Combining ternary phase diagrams and multiphase coupled matrix-based Monte Carlo to model phase dependent compositional and molar mass variations in high impact polystyrene synthesis

High impact polystyrene (HIPS) production starts from a homogeneous grafting of styrene (St) on polybutadiene (PB). However, already at low St conversion, the reaction mixture becomes heterogeneous due to the incompatibility between polystyrene (PS) and PB. First a PS-rich phase appears, dispersed in a PB-rich phase, and then the situation reverses. At even higher St conversions, the PB-rich phase also includes PS occlusions (defined as a third phase). In the present work, a multiphase coupled matrix-based Monte Carlo (CMMC) model is pre-sented, tracking the molecular variations of individual molecules for the first time per phase. This is done ac-counting for (i) phase equilibria as inputted based on a literature-based ternary phase diagram for St-PB-PS mixtures, and (ii) diffusional limitations by means of apparent rate coefficients, demonstrating a preference for the composite kt model for the representation of the gel-effect. The simplified single phase model output is acceptable up to St conversions of 30 %, justifying the previous efforts on tuning modeling parameters also including model validation for high yield general purpose PS synthesis. However, at higher St conversions (up to 60 %), a single phase model predicts too high St amounts in the graft copolymer, and too low PS average chain lengths and dispersities. Furthermore, post-processing at a given St conversion reveals the contributions to the total PS log-molar mass distribution (log-MMD) from the PS-rich phase, the PB-rich phase, and PS occlusions. Moreover, the multimodality of the observed (total) log-MMD is uniquely explained based on these contributions, with specifically a more pronounced second peak due to blocked mass transfer of PS to the PS-rich phase at higher St conversions.