Polymer nanoparticles have found applications in high performance materials and in medical applications. An attractive route to these particles is the use of templates. However, templating methods have the disadvantage that one templating entity is needed for every object that is formed. The ideal template for polymerizations would therefore be a nanoreactor that regenerates itself once the nanoparticle is formed. Nature provides us with a protein assembly that, when further modified, may fulfill this requirement. The thermosome from Thermoplasma acidophilum is a protein complex which encloses two central cavities with a void volume of 130 nm3. Each cavity is accessible via several small pores and one large pore. The large pore is gated by a build-in lid, whose opening and closing can be controlled by ATP. I propose to use the thermosome as a nanoreactor for Atom Transfer Radical Polymerization (ATRP). To this end, an appropriate catalyst will be covalently linked into the cavity of the protein complex. In the thermosome’s closed conformation, monomers, crosslinkers and initiators can enter the cavity through the small pores, and a polymer particle will form inside of the protein. The cavity of the thermosome acts as template in order to generate nanoparticles of defined size and shape. By opening the lid, the particles will then be released into the surrounding media and the nanoreactor regenerated for further reaction cycles. The objective is to run the process in a continuous way, so that the production of the nanoparticles becomes catalytic. To this end, the formation of the particles will be synchronized with the opening-and-closing cycle of the thermosome. Furthermore, the effect of the confined reaction space provided by the cavity will be exploited to achieve an enhanced control over the ATRP reaction in order to synthesize polymers of narrow molecular weight distribution with significantly less copper-based catalyst than normally needed for aqueous ATRP.