Polyphosphazenes polyelectrolytes. The polyphosphazenes can be prepared by substituting pendant groups (e.g., ionic groups or pendant groups that can form ionic groups) onto a reactive macromolecular precursor for example, by reaction between the reactive chlorine atoms on the backbone of poly(dichlorophosphazene) and appropriate organic nucleophiles. In certain examples, one or more charged pendant groups of a polyphosphazene is/are further modified to introduce desired counterions, which can be hydrophobic counterions. The polyphosphazenes can activate distinct Toll-Like Receptors (TLRs) and can be used in methods of stimulating an immune response.