The large size of enzymes and their rich surface chemistry allows to enclose them into nanoscale reaction compartments, such as polymersome nanoreactors and protein cages. BioATRP in polymersomes was shown to fill their lumen with a hydrophilic polymer, thus creating a dense interior which mimics the cytosol of cells. Such polymer-filled nanoreactors could be used to characterize enzymatic reactions in crowded microenvironments and could find application as stabilized polymer nanocapsules for drug delivery purposes. By conducting bioATRP and classic ATRP within the protein cage, we could show that the nanoscale reaction compartment leads to smaller and more narrowly dispersed polymers in comparison to the same reactions in free solution. The protein cage that was used in these studies is the thermosome, a chaperonin that usually helps in refolding unfolded proteins. Thus, the polymers were synthesized in a biological folding chamber, which will be exploited to tune the folding of polymer chains.