Nitroxide-mediated precipitation polymerization of styrene in supercritical carbon dioxide: Effects of monomer loading and nitroxide partitioning on control

Nitroxide-mediated solution and precipitation polymerizations of styrene in toluene and supercritical carbon dioxide (scCO₂), respectively, using the nitroxides N-tert-butyl-N-[1-diethylphosphono-(2,2-dimethylpropyl)]nitroxide (SG1) and 2,2,5-trimethyl-4-phenyl-3-azahexane-3-nitroxide (TIPNO) are presented. Solution polymerizations are compared with simulations using PREDICI software, revealing that differences in the polymerization behaviours between the SG1- and TIPNO-mediated systems cannot be rationalized based on literature rate coefficients and the ideal mechanism for nitroxide-mediated polymerization. Nitroxide and monomer partitioning between the polymer particles and the continuous phase play important roles in the precipitation polymerizations in scCO₂. Loss of control (broader molecular weight distributions) as a result of nitroxide partitioning is accentuated at low monomer loading, and is significantly more pronounced for TIPNO than SG1. However, at higher monomer loading the level of control was superior in scCO₂ compared to in the corresponding solution polymerizations for both nitroxides, most likely caused by an increase in the number of activation-deactivation cycles experienced by any given chain during its growth.