Reactivity ratios for the copolymerization of methyl methacrylate (MMA) and a quaternary ammonium compound N,N-dimethyl-N-{2-[(2-methylprop-2-enoyl)oxy]ethyl}undecane-1-aminium bromide (dMEMUABr) were measured for the first time by using a thermal initiator azobisisobutyronitrile (AIBN) in deuterated dimethyl sulfoxide (DMSO-d(6)) and deuterated chloroform (CDCl3) and a photoinitiator 2-hydroxy-2-methylpropiophenone (HMP) under UV irradiation in CDCl3, respectively. The consumption profiles for the monomers in the thermal initiation copolymerization were monitored by in situ H-1 NMR, whereas the data were analyzed by the Meyo-Lewis, Kelen-Tudos, and Meyer-Lowry methods, showing that the reactivity ratio of dMEMUABr is lower than that of MMA in DMSO-d(6) but much higher in CDCl3. The significantly different reactivity ratios of dMEMUABr in DMSO-d(6) from that in CDCl3 are attributed to micelle formation. These findings enable additional flexibility in the design and development of antimicrobial poly(MMA-co-dMEMUABr) coatings with a desired copolymer chain sequence.