Molecular Weight Distribution in Di Metal Cyanide Catalyzed Polymerization 2: Numerical Simulation of Chain Activation/Deactivation and Diffusion Effects

A kinetic model for the dynamic simulation of Di Metal Cyanide (DMC) catalyzed alkoxylation is proposed. The model covers the characteristic “catch‐up” kinetics, favoring the growth of small polymers, and high molecular weight tailing (HMWT), which broadens the distribution toward large polymers. Computations are based on the direct numerical integration of fundamental reaction kinetics of a living polymerization with chain length dependent reaction coefficients. It is observed that “catch‐up” kinetics can be modeled by molecular weight (MW) dependent propagation and activation constants. The formation of HMWT can be described by MW dependent segmental growth. Thereby, the segmental growth is influenced by chain length dependent deactivation constants and effects of polymer diffusion. Simulations of (semi‐)batch operations return an appropriate reproduction of measured polymer distributions. Processes with continuous addition of starters (CAOS) and single stage continuous (CSTR) processes are compared to (semi‐)batch operations. Kinetics without “catch‐up” mechanism have a strongly broadened MWD in CAOS/CSTR processes, compared to (semi‐)batch operations. Also, the concentration of short chained polymers, which act as catalyst poison, is increased. In contrast, CAOS/CSTR simulations with “catch‐up” mechanism result in a MWD comparable to results from (semi‐)batch operations and there is no accumulation of short chained polymers. The molecular weight distribution of polymers from DMC catalyzed propoxylation exhibits two characteristics. They are extremely narrow, in batch and continuous production, and have a high molecular weight tailing. A dynamic model is developed that shows how molecular weight dependent growth and segment formation can describe these phenomena. The effect of polymer diffusion on molecular weight distribution is also analyzed.