Kinetic modeling of miniemulsion nitroxide mediated polymerization of styrene: Effect of particle diameter and nitroxide partitioning up to high conversion

The miniemulsion polymerization of styrene mediated by N-(2-methyl-2-propyl)-N-(1-diethylphosphono-2,2-dimethylpropyl)-N-oxyl (SG1) at 396 K is modeled up to high conversion as a function of the targeted chain length (TCL) and particle diameter. Thermal self-initiation and diffusional limitations are explicitly accounted for. The importance of the compartmentalization of nitroxide, initiator and macroradicals and of nitroxide partitioning is assessed using 3-dimensional Smith-Ewart equations. Diffusional limitations on termination are important for higher particle diameters only (>～70 nm). The influence of diffusional limitations on deactivation, however, can be significant even for intermediate particles diameters (～40 nm). For a TCL of 300, low particle diameters (<～20 nm) provide theoretically both a better livingness and control over chain length compared to the bulk case at the expense of a significant reduction of the polymerization rate. For a sufficiently high particle diameter (～30 nm), a rate acceleration can be obtained accompanied by an improved livingness but with a somewhat reduced control over chain length. For TCLs higher than 300, better overall average polymer properties can be achieved up to particle diameters of ～ 50 nm. Nitroxide partitioning is shown to lead on average to a limited increase of the polymerization rate without significantly affecting the average polymer properties.