Modified silicone rubber intended mainly for biomedical applications was obtained by in situ polymerization. The process involved swelling of the rubber with a mixture of solvent and monomers, polymerization, and finally evaporating the solvent under reduced pressure. The polymerization system selected was the stepwise hydrogen-transfer polyaddition of 1,4-piperazinediyl diethanethiol to ethylene glycol dimethacrylate, leading to a poly(ester thioether amine). The modified silicone was physically and mechanically characterized by several analytical techniques (Fourier-transform infra-red spectroscopy, differential scanning calorimetry, dynamic mechanical thermal analysis, dynamic contact angle and scanning electron microscopy), and the results obtained were compared with those relative to both the linear guest polymer and the unmodified rubber.