An equilibrium model for diffusion-limited solid-state polycondensation

A model for unsteady-state solid-state polycondensation (SSP) is developed and is applied to the polymerization of poly(bisphenol A carbonate) and pol y(ethylene terephthalate) (PET). The model assumes that diffusion of the re action condensate in the solid polymer is the rate-limiting step in the ove rall polymerization kinetics. Therefore, the reversible polycondensation re action is at local equilibrium throughout the polymer particle at all times . The model is applicable to the three general types of step-growth polymer ization: AB, A(2), and A(2) + B-2 polycondensation. Through comparison with the predictions of a full kinetic model for polycarbonate synthesis, it is demonstrated that the equilibrium model provides an upper bound on molecul ar weight and its rate of increase. Model predictions are also compared to experimental data for PET SSP. These comparisons show that the equilibrium model provides a useful. tool for understanding the effects of temperature and particle size as well as for establishing a lower bound on the diffusio n coefficient of the reaction condensate in the solid polymer.