Antibacterial modification of polymers with potent biocides via an efficient and universal approach is very desirable in both academia and industry. A polysiloxane with both quaternary ammonium and N-chloramine was prepared via a facile three-step synthetic route. Poly(methylhydrosiloxane) (PMHS) was first reacted with 2-(dimethylamino)ethyl acrylate to introduce an ending tertiary amine that was subsequently quaternized with 3-(3-chloropropyl)-5,5-dimethylhydantoin (CPDMH). The 5,5-dimethyl-hydantoin moiety was then transformed into its N-chloramine counterparts by chlorination with tent-butyl hypochlorite to produce quaternary ammonium (quat)/N-chloramine polysiloxane. The CO2-philic quat/N-chloramine polysiloxane was interpenetrated into polyethylene terephthalate (PET) in supercritical carbon dioxide (scCO(2)) and formed a 70 urn biocidal layer. The synthetic procedures and interpenetration results were characterized with Fourier transform infrared spectroscopy (FMIR), H-1 NMR, scanning electron microscopy (SEM), and X-ray photoelectron spectrocsopy (XPS). The incorporation of quaternary ammonium and N-chloramine exhibited improved synergetic biocidal performance against Staphylococcus aureus and Escherichia coli, providing a complete kill of a 7-log reduction of both species within a contact time of 10 min. The polysiloxane interpenetration layer was stable and the rechargeability of lost chlorine was good when the layer was subjected to repeated washing, storage, and UV irradiation. This modification procedure uses ecologically responsible CO2 as solvent and is expected to be applicable to substrates of other chemistry since it does not rely on chemical linkage.