Selective assembly of cyclodextrins on poly(ethylene oxide)-poly(propyleneoxide) block copolymers

This paper presents a computational study on the formation of a molecular n ecklace formed by specific threading of cyclodextrins (CDs) on block copoly mers. Structural as well as energetic principles for the selective complexa tion of alpha- and beta-cyclodextrin with poly(ethylene oxide)-poly(propyle ne oxide) block copolymers (PEO-PPO) are elucidated considering a diblock c opolymer of equimolecular composition (PEO)(4)-(PPO)(4) as guest. A non-sta tistical distribution of CDs, i.e. alpha-CDs primarily located on the PEO c hain and beta-CDs on PPO blocks of the polymer, is based on a variety of st ructural features and energetic preferences considering both potential as w ell as solvation energies. This selectivity becomes already obvious conside ring 1:1 complexes between PEO and PPO monomers and the two CDs, but is inc reasingly evident when calculating higher order ensembles. Besides the host -guest interaction, docking between CDs themselves is an important, also no n-statistical, prerequisite for the self-assembly of highly ordered tubes. The formation of intermolecular hydrogen bonds between adjacent CDs in a tu bular aggregate gives an important contribution to the overall stability of the molecular necklace. The net effect, based on the preferential interact ion between host and guest as well as between the host molecules themselves , results in the formation of a stable, highly ordered macromolecular, mult icomponent aggregate.