7.1 Introduction

Accounts of the preparation and properties of cyclic ethers invariably start with Pedersen's preparation of dibenzo-18-crown-6 and subsequently of a range of substituted and unsubstituted crown ethers, as reported in 1967 ^[1]. This reflects Pedersen's recognition of their potential as complexing agents rather than prior discovery, since cyclic oligomers of ethylene oxide were already known in the 1950s ^{[2] [3]}, and similar compounds had been reported as early as 1937 ^[4]. The smallest crown is 1,4,7-trioxacyclononane, 9-crown-3, and the commonly encountered members of the series are 12-crown-4 and 18-crown-6.

Our purpose here is not to retrace this familiar ground, which is already comprehensively reviewed ^[5]- ^[8], but to summarise recent work on: (i) large unsubstituted crown ethers, i.e. uniform cyclic oligomers of ethylene oxide containing at least ten oxyethylene units; (ii) non-uniform cyclic poly(oxyethylene)s, i.e. cyclic polymers with narrow distributions of chain length, which the polymer community might call monodisperse; (iii) related cyclic polyethers, e.g. poly(oxypropylene) and poly(oxybutylene); and (iv) cyclic block copolymers with hydrophilic and hydrophobic polyether components. As befits our interests, emphasis is placed upon the properties of the cyclic polymers in the bulk state, particularly the crystalline state, and upon the association properties of the block copolymers in aqueous solution. Application of large crown ethers in forming rotaxanes is covered in another review in this volume.