Polymer molecular weight significantly influences end-use product properties such as elasticity, toughness, and tensile strength. A chain transfer agent (CTA), tertiary-dodecyl mercaptan was utilized to control molecular weight. Calorimetry enables product property estimation by exploiting the knowledge inferred from mass and energy balances. We developed a calorimetry model to estimate conversion and molecular weight of polystyrene for emulsion polymerization in batch and semibatch operations. The model also provides the effects of CTA in variable modes of operation. We found that the CTA is significantly able to reduce molecular weight in semibatch compared to batch reactor operation. The calorimetry model was found to be suitable for monitoring the emulsion polymerization process and to track the evolution of molar mass while implementing process optimization and control.