A graft copolymer having metalloporphyrin ionophores covalently attached in a manner that prevents dimer formation is provided. A method of making the graft copolymer comprises polymerizing a functionalized metalloporphyrin monomer, which includes a polymerizable group, with a co-monomer. Methods for synthesizing the polymerizable metalloporphyrin monomers are provided. The graft copolymer can be incorporated into anion-selective membranes for use in anion-detecting sensors, which have improved longevity and response times.