Dispersion polymerization of methyl methacrylate in supercritical carbon dioxide using a pseudo-graft stabilizer: Role of reactor mixing

The free radical polymerization of methyl methacrylate (MMA) in supercritical carbon dioxide (scCO(2)) using a commercially available acid-terminated perfluoropolyether as a stabilizer (Krytox 157 FSL) is analyzed experimentally. A series of polymerizations were carried out under identical conditions; only the rate of stirring was varied. At each specific stirring rate, the complete evolution of conversion and molecular weight was obtained by repeated experiments quenched at different times. The results obtained indicate that the dispersion becomes unstable at a surprisingly moderate stirring rate (above 25 rpm). Clearly, the anchoring interaction of the Krytox stabilizer is relatively weak, and destabilization occurs at modest shear induced by the mechanical stirring. However, when stable conditions are achieved, normal kinetic behavior is observed, leading to a fluffy polymer, with uniform particle size in the range of few microns and relatively broad molecular weight distribution. A particular attraction of these stabilizers is that the anchoring interactions occur via a hydrogen-bonding mechanism which is reversible; thus, no residues of stabilizer are found in the final PMMA product, and the stabilizer is vented with CO2. Apart from the shift of the high molecular weights, likely due to diffusion limitations in the polymer particles at high conversion, this broad molecular weight distribution is a result of the combined action of two reaction loci: the polymer particles and the continuous phase. Even under stable conditions, although the polymer particles are clearly the dominant locus of polymerization, the contribution of the polymerization in the continuous phase cannot be neglected.