Synthesis of a tubular polymer from threaded cyclodextrins

Abstract

MUCH attention has been focused recently on the design and fabrication of large-scale molecular structures¹. Carbon nanotubes formed by an arc-discharge method^2,3 have attracted particular attention. These tubes range from about 1 to 30 nanometres in diameter and a micrometre or so in length. To construct smaller tubes, direct chemical synthesis may be a more convenient approach. The cyclic oligomers of glucose known as cyclodextrins (CDs) would seem to be ideal candidates for the components of a molecular tube: they contain cylindrical cavities about 0.7 nm deep, with diameters of 0.45 nm, 0.7 nm and 0.85 nm for α-CD, β-CD and γ-CD respectively⁴. Lehn has recently reported the use of 'bouquet molecules' built from β-CD with long side chains as artificial ion channels⁵. We have previously prepared rotaxane super-molecules in which CDs are threaded on a polymer chain, and we⁶ and others^7,8, have reported polyrotaxanes with many threaded CDs. Here we report the crosslinking of adjacent CD units in apolyrotaxane to create a molecular tube. By removing the bulky ends of the polymer thread, the tube can be unthreaded and can act as a host for reversible binding of small molecules.