Supramolecular Dissociation of Polymeric Inclusion Complexes Containing Cyclodextrins as a Method of Preparing New Columnar Structures

Abstract

Polymeric inclusion complexes formed from cyclodextrins (CDs) and linear polymers are molecular necklaces (MNs), structures in which dozens of CD molecules are tightly thread onto a polymer chain [1]. These systems clearly exemplify the significance of the geometric match between the size of the CD cavity and the cross section of the polymer. For example, α -CD forms complexes with poly(ethylene oxide) (PEO) but not with poly(propylene oxide) (PPO); β -CD forms complexes with PPO but not with PEO; and γ -CD interacts either with one PPO molecule or with two PEO molecules [2]. In crystals of MNs, CD molecules are arranged in channels, forming a columnar structure. The question arises as to whether this structure will survive after removal of the thread polymer. Similar structures are of interest as new crystalline CD modifications with a common extended channel and, hence, exhibiting new properties. The resolution of the problem calls for a method of removing the polymer from the MN cavity that would leave the type of crystal lattice unaltered. In this paper, we describe a new property of MNs, namely, their ability to dissociate under the action of an organic solvent, selective to the thread polymer but inert to the macrocycle, to produce a new ordered CD form, a columnar CD structure (CD column ). The synthesis of complexes and their properties are described elsewhere [3‐5]. Previously, we studied the formation, composition, and structure of MNs based on α -, β - , and γ -CDs. These MNs are precipitated on mixing water solutions of a CD and a poly(alkylene oxide) (PAO) complementary in size to the CD. The complexes contain one macrocyclic molecule per two monomeric units of PAO (except the γ -CD‐PEO, which contains a double amount of the polymer). As a third component, MNs also include water, which is involved in the formation of intermolecular hydrogen bonds between hydroxyls of neighboring CD molecules. Therefore, we obtained the complexes α - CD‐PEO, γ -CD‐PEO, and β -CD‐PPO, composed of macrocycles threaded on polymer chains and characterized by a lack of covalent bonds between cyclic and thread molecules. To selectively extract the polymers from MNs, we used both polar (acetone, THF) and nonpolar (chloroform, carbon tetrachloride, benzene, tert -butylbenzene, Tetralin) organic solvents inert to CDs. A solvent was added to a MN powder, stirred for several hours, and then the organic phase was separated. For all three types of complexes, the corresponding polymer was identified in the solution within several hours. The polymer content of the β -CD‐PPO-3500 complexes was quantitatively assessed by IR spectroscopy [6].