A computational model for free-radical polymerization reactions in an ideally macromixed batch stirred tank reactor has been developed. The polymerization process may take place using an initiator, a chain-transfer agent, and a prescribed temperature program. The model yields, as a function of time, the monomer conversion, the number- and weight-average molecular weights, and a complete molecular weight distribution (MWD) of both the growing and dead polymer chains. A kinetic scheme typical for free-radical polymerization and the method to generate a complete MWD have been adopted from the literature. We included a fundamental approach to account for the "viscosity effects" such as the cage effect, gel effect (Trommsdorff-Norrish-Smith effect), and glass effect as proposed by Achilias and Kiparissides. An expression to account for the chain transfer to a solvent or to a chain-transfer agent has been added. The model has been successfully verified with two literature cases and demonstrated with two of our own defined cases.