactions and stable structure of the mechanical interlock. And these entrapped CDs can slide and rotate along the polymer chain, which impart poly(pseudo)rotaxanes the great potential to be applied as smart materials. At present, the research on α- and β-CD-based poly(pseudo)rotaxanes has been very comprehensive. As compared to α- and β-CD, γ-CD hold a larger inner cavity and a higher structural deformation capacity. γ-CD not only can be single-chain stranded and double-chain stranded with polymer chains to give rise to(un)matched poly(pseudo)rotaxanes, but also the entrapped γ-CD possess more flexable sliding and rotation capability along the polymer chains. Thus, γ-CD-based PPRs and PRs have attracted attention. To this end this paper attempts to examine the self-assembly of γ-CD with different cross-sectional area of the block copolymers and structures of poly(pseudo)rotaxanes, and form unmatchedγ-CD-based PRs showing single chain stranded inclusion complexation architectures via atom transfer radical polymerization(ATRP).Pentablock copolymers PMA-PPO-PEO-PPO-PMA are synthesized via ATRP of methyl acrylate(MA) initiated by 2-bromoisobutyryl end-capped PPO-PEO-PPO as the macroinitiator. Then it is used to self-assemble with a varying amount of γ-CD to prepare polypseudorotaxanes(PPRs). It is found that when the added amount of γ-CD is lower, the central PEO segments are preferentially bent to pass through the cavity of γ-CD to construct double-chain stranded tight-fit PPRs. With increasing the number of added γ-CD,those γ-CD begin to accommodate the outer PMA segments and slip over them to be randomly distributed along the entire pentablock copolymer chain to give rise to single-chain stranded PPRs. This transformation process from the double-chain stranded to the single-chain stranded PPRs lead to the formation of hydrogels. In addition, these single-chain stranded loose-fit PPRs are transformed into the same comformational polyrotaxanes(PRs) after end-capping via in situ ATRP of 2-methacryloyloxyethyl phosphorylcholine(MPC).PBMA-PPO-PEO-PPO-PMBA pentablock copolymer is synthesized via ATRP of n-butyl methacrylate(BMA) using 2-bromoisobutyryl end-capped PPO-PEO-PPO as the macroinitiator and then allowed to self-assemble with γ-CD in acetone/H2 O at roomtemperature. It is found that γ-CD enable to accommodate and slip over the PBMA blocks to be distributed along the entire pentablock copolymer chain to give access to both loose-fit and over-fit single-chain stranded PPRs. It clearly suggests that the attached PBMA blocks would be not bulky enough as stoppers to end-cap the γ-CD-based PPRs into the PR-based multiblock copolymers.