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 solventbased 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 comonomer) with monomer conversion (X) <72% in toluene resulted in resins with M n 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 M n 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 (M n = 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%.