Radical ring opening polymerisation (RROP) of cyclic ketene acetals (CKAs) is an attractive technique to synthesize branched, (bio)degradable polyesters. However, CKAs as monomers of RROP suffer from hydrolysis lability, making aqueous heterogeneous precipitation polymerisations challenging. In order to explore the precipitation polymerisation of 2-methylene-1,3,6-trioxocane (MTC) and avoid the hydrolysis of monomer, kinetic studies in supercritical CO₂ (scCO₂) were performed. Tuning the reaction pressure and temperature allowed for synthesis in varying viscosity domains. Importantly, we describe the precipitation polymerisation of low T_g PMTC (−65 °C) in scCO₂. In contrast to literature, where a particle-driven polymerisation mechanism has been described for high T_g polymers, a slurry-like precipitation polymerisation was observed for PMTC. Depending on the reaction temperature, polymers with either (i) a defined density of branches (DB) and a scattering of molar mass values, or (ii) a scattering of DB values and relatively defined molar mass were prepared.