We have investigated the anionic ring-opening multibranching polymerization for hyperbranched polyglycerol using slow monomer addition at 120 degrees C. Different molecular masses were targeted, and the reaction mixture was probed at regular intervals for the experimental data. The resulting polymers were characterized by gel permeation chromatography, mass spectrometry, and NMR spectroscopy. A computational PREDICI model of the polymerization was developed to describe the experimental parameter dependencies. The rate coefficients were determined for the thermal and base-catalyzed, intra- and intermolecular reactions by fitting simulated number- and weight-average molecular weights to the experimental values. Although the main reaction was expected to be base-mediated, thermal propagation proved to play a crucial role in the dynamics of the investigated system. Both thermal and base-catalyzed self-initiation significantly increased the dispersity for targeting molecular masses exceeding 10 kDa, whereby the size of the growing polymer species was affected by polymerization kinetics.