In this study, nitroxide mediated polymerization of methacryloisobutyl POSS (POSSMA) and bio‐sourced monomers: isobornyl methacrylate (IBOMA) and C13 methacrylate (C13MA, an alkyl methacrylate with an average chain length of 13 units) was conducted in solution (toluene) and miniemulsion. BlocBuilder‐MA (with 10 mol% acrylonitrile [AN] controlling co‐monomer, for the solvent‐based system) and Dispolreg 007 (for the miniemulsion) were used as the alkoxyamine for initiation and controlling the polymerization. POSSMA/IBOMA/C13MA effective terpolymerization (having 10 mol% AN controlling co‐monomer) with monomer conversion (X) <72% in toluene resulted in resins with Mn up to 21.3 kg mol−1 and Ð < 1.67. Next, terpolymerizations were conducted in dispersed aqueous media to completely remove the organic solvent, resulting in polymers with Mn up to 46.7 kg mol−1 and Ð < 1.65. The successful chain extension of poly(IBOMA/AN) with a mixture of POSSMA/C13MA/AN (Mn = 74.1 kg mol−1 and Ð = 1.55) showed high chain‐end fidelity, exemplified by a clear, monomodal shift in the GPC chromatogram from the macroinitiator. Finally, it was shown that the addition of 20 mol% POSSMA improved the decomposition temperature of bio‐based polymers of IBOMA/C13MA by 15%.