A composition of matter comprises a cationic polymer comprising a polycarbonate chain fragment, the polycarbonate chain fragment comprising a repeat unit comprising a side chain moiety containing a quaternary amine group; and a non-charged polymer comprising a polyester chain segment and a poly(alkylene oxide) chain segment; wherein i) the cationic polymer and the non-charged polymer are amphiphilic and biocompatible, ii) the cationic polymer and the non-charged polymer form a mixed complex by non-covalent interactions in water, and iii) the mixed complex is a more effective antimicrobial agent against at least a Gram-negative microbe compared to the cationic polymer and the non-charged polymer alone when tested using otherwise identical conditions.