One‐Step Synthesis of K+‐Selective Methacrylic‐Acrylic Copolymers Containing Grafted Ionophore and Requiring No Plasticizer

A main drawback in the application of ion sensors that are based on plasticized polymeric membranes is the problem of leaching of plasticizer and ionophore. Leaching problems can be critical in solid-state minidevices, e.g., ion-selective electrodes or optodes, where a thin ion sensing polymer film is often empolyed. To resolve such problems, we have designed simple methacrylic-acrylic copolymers that required no plasticizer and which include immobilized polymerizable but hydrophilic ionophores such as 4-acryloylamidobenzo-15-crown-5 (AAB15C5) and 4-acryloylamidobenzo-18-crown-6 (AAB18C6) that would normally be susceptible to leaching from plasticized PVC membranes, were used as model ionophores for the immobilization studies. These crown compounds are attractive since they can be employed as a monomeric unit in the copolymer recipe and thus no additional synthesis or polymer modification is required. Copolymers suitable for ion-selective membrane application were synthesized by introducing more than 80 wt. % of n-butyl acrylate to yield copolymers of Tg's between –20 to –30 °C. The molecular weight distributions of these copolymers were M¯w <50 000 Da and they contained 2 to 6 wt. % of immobilized crown ether ionophores. The ‘self-plasticising’ methacrylic-acrylic membranes formed with these acryloyl crown ethers demonstrated good potentiometric responses to potassium ion. In contrast, AAB15C5 and AAB18C6 entrapped in membranes in a more classical ion selective membrane recipe, even when a plasticizer was included, were still inferior in their response to those with membranes containing immobilized ionophores. The restricted mobility of the immobilized ionophores in these self-plasticizing polymer matrices did not appear to hinder the normal complexation behavior of these ionophores. This was inferred from the possible formation of the usual 2:1 and 1:1 ionophore-cation complexes of AAB15C5 and AAB18C6, respectively, in these new copolymer matrices.