Organic-inorganic hybrid slide-ring gels: Polyrotaxanes consisting of poly(dimethylsiloxane) and γ-cyclodextrin and subsequent topological cross-linking

A novel polyrotaxane consisting of poly(dimethylsiloxane) (PDMS) and Zγ-cyclodextrin (γ-CD) is synthesized, and the threaded γ-CDs of polyrotaxane are intermolecularly cross-linked in solution to yield a new class of slide-ring gel. Synthesis of the polyrotaxane is achieved by an end-capping reaction of pseudo-polyrotaxane whose both ends are preliminarily activated as p-nitrophenyl ester with p-methoxy-tritylamine derivatives. The obtained polyrotaxane is soluble in N, N-dimethylformamide (DMF) and N, N-dimethylacetamide (DMAc) only in the presence of lithium salts, while PDMS itself is not soluble in these media. Characterization of the polyrotaxane is performed by NMR, FT-IR, and TGA, showing that it is sparsely populated with γ-CDs. Chemical modifications of γ-CDs of the polyrotaxane improve the solubility in various organic solvents and enable further characterizations. An acetylated polyrotaxane is measured by GPC in chloroform to verify the rotaxanation, showing increase of molecular weight and refractive index compared to PDMS. Cross-linking of these polyrotaxanes with and without modifications is carried out with hexamethylene diisocyanate in solution to bridge γ-CDs of the neighboring polyrotaxanes. As a result, an organic-inorganic hybrid slide-ring gel in which PDMS chains are topologically interlocked by figure-of-eight cross-links is materialized after years of the first slide-ring gel consisting of PEG and α-CD reported in 2001.