Chiral Hydrogels from Helical Poly(phenylacetylene)s Cross-Linked by Polyrotaxane with Reduction-Regulated De-Cross-Linking

Abstract

Cross-linking of polymers through polyrotaxanes offers an attractive strategy for fabricating hydrogels with unique network architectures and molecular adaptability. In this study, through the cross-linking polymerization of water-soluble chiral phenylacetylene monomers with reduction-cleavable polyrotaxane as the cross-linker, chiral hydrogels based on hydrophilic helical poly(phenylacetylene)s (PPAs) are prepared. Compared to polar organic solvents, water proves to be a highly efficient polymerization medium, affording high-molecular-mass helical PPAs in high yields. The resulting hydrogels, fabricated through aqueous polymerization with either polyrotaxane or a covalent substance as the cross-linker, have low solid content and exhibit good stability and intrinsic chirality originating from the helical PPA backbone. Their mechanical properties are predominantly governed by the semiflexibility of the helical PPA chains, cross-linking density, and solid content, whereas the dynamic sliding of the polyrotaxane cross-linker exerts a negligible influence─a behavior that contrasts with that of polyrotaxane-cross-linked hydrogels derived from flexible polymer chains. Notably, these chiral hydrogels can transform into sols through the dynamic dethreading of polyrotaxanes in response to a reductive stimulus, affording soluble PPAs with chemical structures and helical conformations nearly identical to those of the parent polymers. Polyrotaxane-mediated de-cross-linking of the chiral hydrogels thus enables controlled degradation with tailored kinetics, while also offering the opportunity to recycle soluble helical polymers for reuse.