Urea-Based Imprinted Polymer Hosts with Switchable Anion Preference

The design of artificial oxyanion receptors with switchable ion preference is a challenging goal in host–guest chemistry. We here report on molecularly imprinted polymers (MIPs) with an external phospho-sulpho switch driven by small molecule modifiers. The polymers were prepared by hydrogen bond-mediated imprinting of the mono- or dianions of phenyl phosphonic acid (PPA), phenyl sulfonic acid (PSA), and benzoic acid (BA) using N-3,5-bis-(trifluoromethyl)-phenyl-Ń-4-vinylphenyl urea (1) as the functional host monomer. The interaction mode between the functional monomer and the monoanions was elucidated by 1H NMR titrations and 1H–1H NMR NOESY supported by molecular dynamic simulation, which confirmed the presence of high-order complexes. PPA imprinted polymers bound PPA with an equilibrium constant K eq = 1.8 × 105 M–1 in acetonitrile (0.1% 1,2,2,6,6-pentamethylpiperidine) and inorganic HPO4 2– and SO4 2– with K eq = 2.9 × 103 M–1 and 4.5 × 103 M–1, respectively, in aqueous buffer. Moreover, the chromatographic retentivity of phosphonate versus sulfonate was shown to be completely switched on this polymer when changing from a basic to an acidic modifier. Mechanistic insights into this system were obtained from kinetic investigations and DSC-, MALDI-TOF-MS-, 1H NMR-studies of linear polymers prepared in the presence of template. The results suggest the formation of template induced 1–1 diad repeats in the polymer main chain shedding unique light on the relative contributions of configurational and conformational imprinting.