A polymerization method for the preparation of an interpenetrating network polymer with poly(ethylene terephthalate) is reported. Two types of monomer, styrene and 4-vinylpyridine, were chosen as hydrophobic and hydrophilic representatives, respectively, in order to show the versatility of this polymerization method. The polymer-immobilized poly(ethylene terephthalate) samples were characterized using Fourier transform infrared spectroscopy, scanning electron microscopy and X-ray photoelectron spectroscopy. The immobilization efficiency of styrene as a function of reaction temperature, monomer-to-initiator molar ratio, reaction time, addition of solvent, surface hydrophilicity and immersion in initiator was studied. The maximum immobilization percentage of styrene reaches 111%. The immobilization efficiency is proportional to polymer molecular weight and sample thickness. Based on these results, this strategy is shown to be an efficient, versatile method for preparing interpenetrating network polymers and can be used as a means to modify the structures and properties of polymeric substrates.