Polyhydroxyalkanoates (PHAs) are biodegradable polyesters produced by many bacterial species under growth-limited conditions when the carbon source is present in excess. It has been recently experimentally demonstrated (Nano Letters 1(9) (2001) 481) that alternating between different carbon sources can lead to the formation of different block copolymer types. This experimental work has been guided by the modeling work presented here as the theoretical considerations permitted evaluation and optimization of the synthesis conditions applied experimentally. To better understand and to optimize the biosynthesis process of different copolymers with desirable properties, we have developed a population balance model that can predict the dynamics of active and inactive PHA polymer chain molecular weight distributions in a nongrowing Ralstonia eutropha cell population. The steady-state version of the model in conjunction with available experimental data was used to compute the steady-state active chain molecular weight distribution and the termination rate as a function of polymer molecular weight for two elongation rate models. For both elongation rate models the steady-state active chain distribution was found to be a monotonically decreasing function of the polymer molecular weight, whereas the termination rate exhibited a maximum. The analytical solution of the steady-state problem was shown to be in excellent agreement with the available experimental data. The population balance model was subsequently used to study the transient dynamics of the process and to predict the experimental conditions, which maximize the production of di- and tri-block copolymer final concentrations. In addition, the structure and molecular weight distribution of the obtained block copolymers were analyzed. Due to the fact that the predicted conditions fall into the range of feasible bioprocessing manipulations, it is expected that such block copolymers can be synthesized. In addition, the proposed model reveals important details of the polymerization process that are difficult to obtain experimentally. Thus, the developed population balance framework should be generally useful for the optimization and control of polymerization processes with similar reaction mechanisms.