Mathematical Model for the Bulk Polymerization of Styrene Using Multifunctional Initiators

A detailed mathematical model for bulk polymerization of styrene (St) using multifunctional initiators –both linear and cyclic- was developed. The model is based on a kinetic scheme that considers chemical and thermal initiation by sequential decomposition, propagation, transfer to monomer, diffusion-controlled termination by combination and re-initiation reactions and is used to predict the evolution of the chemical species and the molecular structure of the obtained polymeric species. The model calculates the evolution of total or average variables (e.g, total radicals, total polymer, molecular weight distributon moments) as well as the evolution of the detailed complete polymer molecular weight distribution, with polymer species characterized by chain length and number of undecomposed labile groups. The mathematical model was adjusted and validated using experimental data obtained from bulk polymerizations of styrene using various peroxide-type multifunctional initiators: diethyl ketone triperoxide (DEKTP, ctrifunctional cyclic), pinacolone diperoxide (DPP, bifunctional cyclic), 1,1-bis(tert-butylperoxy)cyclohexane (L331M80, bifunctional linear) and benzoyl peroxide (BPO, monofunctional linear) at reaction temperatures of 110-130 °C, where initiator decomposition is mostly sequential. The model is able to very adequately predict experimental polymerization rates and complete molecular weight distributions by adjusting the initiator decomposition constants. The model is used to theoretically evaluate initiator structure and functionality, as well as reaction conditions in a bulk St polymerization process. For a given initiator, the model predicts the existence of optimum temperature and initial initiator concentration resulting in high productivity and increased product quality, which was experimentally validated.