The leading role of cation–π interactions in polymer chemistry: the control of the helical sense in solution

Abstract

Cation–π interactions determine the helical sense adopted by a polyphenylacetylene bearing (R)-α-methoxy-α-phenylacetamide as a pendant group (poly-1). These interactions take place when small amounts of Li⁺, Na⁺ and Ag⁺ salts dissolved in donor cosolvents are added to the polymer dissolved in a non-donor solvent and the corresponding helical polymer–metal complexes (HPMCs) are formed. Extensive spectroscopic studies (i.e., ⁷Li, ²³Na, ¹H NMR and IR among others) indicate that the pendants act as dipodal receptors for metal cations, where cation–π interactions play a major role in controlling the M or P helicity of the polymer. Although all the cations tested generate cation–π binding, each metal presents its own particular and distinctive behaviour, based on polymer/cosolvent ratios and the type of cosolvent. Manipulation of these variables allows the selective disruption or activation of the cation–π interactions (i.e., a controlled switch on/off mechanism) and the selection of the helical sense on an à la carte basis. The 3/1 helical structure of poly-1 can cause a novel “triple cascade effect” of cooperative cation–(π)_n–π interactions, which further induce the high chiral amplification response found in these HPMCs. These non-covalent interactions are not found when divalent cations are used instead.