Secondary reactions have a marked effect on butyl acrylate polymerization rate and polymer structure under the high-temperature free radical reaction conditions typically used for production of solvent-based acrylic coatings resins. The low monomer concentrations characteristic of semibatch starved-feed reactor operation lead to significant rates of intramolecular transfer, resulting in a tertiary radical center capable of termination, monomer addition, or beta-scission. The rate coefficients for these reactions are estimated from monomer concentrations, polymer molecular weights, and NMR analyses of quaternary branch points and macromer end groups for a series of semibatch experiments with varying monomer and polymer concentrations. A classical free radical polymerization model is insufficient for the description of this complex system. Thus, a mechanistic model including the additional reactions has been formulated and is shown to provide a good representation of the experimental results.