Covalently-bonded hyperbranched poly(styrene-divinylbenzene)-based anion exchangers for ion chromatography

A number of covalently-bonded hyperbranched poly(styrene-divinylbenzene)-based (PS-DVB) anion exchangers having functional ion exchange layers of different branching degrees are prepared and investigated. The attachment of the hyperbranched functional layers to the substrate surface is realized via anchor secondary amino groups inserted into the polymeric backbone by means of acylation with acetic anhydride followed by reductive amination with methylamine. Further modification of the obtained secondary amino groups is provided by repeating the steps of alkylation with 1,4-butanediol diglycidyl ether (1,4-BDDGE) and amination of the terminal epoxide rings with methylamine (MA). The variation of the number of cycles including modification with 1,4-BDDGE and MA results in selectivity alterations for the obtained anion exchangers. Chromatographic parameters of the obtained stationary phases are evaluated using the model mixtures of anions (F^-, HCOO^-, Cl^-, EtCOO^-, BrO₃^-, NO₂^-, Br^-, NO₃^-, SO₄^2-, PO₄^3-) with hydroxide and carbonate/bicarbonate eluents. The anion exchangers show the increase of NO₂^-/EtCOO^- and NO₂^-/BrO₃^- selectivity and the decrease of EtCOO^-/Cl^- selectivity with increasing the number of modification cycles. In case of anion exchanger obtained after three modification cycles, the calculated values of column efficiencies for polarizable NO₂^- and Br^- are up to 18,000 and 16,000N/m, respectively.